Build the Modular Pinhole Camera

Purpose

To build the camera specified in the design lesson — a light-tight box with an interchangeable pinhole carrier, a shutter, a tripod bush, two stacking extension frames, a flat back and a curved insert — and to finish the session holding a table of measured focal distances rather than a set of drawn ones.

The build is the easy half. The half that decides whether the rest of this part works is the measuring and the inspecting at the end: a camera you have not measured has no f-number, and a camera you have not inspected has no negatives. Budget the full five hours, and put the last hour where this page puts it.

Three routes are given and all three are complete. Plywood is the reference and takes about five hours. Board reaches the same interior with a knife in about three. Tin reaches a working camera with one focal distance in about ninety minutes. None of them is a consolation prize: the optics are set by the hole, the distance and the sheet, and none of those cares what the walls are made of.

Learning objectives

By the end of this session you should be able to:

• set out a box from a computed drawing and cut it square, checking squareness by measurement rather than by eye;
• explain, from the light-trap rule, why every opening joint on the camera is made as a spigot and a socket, and build three of them;
• fit an interchangeable pinhole carrier that centres the hole on the format and repeats its position when a plate is swapped;
• make a shutter that does not leak when closed and does not displace the camera when operated, and prove both;
• measure the true pinhole-to-emulsion distance for every configuration the camera offers, and recompute the f-number, the angle of view and the corner falloff from the measured value;
• run a dark-adapted light-tightness inspection, find leaks in the order they usually appear, and seal them;
• state what you built that differs from the drawing, and predict the optical consequence of each difference before any film is exposed.

Prerequisites

Designing the modular camera — you need your own version of its table, for your format and your register, with the three focal distances and their f-numbers computed.

Making and measuring pinholes — the plates and their carriers exist and the register is filled in. If it is not, the camera will be finished before its holes are, and the fitting steps here have nothing to fit.

The cutting discipline from the camera obscura: steel rule, cutting mat, blade retracted every time the knife is set down.

Safety classification

Level A. The hazards on this page are the hazards of a domestic workshop — cutting tools, a drill, wood dust, sharp sheet metal and an adhesive — and every one of them is controlled by ordinary workshop practice plus eye protection. Nothing here needs a fume cupboard, a respirator as a matter of course, or any control the safety classification reserves for higher levels.

What is not a hazard here, and why. This page handles no photographic chemistry at all. There is no developer, no stop bath, no fixer, no silver salt, no solution of any kind, and consequently no chemical splash risk, no eye-irrigation requirement, no glove selection problem, no ventilation requirement arising from a vapour, and no liquid waste stream. That is worth stating plainly rather than leaving to inference, because the reflex when a course has taught chemical safety for four parts is to import it wholesale, and importing controls that do not apply is its own failure: it wastes attention that the real hazards need. The real hazards here are a sharp blade near a hand and fine dust near a lung.

Two substances could enter the room, and the course’s design removes both. Solvent contact adhesive and cyanoacrylate are not required for any joint on this camera — poly(vinyl acetate) woodworking adhesive and mechanical fastening do every job — and an aerosol paint is not required either, because brush-applied water-based matt black acrylic blackens the interior perfectly well. HSE’s first control measure, ahead of every other, is to eliminate the harmful substance and use a safer one; here that is available for nothing, so the course takes it. If you choose to use either anyway, the control is the product’s own label and safety data sheet, read before the tube is opened, plus open-window working.

Hazards

Cuts, from blades and from sheared metal. A craft knife parts skin more easily than it parts plywood, and tin snips leave an edge that behaves like a blade. HSE’s guidance is written for catering, so take the general principles: keep the blade sharp, because a blunt one needs force and force is what slips; cut on a stable surface; store the tool securely the moment you set it down; never leave it loose where it can be knocked off; never try to catch a falling one. Add the craft rules: cut on a mat against a steel rule, with the stroke moving away from the hand holding the work, and retract the blade every single time. File and tape every cut metal edge before you handle the piece a second time. For a cut, NHS guidance is pressure to stop the bleeding, rinse to clean, a sterile dressing to cover, and no attempt to remove anything embedded yourself.

Eye injury from flying particles, when drilling, sawing and above all cutting or filing metal. OSHA’s rule is the clear statement of the principle: eye or face protection where there is a hazard from flying particles, with side protection where there is a hazard from flying objects. Wear spectacles or goggles to a recognised standard for the whole of the cutting and drilling work, not just the metal parts.

Wood dust. HSE classify all wood dust as a substance hazardous to health: it can cause asthma, dermatitis and irritation of the eyes, nose and throat, and hardwood dust can also cause a rare nasal cancer. It is also flammable. The greatest risk is from the fine fraction, which reaches deep into the lung and travels furthest from the work. Sawing and powered sanding are named among the operations that produce the highest exposures. The workplace exposure limits are 3 mg/m³ for hardwood and 5 mg/m³ for softwood as eight-hour time-weighted averages, and because wood dust is both an asthmagen and, for hardwoods, a carcinogen, exposure must be reduced as far as is reasonably practicable even below those limits. Those are limits for people at work, and a person building one camera on a Sunday is not; the biology, however, does not know that. The controls that transfer are: cut and sand outdoors or at an open window with the door to the rest of the house shut; use a hand saw rather than a power saw if you have the choice, because it makes coarser dust and less of it; damp-wipe or vacuum, never dry-sweep and never blow with compressed air, both of which HSE name as things to avoid because they put settled dust back into the air; and choose plywood over medium-density fibreboard, which cuts to a much finer dust.

Adhesive. PVA is the adhesive for this build and it is a water-based emulsion; the control is ordinary care and washing hands. If you use cyanoacrylate for a small repair, PubChem’s aggregation of the ECHA notifications for ethyl 2-cyanoacrylate carries H302, harmful if swallowed, and H315, causes skin irritation, both with the signal word Warning. It also bonds skin, and the label on the tube is the authority for that and for the first-aid instruction; read it before you open it, and keep the work below the level of your face, since an unprotected eye is what that warning is about.

Paint, and what it might do to paper afterwards. Water-based matt black acrylic is a household product; use it as its label says. The photographic question is separate and is worth being precise about. FOMA instruct that unexposed film be stored “out of reach of harmful vapours, gases and ionizing radiations”, which is a manufacturer’s statement that vapours can damage sensitised material. Whether a particular paint or adhesive outgasses enough to fog or stain a sheet of photographic paper inside a closed camera is not documented anywhere this course has been able to find, so the following is a precaution and not a finding: leave the camera open in a warm airy place until every surface is hard and odourless — a day for acrylic, longer if it is thick — and make the first sheet you put in it an unexposed control that goes straight to the developer.

