Commissioning the Camera: Light-Tightness, Flare and the First Negatives

A camera that has never been tested is a hypothesis with a tripod bush. This session turns it into an instrument, and it does so in the order an instrument is always commissioned: first prove that it excludes what it is supposed to exclude, then measure the stray signal it adds, then calibrate it, and only then use it to test something.

Purpose

Fourteen sheets of resin-coated paper, exposed in one afternoon and processed together in one session, that answer four questions in sequence: is the camera light-tight, how much flare does it add, what exposure does your paper actually need, and do the two optics lessons predict what the negative shows.

The hypothesis. The camera admits light only through the pinhole; the exposure computed from the exposure lesson is right to within one stop; and the sharpness and corner falloff computed from the geometry and diffraction lessons are what the negatives show. Each of those three is separately falsifiable, and the session is designed so that a failure of one does not hide a failure of another.

The control. One sheet from the same box, cut at the same time, carried in the same dark slide, never exposed to anything, and processed in the same solutions in the same session as everything else. Its density is the base fog of that box in that developer on that day, and every other sheet is read against it. Without it a fogged sheet and a fogged box look identical.

The variable, one per series. The fog test changes nothing but time in sunlight. The bracket changes nothing but exposure time. The diameter series changes nothing but the pinhole plate, at one focal distance, with exposure scaled to hold the light on the paper constant. The focal-distance series changes nothing but the frame stack, with one plate. That discipline is why the camera was built modular: a camera whose hole is a hole in its own front panel cannot run any of these comparisons, because changing the hole would mean building a second camera and a second camera differs in a dozen ways you did not intend.

Learning objectives

By the end of the session you will be able to:

• Load a paper negative under a safelight, emulsion towards the pinhole, and record its orientation so that a mark on the negative can be traced back to a place on the camera.
• Design and run a fog and leak test with an explicit duration and an unexposed control, and read base fog, edge fog and leak streaks apart from one another.
• Say what fog is at the level of the crystal, without re-deriving Part IV.
• Measure the flare a camera adds by photographing with a bright source just outside the frame, with and without a shade, and say why flare raises shadow density rather than highlight density.
• Compute a starting exposure, bracket it, and turn the result into a first effective paper speed with the arithmetic shown.
• Scale an exposure by the square of the f-number ratio so that a pinhole-diameter series changes sharpness and nothing else.
• Read a paper negative by eye against a step ladder, and state honestly what that method can and cannot resolve.
• Close a full exposure-log row on every sheet, prediction first.

Prerequisites

The camera itself, finished and calibrated: build the modular pinhole camera, including its three-pass dark-adapted inspection, and the pinhole register with at least three measured plates. The arithmetic comes from pinhole exposure and reciprocity correction, and every exposure on this page is computed there rather than here.

The processing is Part IV’s and is not re-taught: the latent image made visible establishes the developer-stop-fixer-wash tray routine, the tongs discipline and the safelight test, and the latent image establishes what a developable centre is. If your safelight has never been tested against ILFORD’s four-step method, test it before this session and not after: a fog test run under an unsafe safelight measures the safelight.

Safety classification

Level A, on both halves of the session. The criteria of the rubric that decided it:

• Substances. Working-strength dilutions of three proprietary photographic solutions at the makers’ own dilutions, in quantities of one litre or less: an alkaline paper developer that ILFORD describe as a dimezone-s and hydroquinone developer, a citric acid stop bath at 1+19, and an ammonium thiosulfate rapid fixer at 1+4. Nothing is mixed from powder, nothing is heated, and nothing is concentrated.
• Energies. None indoors beyond a safelight. Outdoors, the energy source is the sun.
• Procedures. Trays of dilute solution at room temperature moved with tongs; a tripod on a pavement or in a garden. The failure mode is a spoiled sheet.
• Waste. Used fixer is silver-bearing and goes to the stream Part II established.

What is not a hazard here, and why. No part of this session generates a vapour, a gas or an aerosol. The stop bath is citric rather than acetic and its maker describes it as low odour; the developer and fixer are cold aqueous solutions with no volatile component; nothing is agitated vigorously enough to throw a mist. The silver spends the whole session either locked in a hardened gelatin layer on a sheet of paper, where it is insoluble and cannot be inhaled or absorbed, or dissolved in the fixer — and that second state is precisely why the fixer is collected rather than poured away. Compare this with the halide experiment in Part IV, where silver nitrate is handled as a solution and the classification rises to Level B. Same element, different physical state, different level; that comparison is the hazard assessment.

The outdoor half brings hazards that are real but are not chemical, and they are the ones most likely to hurt you today: the sun, a tripod in a public place, and a long exposure during which you are standing still and not paying attention to traffic.