The drill. Clamp the work; never hold a small part in your fingers against a bit. Do not wear gloves at a rotating chuck — a glove that catches takes the hand with it — and tie back long hair.

Required PPE

Eye protection, worn for all cutting, drilling and filing, and without exception for the metal work. This is the one item on the list that is not optional and not substitutable.

A steel rule and a self-healing cutting mat, which are protective equipment on this page rather than conveniences. A plastic or wooden rule lets a blade ride up over its edge and into the hand holding it; a steel one stops it. The mat holds the work and keeps the blade from skating.

Respiratory protection, only if the work warrants it. Cutting a dozen small plywood parts with a hand saw at an open window does not. Machine-sanding, or any prolonged sanding indoors, does: HSE specify a respirator with an assigned protection factor of at least 20 — “suitable disposable respirators are often described as FFP3” — CE or UKCA marked, and fit-tested to the individual, noting that facial hair or glasses lift the seal and let contaminated air in. A mask that does not fit is not protection, it is the appearance of protection.

Not required, and named so that their absence is a decision rather than an oversight: chemical gloves, because nothing here is a chemical splash; a fume cupboard or extraction, because the two vapour-producing products the build might have used have been designed out; and an apron, because there is nothing to spill.

Parts

Quantities are for the reference 4 × 5 inch camera with an interior of 135 × 110 × 50 mm. Sizes are from the design lesson’s cutting list; recompute them if your format differs.

Item	Quantity	Notes
Birch plywood, 6 mm	one panel 300 × 600 mm	The whole body and both frames come out of this one panel; the cutting plan below shows how
Birch plywood, 4 mm	one panel 300 × 300 mm	Back tray and pressure plate
Birch plywood or stiff card, 3 mm	one panel 300 × 300 mm	Collars, carrier track, paper stops
Black foam board 5 mm, or 2 mm greyboard	2 sheets A2	Board route only, in place of the plywood
Rectangular tin with a press-fit lid	1	Tin route only; deeper than 40 mm and larger than the sheet you intend to use
PVA woodworking adhesive	100 ml	The only adhesive the build requires
Matt black acrylic paint, water-based	100 ml	Interior of everything, and both faces of every pinhole plate
Black self-adhesive felt or flocking sheet	one A4 sheet	The two long interior walls and the dark-slide slot
Black photographic or gaffer tape	1 roll	Sealing found leaks; taping metal edges
¼-inch 20 tpi UNC T-nut, with a bolt to match	1	The tripod bush
Small neodymium magnets, 6 mm	4	Two for the carrier, two for the cap shutter
Black polypropylene or painted aluminium sheet, 0.5 mm	150 × 130 mm	The dark slide
Thin closed-cell foam, 3 mm	100 × 125 mm	Faces the pressure plate
Pinhole plates in their carriers	3 or 4	From the pinhole register; not made here
Second-hand 4 × 5 inch double dark slide	1, optional	Only if you will shoot sheet film

Tools

All routes: a steel rule at least 300 mm; a try square or engineer’s square; a sharp pencil and a fine marker; a cutting mat; a craft knife with spare blades; a bradawl; abrasive paper, 120 and 240 grit; a damp cloth; a tape measure; eye protection; a stiff card or a straight steel rod about 200 mm long for the film-plane measurement; a torch.

Plywood route, additionally: a fine-toothed hand saw (a Japanese pull saw or a tenon saw and mitre box gives the squarest cuts by hand); four small clamps or half a dozen spring clips; a drill with a bit to suit the T-nut’s shank; a sanding block.

Board route, additionally: a heavy steel straight-edge at least 500 mm, because a long cut in one pass is squarer than three short ones; a fresh packet of blades, since foam board blunts a blade in about a metre of cutting.

Tin route, additionally: tin snips; a half-round file; a hammer and a nail or centre punch; a scrap block of softwood to back the metal while you punch it.

Not needed: a table saw, a router, a bandsaw. Every cut on this page can be made with a hand saw or a knife, and the design was drawn that way on purpose.

Estimated cost

Cost band ££. The plywood, the T-nut, the magnets, the felt and the tape are the whole of it, and the single item that can double the total is a second-hand 4 × 5 inch film holder — which is optional and is not needed for any exposure in this part except the sheet-film run.

This page quotes no prices, because it cannot verify a current one. Dated UK figures live in the laboratory planner, where they can be corrected without rewriting the text, and the three cost tiers are the same three the part overview sets out: the tin route is effectively free, the board route is pocket money, and the plywood route with a bought film holder is the top of the band. A commercially made pinhole body is an accepted alternative to all three; if you buy one, you still owe the measuring and the inspection in this session, because a bought camera’s focal distance is a claim until you have measured it.

Estimated consumables cost

A camera is built once, so most of the Parts table above is capital rather than consumption. What is genuinely used up in the build is the adhesive, the paint, the felt, the foam and the tape — and the plywood, which is bought as a panel and cut to nothing.

Consumed	This session	Sourced price	Cost this session
PVA woodworking adhesive	100 mL of a 5 L container	£11.99–£14.29 per 5 L	£0.24–£0.29
Birch plywood, 6 mm and 4 mm	one 300 × 600 mm panel and one 300 × 300 mm	Priced by the sheet in the planner, not by the offcut	—
Matt black acrylic paint	100 mL, interior and both faces of every plate	None. matt-black-paint carries a cost band and no dated figure	—
Black self-adhesive felt or flocking	one A4 sheet	None. A named price gap: camera build hardware — the T-nut, the magnets, the felt, the foam, the sheet and the tin	—
Thin closed-cell foam, 3 mm	100 × 125 mm	None. A named price gap: camera build hardware — the T-nut, the magnets, the felt, the foam, the sheet and the tin	—
Black photographic or gaffer tape	about one roll across the build and commissioning	None. tape-and-adhesives carries a cost band and no dated figure	—
¼-inch UNC T-nut, 6 mm neodymium magnets	one T-nut, four magnets	None. A named price gap: camera build hardware — the T-nut, the magnets, the felt, the foam, the sheet and the tin	—
Black polypropylene or aluminium sheet, 0.5 mm	150 × 130 mm, the dark slide	None. A named price gap: camera build hardware — the T-nut, the magnets, the felt, the foam, the sheet and the tin	—
Abrasive paper, 120 and 240 grit	one sheet of each	None. A named price gap: pinhole-making stock — shim, wet-and-dry abrasive paper, a pin vice and a bought laser-drilled pinhole	—
Craft-knife blades	a packet on the board route; foam board blunts a blade fast	None. A named price gap: a self-healing cutting mat, steel rule and craft knife	—

The priced rows come to £0.24 to £0.29 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 8 of the 10 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

The plywood is the only row with a sourced figure behind it and it is a sheet price, not an offcut price, so it is left out of the arithmetic rather than guessed at: the planner carries £11.00–£21.50 for a sheet of thin hardwood plywood, of which this build uses part of one. The second-hand 4 × 5 in double dark slide is optional, is a named price gap, and is the single item that can double the band above.

Build

Stage 0 — Set out, all routes (20 minutes)

Write your own numbers on a sheet of paper before you touch a tool: interior dimensions, the three focal distances, the plate letter and diameter for each, and the f-number each combination gives. Everything below refers to those, not to the reference camera’s.

Mark out every part on the material in pencil, all of it, before cutting anything. Two things come out of this that do not come out of cutting as you go: you find out whether the material is big enough, and you cut along shared lines, which is what makes parts identical.

Mark a grain direction and an outside face on every plywood part, and keep the good face outwards. Plywood is not symmetrical, and a panel that is planed on one side only will bow towards the rough side when it is glued.

Cutting plan: the whole camera out of one 300 × 600 mm panel of 6 mm plywood

1. 1122 mm strip → front panel, 147 mm long — window and carrier track are cut into it later, after the body is square
2. 250 mm strip → body: 2 × 147, 2 × 110 — 50 mm is the wide focal distance; get this strip right and the camera is right
3. 330 mm strip → frame A: 2 × 147, 2 × 110 — 50 + 30 = 80 mm, the two-stop-corner setting
4. 440 mm strip → frame B: 2 × 147, 2 × 110 — 80 + 40 = 120 mm, the setting that can take a shift
5. 550 mm spare — test cuts, and the part you will inevitably re-make

Rip along the length first, crosscut second. Total strip widths 242 mm plus four kerfs; a 300 mm panel leaves 50 mm in hand.

Stage 1 — Cut the parts (45 min plywood, 30 min board, 20 min tin)

Plywood. Rip the four strips along the 600 mm length, supporting the panel on both sides of the cut. Then crosscut each strip to length against a square. Cut a hair outside the line and sand back to it; a saw cut that lands exactly on a pencil line is a saw cut that has taken the line’s width off the part. Check each pair of identical pieces against each other and sand the longer one down until they match — matched pairs matter more than absolute dimensions, because two sides of unequal length build a box that cannot be square.

Board. Cut with the blade held vertical and the rule pressed down hard, in three or four passes of increasing depth rather than one heavy one. Foam board tears if you hurry and the torn edge will never be light-tight. Cut every part 0.5 mm oversize and trim, because a board edge compresses under the rule and the first cut is always slightly short.

Tin. Choose the face that will carry the pinhole — the lid if the lid is the largest flat face, otherwise the base. Mark its centre by drawing both diagonals. Cut a 20 × 20 mm window there with the snips, working from a drilled or punched starting hole, and file every edge of that window flat and smooth, then run a strip of black tape around it. That window is not the aperture: the pinhole plate in its carrier will cover it from the inside. Eye protection for all of this, without exception.

Stage 2 — Assemble the body, square and rigid (60 min plywood, 40 min board, 20 min tin)

Squareness is the property that everything else depends on: a body that is 2° out of square puts the film plane 2° out of perpendicular, which makes the focal distance different at one edge of the sheet from the other, and no amount of measuring afterwards recovers it.

Assembling the body square, in six stages, with the check that proves it

1. 1Sides on edge, spacer block between — cut the block to the interior width and keep it: it sets the frames too
2. 2Glue and clamp the bottom — ends flush with the outer faces of the sides
3. 3Turn over, glue the top — a four-sided tube, 147 × 122 external, 50 deep
4. 4Measure both diagonals; rack until equal — equal diagonals is square. Equal sides is not.
5. 5Front panel on, still blank — cut the window after the glue has cured, not before
6. 6Shoulder collar, 15 mm from the rear edge — the stop that every socket butts against, and therefore the datum for the focal distance

Plywood shown. The board route is identical except that a 20 mm fillet strip is glued into every internal corner at stages 2 and 3, and the tin route skips to stage 6.

Plywood. Cut a spacer block to exactly your interior width and keep it for the rest of the build; it will set the frames as well. Stand the two sides on edge on a flat board with the spacer between them, glue and clamp the bottom across, turn the assembly over and glue on the top. Rack the tube gently until it passes both of the checks below, clamp, and leave it overnight if you can. Then glue on the front panel, still blank.

Two checks, and they test different things. The first is the diagonals of the open front: equal diagonals means the opening is a rectangle, and equal side lengths does not, because a parallelogram has four sides of exactly the right length and no right angle in it. What a parallelogram opening costs you is a back tray that cannot seat without a wedge-shaped gap at two corners — which is a light leak — and a sheet that sits skewed in the frame, so that a level horizon is not parallel to the edge of the negative.

The second check is that the four rear edges lie in one plane: stand the tube on a flat board on its rear edges and try to rock it, or lay a steel rule across each pair of opposite edges. This is a different failure with a different cause — walls cut to unequal lengths rather than a racked tube — and a different consequence: rear edges that are not coplanar tilt the film plane relative to the optical axis, so the focal distance differs from one edge of the sheet to the other and with it the magnification and the exposure. A camera has to pass both.

Board. The same sequence with one addition: glue a 20 × 20 mm fillet strip of the same board into every internal corner, running the full length of the joint. A butt joint in board has almost no glued area and will open; a filleted one has three surfaces in contact and will not. Tape the outside of every joint with black tape once the glue is dry, both for strength and because a taped joint is a sealed joint.

Tin. The body already exists and is already square. Your work is the light path: run black tape around the inside of the lid’s rim, and check the lid’s fit by eye in a dark room before going further.

The shoulder collar, on the plywood and board routes. Glue a collar of 3 mm board or card around the outside of the rear end so that its rear edge stands 15 mm forward of the rear edge of the body. That 15 mm is the overlap the back and the frames will slip over, which makes the joint the light trap the design lesson specifies; and the collar’s front face is the stop that fixes where those parts seat, which makes it the datum for the whole focal distance. Cut it carefully and check it with the square.

The tin needs no collar, because its lid already is one: a press-fit rim sliding over a flange is a spigot in a socket, and the flange is the stop. Blacking the inside of that rim, which Stage 3 covers, is what turns it from a cover into a light trap.