Hazards

Paper developer: skin sensitisation and eye damage. Hydroquinone’s aggregated GHS classification includes skin sensitisation and serious eye damage, and the working solution is alkaline. Sensitisation is cumulative and irreversible, which is why ILFORD’s own instruction is tongs rather than fingers, and why this session uses gloves as well over roughly fifty immersions.

Stop bath and fixer: irritants. Citric acid at 1+19 and ammonium thiosulfate at 1+4 are mild irritants that will damage an eye as readily as a stronger solution.

Cross-contamination. A trace of fixer in the developer gives inconsistent results or blank prints — ILFORD say so in their own beginners’ sheet. On a page whose whole purpose is to compare fourteen sheets against each other, a contaminated bath does not spoil one sheet, it spoils the experiment.

The sun, looked at. The flare test puts a bright source just outside the field of view, and the easiest way to aim it is to sight along the camera. Do not. Aim by the shadow the camera casts, or by a hand held at arm’s length, and never place your eye where the pinhole is pointing when the sun is near the axis. Ultraviolet and visible radiation from the sun can damage the retina, and the damage is painless while it happens.

Sun exposure over four hours. The outdoor half is a long session in the open. Shade, a hat, covered arms and sun cream are the controls, and the same outdoor working practice Part I set out for the camera obscura session applies here.

The tripod. Three legs at ankle height in a public place, with a person standing beside them staring at a wall for three minutes, is a trip hazard for other people. Set up where you are not in a line of travel, and keep a hand on the camera in wind.

Required PPE

• Nitrile gloves for all wet work. HSE’s COSHH essentials sheet for manual film and plate development takes single-use nitrile as splash protection where the safety data sheet gives nothing more specific, and that is the basis used here.
• Eye protection, on from before the first bottle is opened.
• Print tongs, one pair per tray, marked, never swapped. On this page tongs are PPE and not a convenience: they are the control that keeps a sensitiser off your skin.
• An apron or old clothes, because developer stains.
• Outdoors, sun protection is the PPE: a hat, sleeves and sun cream. It is protection against radiation rather than against chemistry, and it is needed for exactly the hours the chemistry is not.

Ventilation

Ventilation is a control here and it is the ordinary one: process in a room with an openable window or an extractor, and not in a sealed cupboard. ILFORD’s own guidance is that darkroom areas should be well ventilated. A fume cupboard is not among the controls, because nothing in the session produces a vapour that would need extraction — the solutions are cold, dilute and aqueous, and no gas is generated. Air movement is here for the general reason instead: three trays in a small dark room make the air stale, and a person working carefully in stale air stops working carefully.

Materials

Item	Quantity	Notes
Variable-contrast RC paper, cut to your format	16 sheets	Fourteen for the plan, two spare. ILFORD MULTIGRADE RC or a Foma equivalent.
Opaque card, black	1 sheet	For the pinhole cap and the flag used in the flare test.
Black photographic or gaffer tape	1 roll	For sealing leaks found during the session.
Pencil, and a printed exposure-log sheet with fourteen rows	—	Pencil, because ink runs and because you will be writing with gloves on.
Light-tight box or paper safe for exposed sheets	1	Anything a box of paper came in.
A printed step ladder on your own paper	1	Made in the preparation stage; see below.

Chemicals

Chemical	Quantity	Form
Paper developer concentrate	100 mL	Diluted 1+9 to make 1 L of working solution. ILFORD describe MULTIGRADE as a dimezone-s and hydroquinone developer, used at 1+9 for 1 minute at 20 °C.
Stop bath concentrate	50 mL	Diluted 1+19 to make 1 L. ILFOSTOP is a low-odour citric acid stop; 10 seconds at 18–24 °C.
Rapid fixer concentrate	200 mL	Diluted 1+4 to make 1 L. An ammonium thiosulfate rapid fixer; 30 seconds for RC paper at 18–24 °C.
Water	about 15 L	Dilution and washing.

Every one of those times, temperatures and dilutions is ILFORD’s published figure for MULTIGRADE RC paper in MULTIGRADE developer. If you are using another maker’s paper or another developer, use that maker’s numbers and record which sheet you took them from; this course does not carry one maker’s times across to another’s material, and neither should you.

Equipment

The camera with its three plates, its frames and its flat back; a tripod; a timer that will run to ten minutes; a light meter or a phone meter, or the published daylight table if you have neither; three processing trays and a fourth for washing; a 1 L graduate and a dedicated mixing jug; a thermometer reading to 0.5 °C; three pairs of print tongs; a tested safelight; a loupe of about 8× to 10×; and a light box or a bright window for reading the dry sheets.