Stage 3 — Blacken and flock (30 min plus drying)

Wall’s 1912 instruction still stands: “The interior of all cameras, dark slides, and lens tubes should be coated with a dead black to prevent the reflection of light and consequent fog on the plate.”

Paint every interior surface of the body, both frames, the back, the front panel’s inner face and the inside of the shutter. Two thin coats, sanded lightly between them with 240 grit, beat one thick one: a thick coat of acrylic on plywood raises the grain and leaves a slightly glossy skin, which is the opposite of what you want. Get into the corners with the brush end-on.

Then flock the two long walls — the ones that face the film across the widest angle — with self-adhesive black felt. Those are where the grazing-incidence rays land, and a fibre pile absorbs several times over where paint absorbs once. Felt on the top and bottom walls as well is a bonus; felt on the front panel’s inner face is not, because the carrier has to sit flat on it.

The single most valuable square centimetre in the camera is the inner face of the pinhole plate, which is at the exact centre of the picture’s geometry and returns whatever it reflects into the middle of the frame. The pinhole laboratory already painted both faces of every plate. Check them again under the loupe now, and touch up anything that has chipped in storage.

Leave everything open in a warm, airy place until it is hard and odourless. See the note in Hazards about outgassing: the wait is a precaution rather than a documented necessity, and it costs a day.

Stage 4 — Fit the pinhole carrier (30 min)

🛠Centre it on the format, not on the panel

Find the centre of the back opening, not the centre of the front panel. On a box that is 0.5 mm out of square those are different points, and it is the film that has to be centred on the hole.

Method: fit the back dry, mark the centre of its aperture with two pencil diagonals, and lay a straight-edge along the top of the body from that mark to the front panel. Square down. That is the carrier’s centre line. Repeat from the side. Where the two lines cross is where the hole goes.

Cut a 40 × 40 mm window in the front panel, centred on that point. It is deliberately much larger than the plate’s own 6 mm window, and the design lesson gives the reason with a number: the carrier sits on the panel’s inner face, so the panel’s 6 mm thickness stands in front of the hole as a short tunnel, and it passes light only out to arctan(W/2T). At the wide setting the corner needs 58.4°, tan 58.4° = 1.63, so the window has to be at least 2 × 6 × 1.63 = 19.5 mm across. Twenty is the minimum; forty leaves room for a carrier that is a millimetre off centre and for a wider format later.

Glue the two carrier track strips — 3 mm thick, 8 mm wide — to the panel’s inner face, spaced to take a plate carrier with about 0.3 mm of side play. Glue a stop at the centred position, so the carrier seats in the same place every time, and a second stop 20 mm to one side for the shift position. Set a small magnet into the panel behind each stop, or fit a leaf of springy card, so a seated carrier is held rather than resting.

Board and tin. The track works the same way on both, glued to the inner face of the front wall with PVA. On a tin, degrease the metal with a little washing-up liquid and dry it before gluing, and add a strip of black tape over each track strip so that a lifted strip cannot fall onto the paper. The tin’s own front wall is a few tenths of a millimetre thick, so the 20 mm window cut in Stage 1 clears the widest cone the camera can see with room to spare.

Test-fit every plate in the register, one after another. Each should slide in, seat against the stop with a small click, and come out without forcing. Write the plate letter and the fit — easy, tight, loose — in the notebook. A carrier that needs forcing will eventually be forced in the dark with the camera on a tripod, and that is how a camera gets knocked off a wall.

Stage 5 — The shutter, and proving it (30 min)

The design lesson gives three mechanisms and recommends the magnetic cap for exposures over about a minute, because once it is off, nothing is touching the camera. Make the cap first; make a sliding blade as well if you want short exposures on film.

The cap. A disc or square of 3 mm ply, 60 × 60 mm, matt black on its inner face, with two magnets set into it and two steel washers glued to the front panel to meet them. It must overlap the 40 mm window by at least 8 mm all round, which 60 mm does. Drill a small hole in one corner and tie a 600 mm cord to it, with the other end to the tripod: a cap that is dropped in long grass at dusk is a session over.

The sliding blade, if you make one, runs in a channel formed by two 3 mm strips glued to the outside of the front panel, with a third strip closing the top. Cut the blade from the same 0.5 mm sheet as the dark slide. It must overlap the window by 8 mm at every edge in the closed position, and its finger tab must be long enough that your hand is 100 mm from the camera when you pull it.

The shutter in section, the rebate that stops an edge leak, and the lamp test

1. 1Blade lying flat: leaks — the gap between blade and panel points straight at the hole
2. 2Blade in a rebated channel: does not — the passage turns 90° before it reaches the hole; overlap ≥ 8 mm all round
3. 3The lamp test — lamp 300 mm away, swept from head-on to nearly edge-on; observer dark-adapted, looking in through the open back

Shallow angles find the leaks. A lamp held straight in front of a shutter proves only that the shutter is opaque, which was never in doubt.

Prove it now, before the back is finished. Take the camera into a dark room with the back open, let your eyes adapt for five minutes, and have somebody sweep a bright desk lamp around the closed shutter at 300 mm, from head-on to nearly edge-on, while you look into the open back. Any line or glow you can see is a leak. Shallow angles are what find leaks, which is why the lamp is swept rather than held. If the simple blade leaks, that is the signal to cut the rebated channel; if the channel leaks at its mouth, add a felt strip inside the mouth.

Stage 6 — The tripod bush, balance and stance (20 min)

Assemble the camera in its heaviest configuration — both frames on, and the film-holder back if you have one — and balance it across a pencil laid on the bench. Mark where it settles. That is behind the geometric centre, and it is where the bush goes: a camera mounted at its balance point does not try to nose over when the head is loosened.

Drill a hole to suit the T-nut’s shank at that point, from the outside, with the work clamped and a scrap block underneath so the bit does not tear out through the inner face. Press the T-nut in from the inside, so that the thread pulls the flange against the wood rather than out of it, put a smear of PVA under the flange, and draw the spikes home by running a bolt through from outside and tightening against a washer. Wipe the squeeze-out before it dries.

On board, a T-nut has nothing to grip. Glue a 50 × 50 mm pad of 6 mm plywood to the inside of the base first and fit the T-nut through both. On a tin, the same pad is glued inside with the T-nut through a drilled hole in the metal, and the metal edge of that hole is filed and taped.

Check two things on the tripod: that the camera does not sag when the head is tilted down, and that it stands flat and stable on the ground on its own base, because a great deal of pinhole photography happens at ankle height and the tripod is often the thing you left at home.

Stage 7 — The backs (45 min)

The flat paper back. Build the tray first: a 4 mm panel with four walls 15 mm deep, sized to slip over the body’s 15 mm shoulder with about 0.5 mm of clearance. Then, inside it, glue four paper stops of 3 mm material defining an aperture of exactly your format — 101.6 × 127 mm for 4 × 5 inch. Measure that aperture with the rule after the glue is dry, and if it is 1 mm oversize, leave it: an oversize aperture loses nothing, an undersize one crops the negative.

The pressure plate is a 4 mm panel about 4 mm smaller than the format each way, faced with 3 mm closed-cell foam, held against the sheet by two small leaf springs of springy card or by a strip of foam behind it. Its job is to hold the sheet flat against the stops. A sheet that bows 3 mm in the middle has a focal distance 3 mm shorter there than at its edges — 6 per cent at the wide setting — and the negative will be softer and slightly denser in the centre for no reason you can otherwise explain.

The dark-slide slot runs across one short end of the tray: two strips leaving a 1 mm gap, lined with felt on both faces so the slide is gripped as it passes. Cut the slide 20 mm wider than the opening so it cannot be pulled all the way out by accident, and paint a white index line on it at the “closed” position.

The tin’s back is its lid. A press-fit lid is already a spigot and a socket — the rim slides over the tin’s own flange and the light has to turn a corner to get past it — so the work is to black it and to hold the paper. Paint the inside of the lid and the inside of its rim matt black, run black tape around the outside of the joint each time it is closed, and glue four card paper stops to the inside of the lid defining your format, with a pad of 3 mm foam between them so that the sheet is pressed flat when the lid goes on. A tin gets no dark slide: you load it in the darkroom, close it, tape the joint and carry it out with the shutter shut, which is what a tin camera has always done. That is a real limitation and it is the reason the tin route gives you one sheet per darkroom visit.

On board, build the tray from the same board with fillet strips in the corners, and face the inside of the tray wall that meets the shoulder with felt: board edges are never quite square, and the felt takes up what the knife left.

The film-holder back. Replace the tray with a flat frame that presents a flat, true seat for the holder, and a hinged pressure panel sprung against it. The seat must bear on the holder’s own raised light-trap ridge — the holder seals itself, and your job is to hold it square without twisting it. Build the seat, offer the holder up, and look at the joint against a lamp before you trust it.

Measuring the true pinhole-to-emulsion distance, for every configuration

1. 1Rod through the carrier window — plate removed; mark the rod flush with the outer face of the front panel
2. 2Flat back: rod touches the front face of the paper stops — that is where the emulsion sits, not where the tray panel sits
3. 3Film holder: rod touches the septum — several millimetres behind the holder’s outer face — measure, never assume
4. 4Subtract the plate and carrier thickness — and write the result in the register beside the design value

Repeat for body alone, body plus frame A, and body plus both frames, and again for each back. Six numbers, and every f-number in the part depends on them.

Measure the focal distance now, for every configuration. Push a straight rod or a strip of stiff card in through the carrier window until it just touches the emulsion surface — the front face of the paper stops with the flat back, the septum with a holder — mark it flush with the outer face of the front panel, withdraw it, and read the length. Subtract the plate and carrier thickness. Do it six times: three frame stacks by two backs. Every f-number, angle of view and corner-falloff figure in the rest of this part comes from these six numbers, and not from the drawing.

Stage 8 — The extension frames (30 min)

Each frame is the body’s construction in miniature: four walls glued around the same spacer block, so that its interior matches the body’s exactly, with the diagonals checked before the glue sets. Then glue a collar of 3 mm material around the outside so that it projects 15 mm forward of the frame’s front face — that is the socket that slips over the body’s shoulder — and so that its rear edge is flush with a point 15 mm forward of the frame’s rear edge, which becomes the shoulder for the next part.

On board, change the joint rather than the tolerance. Foam board has a compressible core, so the knife bevels the edge as it cuts and the wall thickness wanders; a socket sized to 0.5 mm of clearance will bind in one place and gape in another. Make the frame as a plain collar with no socket at all, and join it to the body with a wrap-over sleeve: a strip of thin card 40 mm wide, wrapped around the outside of both parts across the joint and taped. It gives the same two right-angle turns, tolerates a wandering edge, and can be re-made in two minutes.

On a tin, one collar is the realistic limit. Cut a collar from stiff card to the outside profile of the tin’s flange, 30 mm deep, so that the tin’s flange enters it at one end and the lid’s rim slides over it at the other. Tape both joints. Measure the focal distance with the collar in and write the difference on the collar. A second collar stacked on the first will usually be too floppy to stay light-tight, so the tin route offers two focal distances rather than three — which is a real constraint on the focal-distance series later in this part, and one to plan for rather than discover.

Two rules that decide whether the frames are worth having.

The mating faces must butt, so that each frame adds exactly its own length. If a frame seats on the end of its spigot instead of on its shoulder, it adds whatever the spigot happens to be, and it will do so differently each time it is assembled.

Each frame is labelled with its measured contribution, in white pencil on the outside, not its nominal one. Assemble the camera with the frame in, measure the focal distance by the rod method, subtract the body-alone figure, and write that difference on the frame. If frame A says 29.6 mm rather than 30, write 29.6.

Stage 9 — The curved insert (30 min)

The insert is a partial cylinder whose radius equals the wide focal distance, with the pinhole on its axis. For the reference camera that is R = 50 mm, wrapping the sheet’s 127 mm dimension.

🧮Cutting the former: the arc arithmetic, once

For an arc of length s on a radius R: half-angle s/2R, chord 2R sin(s/2R), depth R(1 − cos(s/2R)).

With s = 127 mm and R = 50 mm: half-angle 1.27 rad = 72.8°, so the wrap is 145.5°. Chord 95.5 mm. Depth 35.2 mm. The leading edges of the sheet stand 14.8 mm behind the pinhole plate.

Check both against your box before cutting: the chord must fit the interior width (95.5 in 135 — 20 mm each side to spare) and the depth must fit the interior depth (35.2 in 50). It does, at the wide setting. It would not at 5 × 7 inch on the same body, where a 177.8 mm arc on a 50 mm radius wraps 204° and folds back on itself — which is the arithmetic telling you to use a longer focal distance or a smaller sheet, and worth doing before you find out with a knife.

The curved insert: the two end formers, the skin, and how the sheet is held

1. 1End former, cut two together — arc R = 50 mm through 145.5°; chord 95.5 mm; depth 35.2 mm; flat base below the chord
2. 2Formers 101.6 mm apart, three spacers, thin skin — skin about 130 mm long so the paper has something to sit on beyond each end of the arc
3. 3Sheet sprung inside, emulsion inwards — clip at each end; fibre paper takes the curve more easily than RC

R equals the focal distance and the pinhole sits on the axis — that is the whole optical specification, and everything else is joinery.