Estimated cost

Cost band £. Sixteen sheets of resin-coated paper and a few hundred millilitres of three concentrates that will each do several more sessions; everything else is equipment the course already owns. Dated prices live in the laboratory planner rather than in this text, so that they can be corrected without rewriting the page.

Estimated consumables cost

Sixteen sheets of paper and a few hundred millilitres of three concentrates. Everything else in this session is the camera, the tripod and the wet bench, all of which the course already owns by this point.

Consumed	This session	Sourced price	Cost this session
Variable-contrast RC paper	16 sheets: 14 for the plan, 2 spare	£16.06–£44.71 per 25 to 100 sheets, 5 x 7 in, variable contrast RC (£0.45–£0.64 a sheet)	£7.15–£10.28
Paper developer concentrate	100 mL, diluted 1+9 to make 1 L	£10.52–£20.03 per 500 ml to 1 L of concentrate, diluted 1+9	£2.00–£2.10
Stop bath concentrate	50 mL, diluted 1+19 to make 1 L	£10.66–£12.18 per 500 ml of citric acid concentrate, diluted 1+19	£1.07–£1.22
Rapid fixer concentrate	200 mL, diluted 1+4 to make 1 L	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£4.21–£5.20
Black photographic or gaffer tape	a few strips, for leaks found in the session	None. tape-and-adhesives carries a cost band and no dated figure	—
Opaque card and a printed log sheet	one sheet, one page	None. card-and-paper-stock carries a cost band and no dated figure	—
Water	about 15 L	Metered supply; the planner prices no water	—

The priced rows come to £14.43 to £18.80 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: 2 of the 7 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.

A litre of each working solution will do several sessions, so the three concentrate rows are what is poured rather than what is destroyed; the paper is the row that scales with how many times you repeat the commissioning plan.

Waste streams

Used fixer is silver-bearing and is the one stream that must be collected, together with the first change of wash water off the sheets, into the labelled silver waste container Part II established. Used developer and used stop are not silver-bearing. Fixed and washed paper, including the spoiled sheets, is ordinary household waste. Local regulations govern, and they differ between authorities even within the United Kingdom; find out what yours accepts before the session.

Alternative route

Without a darkroom. A windowless bathroom at night with a towel along the door is the accepted route. Loading the camera can be done in a changing bag if the bag is large enough to take the camera and the paper box; test that with the lights on before the session.

Without a safelight. Load and process in total darkness. You lose the ability to watch the image appear, which costs nothing here because every sheet is developed for a fixed time rather than by inspection.

Without a tested safelight. Do not guess. Run ILFORD’s four-step test — the method is in Part IV’s darkroom session — before this session, because a safelight that fogs paper will produce exactly the result this page is trying to attribute to the camera.

Without a tripod. A wall, a fence post, a bean bag or a folded coat will hold the camera still for three minutes. What matters is that it does not move and that you can put it back in the same place; mark the position.

Preparation

1. Compute every exposure before you leave the house, and write it in the log. The four numbers this session runs on, for the reference camera on 4 × 5 inch paper with the paper taken at ILFORD’s pessimistic ISO 3, and Kodak’s daylight row for bright sun with distinct shadows (1/500 at f/16 for ISO 400):

Configuration	N	Speed factor	Aperture factor	Computed t, bright sun
f = 80 mm, 0.30 mm plate	f/267	400/3 = 133.3	(267/16)² = 278	74 s
f = 120 mm, 0.20 mm plate	f/600	133.3	(600/16)² = 1406	375 s
f = 120 mm, 0.30 mm plate	f/400	133.3	(400/16)² = 625	167 s
f = 120 mm, 0.40 mm plate	f/300	133.3	(300/16)² = 352	94 s
f = 50 mm, 0.30 mm plate	f/167	133.3	(167/16)² = 109	29 s

Substitute your own measured focal distances and plate diameters; the point is the arithmetic, not these five numbers. As a sanity check, HARMAN publish guide times of one to two minutes in bright summer sunshine for a slower paper in a pinhole camera of unstated f-number, and the 74-second figure sits inside that band. A computed answer of six seconds or forty minutes would mean an arithmetic error rather than a discovery.

2. Make a step ladder on your own paper. Under the safelight, lay a sheet on the bench and give it a doubling exposure series under a steady lamp — six steps at 1, 2, 4, 8, 16 and 32 seconds, each made by sliding an opaque card along — then process it exactly as everything else on this page will be processed. You now hold a six-step ladder in stops of exposure on the material you are actually using, and it is the instrument you will read the negatives against.

📓What a step ladder is, what a step tablet is, and which one this course can specify

These are not the same object and the course keeps them apart.