Whether a tin can take one. It can, if the arithmetic allows. The insert’s radius must equal the tin’s measured focal distance, and it needs a chord that fits the tin’s internal width and a depth that fits its internal depth. Work the same three expressions for your own tin before cutting card: for a tin 60 mm deep taking a 127 mm arc, the half-angle is 127/120 = 1.058 rad = 60.6°, the chord is 2 × 60 × sin 60.6° = 104.5 mm and the depth is 60 × (1 − cos 60.6°) = 30.6 mm, so it needs a tin at least 105 mm wide inside. What a round tin cannot do is stand in for the insert: putting the hole in the wall of a cylinder puts it on the cylinder’s surface rather than on its axis, so the distances from hole to film are not equal and none of the cos-fourth simplification applies. That is a different and interesting optic, but it is not a curved back.

Cut the two end formers together, taped face to face, so they are identical; separate them, stand them 101.6 mm apart, join them with three spacer strips, and skin the curved edges with thin card. Paint it matt black inside and out. Then measure it: push the rod through the pinhole and check that it touches the skin at 50 mm at the centre of the arc, and check with a rule that the distance to each end of the arc is the same. If they are not, the axis is not where the hole is, and the whole advantage is lost.

Stage 10 — Dry commissioning: the dark-adapted inspection (30 min)

This is the step that decides whether anything else you did counts, and it is the step most often skipped because it looks like nothing is happening.

Kodak’s darkroom guidance gives the method and the criterion for a room, and it transfers exactly to a camera: stay in the dark for five minutes with the lights off, and if you still cannot see a sheet of white paper against a dark background, the space passes. Small leaks are then eliminated with black tape. Your eyes are the instrument, and they need those five minutes.

Run it in three passes, in this order.

1. From the inside, back open. Sit in a dark room with the camera in your lap, back off, shutter closed, and look down into it while a helper sweeps a bright lamp slowly around the outside at 300 mm — head-on, then oblique, then almost edge-on, all round, including from below. Mark every glow with a pencil on the outside of the camera.
2. Fully assembled, looking at the seams. Fit the back, put the lamp inside the camera instead — a small torch through the back opening before it is closed, or the camera assembled around a torch — and look at it from outside in the dark. This finds the leaks the first pass cannot, because light coming out of a seam is far easier to see than light going into one.
3. Every configuration. Repeat with each frame stack. Frames add two more joints each and they are new joints; a camera that passes at 50 mm can leak at 120.

Where leaks appear, in order of likelihood. The shutter’s edges, especially at shallow angles. The corners of the body, where three surfaces meet and one of them was cut a degree out. The mouth of the dark-slide slot. The seam between the back tray and the shoulder collar at the two short ends, which is where a tray racks if it is a whisker out of square. Screw and bolt holes, including the tripod bush. And on the board route, the cut edges themselves.

Seal progressively and record what you sealed. Black tape over a found leak, then re-run the pass that found it. A camera that needs tape in four places is a normal first camera; a camera that needs tape in fourteen has a structural problem, and the honest response is to find the racked joint rather than to keep taping.

✕The inspection is not the light-tightness test

A dark-adapted eye is sensitive, but a sheet of photographic paper accumulates. A leak too faint to see in five minutes can fog a sheet over an eight-minute exposure in open shade, because the paper is integrating and your retina is not.

So this inspection is a necessary condition, not a sufficient one. The sufficient test is the first exposure in the commissioning experiment (commissioning-the-pinhole-camera): a sheet loaded, the shutter kept closed, the camera left in full sun for a fixed time, and the sheet processed alongside an unexposed control. What you are doing here is removing the leaks that are easy to find, so that the expensive test has a chance of passing.

Stage 11 — Document the build (20 minutes)

Photograph the camera: the outside, the interior after flocking, the carrier with a plate in it, and the camera on the tripod. Those images date the build and settle later arguments about what changed.

Then write the two tables the rest of the part runs on.

Table 1 — configurations. One row per frame stack and back: measured f, and for each plate in the register, the f-number, the diagonal angle of view and the corner loss in stops. All computed from the measured f.

Table 2 — deviations, with a predicted consequence for each. This is the table that turns a build record into an experiment. For every way in which what you made differs from what you drew, write the deviation, the number, and what you expect it to do to a negative. Examples, with the reasoning supplied so the form is clear:

Deviation	Measured	Predicted optical consequence
Frame A came out 1.4 mm short	contributes 28.6 mm, not 30	f = 78.6 mm not 80: f/262 not f/267 (0.05 stop, ignorable), diagonal angle 91.9° not 90.9°
Body 1° out of square, front panel leaning back at the top	film plane 1° off perpendicular	f differs by about 1 mm top to bottom: negligible for exposure, but the top of the frame is very slightly more magnified
Carrier stop set 1.5 mm left of the format centre	permanent 1.5 mm shift	corner losses become asymmetric; at the normal setting the worst corner on one side loses under a tenth of a stop more than its opposite number
Pressure plate foam too thin, sheet bows 2 mm	f 2 mm short at centre, 4 per cent	illuminance goes as 1/f², so the centre receives about 8 per cent more light than the edges; expect a slightly denser, slightly softer middle

The point of writing predictions rather than observations is that a prediction can be wrong. When the commissioning negatives come back and the right-hand corners are darker, you have confirmed a chain of reasoning; when they are not, you have found something more interesting than a tidy camera.

Testing and calibration

Six checks, all of which should pass before the camera goes near a sheet of paper. Record the result of each.

Check	Method	Pass criterion
Squareness	Measure both diagonals of the front opening and of the back opening	Equal within 1 mm
Interior dimensions	Rule across the interior at three points along the depth	Within 1 mm of the drawing, and consistent front to back
Carrier centring	Fit a blank carrier with a 1 mm hole, shine a torch through it in a dark room onto a card taped over the back stops, and mark where the spot lands	Spot within 2 mm of the marked format centre
Carrier repeatability	Remove and refit the same plate five times, marking the torch spot each time	All five marks inside a 1 mm circle
Focal distance	Rod method, for all six configurations	Recorded, not compared to a target; the measured number is the specification
Light-tightness	The three-pass dark-adapted inspection above	No visible glow at any angle, in any configuration

Then do the arithmetic check the design lesson describes, which catches errors the physical checks cannot. For each configuration take the measured f, divide by the adopted plate diameter from the register to get N, and separately compute the diagonal angle as 2 arctan(D/2f) where D is your format diagonal. Compare both with the design values. A mis-cut frame shows as a consistent offset in both quantities; a mis-recorded pinhole diameter shows in N only. Two independent checks on one measurement, for the cost of two lines of arithmetic.

📓What the register looks like when the session is over

Three tables in the notebook, and the camera is not finished until they exist: the configuration table (six rows: measured f, N for each plate, angle of view, corner loss); the deviation table with a predicted consequence per row; and the inspection record (which pass found which leak, what was done, and whether the re-test passed).

Cross-reference the plate letters to the pinhole register rather than repeating the diameters. One number, one home.

Clean-up and waste

Vacuum or damp-wipe the bench and the floor; HSE name dry sweeping and compressed air as things to avoid, because both put settled dust back into the air where it can be breathed. Bag the dust and the sanding paper together and put them in general household waste — wood dust is combustible, so do not leave a bag of it against a heat source. Plywood offcuts are wood and go with the wood waste your local authority takes; sharp metal offcuts from the tin route go into a closed tin, not loose into a recycling bag where somebody sorting it will put a hand in. Wash the brush in water while the acrylic is wet. Dried acrylic on a brush or a rag is a solid and goes to general waste.

Local regulations govern waste disposal, and they differ between authorities even within the United Kingdom; check what yours accepts before you carry a bag to a recycling centre.

Troubleshooting

What you find	Likely cause	What to do
Diagonals differ by 4 mm and the glue has set	The tube was racked when it was clamped, so the opening is a parallelogram	Do not force it. Trim the paper stops to sit parallel to the walls rather than to the drawing, and pack the two open corners of the back seat with felt so the tray still seals; the picture will be very slightly skewed on the sheet and nothing else changes
The tube rocks when stood on its rear edges	The walls were cut to unequal lengths, so the rear edges are not coplanar	Plane or sand the rear edge until a rule sits flat across every pair, then fit the shoulder collar to the corrected edge; it is the film plane that has to be square to the axis
The back tray rocks on the shoulder	The collar is not the same distance from the rear edge all round	Sand the collar’s front face flat rather than adding packing; a packed joint is a joint that will move
A frame will not go on, or goes on hard	Socket collar glued a fraction proud, or the frame’s exterior is slightly oversize	Sand the frame’s outside, never the socket’s inside, so that the seating faces stay true
Light visible along one edge of the shutter at shallow angles only	The blade is lying flat rather than running in a channel	Cut the rebated channel (stage 5); tape on the outside will not fix a gap that points at the hole
Light visible at a body corner	A cut a degree out, leaving a wedge-shaped gap	Black tape over it, then re-run the inspection. If three corners do the same, the box is racked and the rear-edge remedy above is the real fix
Torch spot lands 5 mm off centre	Carrier track set out from the front panel rather than from the back opening	Move the stop, not the window; the window is oversize precisely so that the stop can be moved
The torch spot moves each time a plate is refitted	The carrier is loose in the track, or the magnet is not seating it against the stop	Reduce the side play with a shim of paper on one track strip; add the second magnet
Measured f differs from the drawing by more than 2 mm	Usually the shoulder collar’s position, sometimes the plate thickness	Do not adjust the camera. Record the measured value and recompute; that is what the register is for
Paper bows visibly in the loaded back	Pressure plate foam too thin or springs too weak	Thicker foam, or a second leaf spring at the middle of the long side
RC paper springs off the curved former	RC base is stiffer than fibre on a 50 mm radius	Add a card tongue at each end of the arc, or use fibre paper for the curved-back work
Black tape leaves a residue inside after a season	Non-photographic tape	Replace with photographic or gaffer tape and clean the residue off before it collects dust that will later fall on a sheet

Questions

1. Your two body sides came out at 110.0 and 111.5 mm and you glued them up anyway. Describe, using the diagonal check, what shape the front opening now is, and say which of the six calibration checks would catch it and which would not.
2. The rod method gives 51.8 mm for the body alone with the flat back, against a design value of 50 mm. Recompute N, the diagonal angle of view and the corner loss for the 0.25 mm plate, and say whether any of the three changes is worth acting on.
3. Why does this page tell you to measure the carrier’s centre from the back opening rather than from the front panel, and what would go wrong if you did it the other way on a box that is 1 mm out of square?
4. The dark-adapted inspection passes in every configuration, but the first fogging test in the commissioning experiment shows a broad grey band along one long edge of the sheet. Give two explanations that are consistent with both results, and say what evidence would separate them.
5. A friend builds the same camera from 5 mm black foam board and cannot get a frame to stack without the joint opening. What is the underlying difference between board and plywood at this joint, and what change to the design would you make for board rather than trying to build the plywood detail more accurately?
6. You have three plates, 0.20, 0.30 and 0.40 mm, and three measured focal distances, 50.4, 79.1 and 119.6 mm. Tabulate all nine f-numbers, then say which three combinations you would actually use and why, referring to the optimum-pinhole band.

Further experiments

Build the same camera twice, in two materials, to the same interior dimensions. Photograph the same subject with both on the same afternoon with the same plate. Any difference is the body, since nothing else changed — and the most likely differences are flare and squareness, both of which you can then chase.

Measure your own squareness error and predict its effect before you look. Set a small square against the film plane and a straight-edge along the optical axis, estimate the angle, compute the difference in focal distance between the top and bottom of the sheet, and convert it to a difference in magnification. Then photograph a wall of brickwork and see whether you can find it.

Test the flocking claim. Make two identical inserts for one long wall, one painted matt black and one felted, and photograph a scene with a bright window just outside the frame, twice, changing only the insert. Read the shadow density in the corner nearest the window on both. This is a controlled one-variable test of an assertion this course makes on mechanism alone, and it is the kind of finding worth writing up.

Cut a third extension frame, deliberately 3 mm off its nominal length, and do not label it. Put it in the set. See how long it takes, and what evidence it takes, before the register catches it. This is a cheap lesson in why measured values beat drawn ones, and it is much less painful to learn on purpose.

Make a second carrier stop at a shift you computed rather than the 20 mm given here. Work out from the falloff table the largest shift your long setting can take before the worst corner passes 45°, cut the stop there, and see whether the corner really does hold.

Check your understanding

Question 1. You measure the front opening of your glued-up body and the two diagonals come out at 189 mm and 194 mm. What have you built, what does it cost you, and what would you have to measure to find out whether the film plane is also tilted?

• A box that is 5 mm too big; trim the sides
• A parallelogram rather than a rectangle: the back tray cannot seat without a wedge gap at two corners and the sheet sits skewed in the frame. Whether the film plane is also tilted is a separate question, answered by standing the tube on its rear edges and checking that it does not rock
• Nothing is wrong; diagonals always differ slightly on plywood
• A box that is 5 mm too small; pack the joint

Show the answer and why

Answer: A parallelogram rather than a rectangle: the back tray cannot seat without a wedge gap at two corners and the sheet sits skewed in the frame. Whether the film plane is also tilted is a separate question, answered by standing the tube on its rear edges and checking that it does not rock

Equal side lengths do not make a rectangle — a parallelogram has four sides of exactly the right length and no right angles at all — and the diagonal check is the only simple measurement that separates the two. A 5 mm difference across a 147 by 122 mm opening is about one and a half degrees of rack. What that costs is a rectangular back tray that no longer matches a parallelogram opening, so two diagonally opposite corners of the seat stand open as wedges, which is exactly the light leak the inspection will find; and a sheet held by rectangular paper stops that is now rotated relative to the walls, so a levelled camera does not put a level horizon parallel to the edge of the negative. Notice what racking does not do: if the four walls were cut to the same length, the rear edges are still coplanar and the film plane is still perpendicular to the axis. That failure has a different cause — walls of unequal length — and its own test, which is standing the tube on its rear edges and trying to rock it. Two shapes of error, two checks, and a camera has to pass both.

Question 2. The rod method gives a measured focal distance of 53 mm where the drawing says 50, with the 0.25 mm plate fitted. What should you do?

• Rebuild the body 3 mm shallower
• Use f/200 as designed; 3 mm is inside any home-made camera’s tolerance
• Record 53 mm in the register and use f/212, which is 0.17 stop slower than f/200, and note that the diagonal angle of view is 113.8° rather than 116.8°
• Add 3 mm of packing behind the paper stops

Show the answer and why

Answer: Record 53 mm in the register and use f/212, which is 0.17 stop slower than f/200, and note that the diagonal angle of view is 113.8° rather than 116.8°

The measured value is the specification. N = 53/0.25 = 212, and the exposure change is (212/200)² = 1.124, which is 0.17 stop — well inside the bracket you will shoot anyway, and therefore not worth a rebuild. What is worth recording is that it is a systematic offset: it applies to every exposure this configuration ever makes, in the same direction, so it will quietly bias your personal paper-speed estimate if it is never written down. The angle of view moves too: 2 arctan(162.6/106) = 113.8° rather than 116.8°, three degrees narrower, which matters for framing a building without a viewfinder. Options 1 and 4 both treat the drawing as the truth and the camera as the error, which inverts the whole discipline of this part.

Question 3. Why does this page insist on eliminating solvent contact adhesive and aerosol paint from the build rather than simply specifying ventilation for them?

• Because ventilation is impossible in a domestic workshop
• Because HSE’s hierarchy of control puts elimination and substitution ahead of every other measure, and here a water-based alternative does the same job, so the hazard can be removed rather than managed
• Because solvents are banned for hobby use in the UK
• Because PVA is a stronger adhesive than contact adhesive in every application

Show the answer and why

Answer: Because HSE’s hierarchy of control puts elimination and substitution ahead of every other measure, and here a water-based alternative does the same job, so the hazard can be removed rather than managed

HSE’s brief guide to COSHH lists control measures in order of priority and puts two forms of removal first: eliminate the harmful substance and use a safer one, and use a safer form of it. Ventilation and personal protective equipment come further down that list, and the list is in that order because a control that depends on somebody remembering to open a window fails when they forget, while a control that consists of not buying the tube cannot fail. Here the substitution is free: PVA glues plywood and board perfectly well, and brush-applied water-based acrylic blackens an interior as effectively as an aerosol. Option 4 overstates the case — contact adhesive is better than PVA for some joints, just not for any joint on this camera — and the page is careful to say that if you use either product anyway, its own label and safety data sheet are the authority.

Question 4. The camera passes the dark-adapted inspection in every configuration, but the first fogging test still comes back with raised density along one edge. Why is that not a contradiction?

• Because the paper was faulty
• Because a dark-adapted eye responds to the light present at an instant, while a sheet of photographic paper integrates over the whole exposure, so a leak too faint to see in five minutes can build a visible density over eight minutes
• Because the inspection was done with the back open and the test with it closed
• Because a leak only appears when the camera is warm

Show the answer and why

Answer: Because a dark-adapted eye responds to the light present at an instant, while a sheet of photographic paper integrates over the whole exposure, so a leak too faint to see in five minutes can build a visible density over eight minutes

The two instruments are measuring different quantities. Your retina reports an approximately instantaneous rate, and after five minutes of adaptation it is very good at that; but it also adapts away a steady faint glow. Photographic paper multiplies rate by time — the exposure it receives is illuminance times duration — so a leak that is below your visual threshold at any instant can deliver a substantial total over a ten-minute exposure. That is exactly why this page calls the inspection a necessary but not a sufficient test, and why the commissioning experiment repeats the question with a sheet of paper as the detector. The practical consequence is worth keeping: the inspection is for finding leaks cheaply and quickly, and the paper is the arbiter.

Question 5. Your friend cannot get the extension frames to stack cleanly in 5 mm foam board. What is the underlying problem, and what design change fits the material?

• Foam board is not rigid enough; use two layers
• The spigot-and-socket joint relies on a thin, accurately cut collar and a stable wall thickness, and foam board compresses under the knife so its edges are neither square nor consistent; give the board route a wrap-over sleeve of card taped in place instead, which achieves the same two turns without depending on a precise edge
• Foam board is not light-tight, so no joint will work
• The frames should be glued permanently, which removes the problem

Show the answer and why

Answer: The spigot-and-socket joint relies on a thin, accurately cut collar and a stable wall thickness, and foam board compresses under the knife so its edges are neither square nor consistent; give the board route a wrap-over sleeve of card taped in place instead, which achieves the same two turns without depending on a precise edge

The plywood detail works because 6 mm birch ply has a stable thickness, holds a square edge, and lets a 3 mm collar be glued to a known position. Foam board has a compressible core: the knife squeezes it as it cuts, so the edge is slightly bevelled and the effective thickness varies along a cut. A joint whose light-tightness depends on 0.5 mm of clearance against a 15 mm overlap will therefore sometimes bind and sometimes gape. The right response is to change the joint rather than to demand more accuracy from the material: a sleeve of thin card wrapped around the outside of both parts and taped gives the same two right-angle turns, tolerates a wandering edge, and can be re-made in two minutes. Option 4 would work and is the wrong trade, because gluing the frames permanently destroys the modularity that the focal-distance series depends on.

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