A step tablet is a manufactured strip of known transmission densities. Stouffer’s T2115 is the standard article: 21 steps, a density increment of 0.15 — which is half a stop — a maximum density of 3.05, on a strip half an inch by five inches. It is a real instrument with published numbers, and it is what you will lay on a light box beside a film negative, because film is read by transmission.

A printed reflection grey scale of stated densities is the equivalent article for reading something opaque, and this course has not obtained a data sheet for one it can specify. It therefore quotes no reflection densities and names no product. What it gives you instead is the step ladder above, whose steps are known in stops of exposure rather than in density, made on your own paper in your own developer. That is enough to say “this shadow is about two steps above base” and not enough to say “this shadow is density 0.42”. Say the first and never the second; measured density waits for the densitometer in Part XV.

3. Lay the darkroom out with the lights on, trays left to right — developer, stop, fixer, wash — mixed to the makers’ dilutions and brought to 20 °C ± 1 °C with a thermometer rather than by feel.

4. Cut and load in the dark. Under the safelight, cut the paper to your format and load one sheet into the back, emulsion facing the pinhole. The emulsion side of RC paper is the shinier one and curls concave; find it by feel, in the light, on a spare sheet, before you rely on the judgement in the dark. Close the dark slide, and pencil a small mark on the outside of the dark slide showing which corner of the sheet is the top-left as the camera sees it. That mark is what will later let you say “the fog band is along the hinge edge” instead of “the fog band is along an edge”.

ILFORD’s safelight recommendation for MULTIGRADE papers is not more than four minutes of direct illumination at not less than 1.2 m, so loading is a two-minute job by design, not a leisurely one. They also state that no significant change in picture quality is seen when the paper is left for 24 hours between exposure and processing, which is what makes an afternoon of exposures followed by an evening of processing a legitimate way to work.

The commissioning sheet set: fourteen sheets and what each one proves

1. 1Sheets 1-3: light-tightness — control (never exposed) · shutter closed, 30 min full sun · shutter open, pinhole capped, 30 min full sun
2. 2Sheets 4-5: flare — bright source 50-60° off axis, outside the frame; without and with a flag
3. 3Sheets 6-10: the bracket — f = 80 mm, 0.30 mm plate: 38 / 53 / 75 / 106 / 150 s — one stop either side in half-stop steps
4. 4Sheets 11-13: pinhole diameter — f = 120 mm: 0.20 mm at 375 s, 0.30 at 167 s, 0.40 at 94 s — exposure scaled by N²
5. 5Sheet 14: focal distance — 0.30 mm plate at f = 50 mm, 29 s; compared with sheet 12 at f = 120 mm

Read top to bottom: nothing in a lower group is trustworthy until the group above it has passed. A bracket run in a leaking camera measures the leak.

Procedure

Stage 1 — The fog and leak tests (70 minutes, mostly waiting)

Sheet 1, the control. Load a sheet into the back under the safelight, close the dark slide, and put the loaded back straight into the light-tight box. It will be unloaded at the end of the afternoon and processed with everything else. It never goes near the camera and never sees daylight.

Sheet 2, the closed-shutter test. Load a sheet, fit the back, check the shutter is closed and the dark slide withdrawn, and stand the camera in full sun for thirty minutes, turning it every ten minutes so that every face gets direct sun. Then close the dark slide and return it to the box.

!Where thirty minutes comes from, since nobody publishes a number

This course could find no manufacturer instruction and no documented pinhole practice specifying a leak-test duration, and it will not borrow one from an unattributable source. So it defines its own and shows the reasoning, which is what you should do whenever a test needs a number nobody has published.

The test has to be longer than any exposure the camera will be given, or it proves nothing about real use; and it has to be short enough to run twice in an afternoon. Thirty minutes in full sun is 24 times the 74-second bright-sun working exposure computed above, and about three times the nine-minute open-shade exposure that the exposure lesson works out as the longest this camera is likely to be given on paper in daylight. A leak that puts no visible density on a sheet in thirty minutes of direct sun will put none on a sheet in nine minutes of shade, with a factor of three in hand.

State your own duration in the log with the working exposure it is a multiple of. A test whose number has no argument behind it is a ritual.

Sheet 3, the shutter test. Load a sheet, tape an opaque card cap over the pinhole from the outside, open the shutter, and repeat the thirty minutes in full sun. Sheet 2 tested every path into the camera except the one the shutter closes; sheet 3 tests that one and nothing else. Two sheets, two disjoint sets of leak paths, and the pair tells you which.

✕Why the dark-adapted inspection is not this test

Stage 10 of the build had you sit in the dark for five minutes and look for a glow. That was a necessary condition and not a sufficient one, and the reason is arithmetic rather than diligence: a sheet of paper integrates and a retina does not. Your eye reports the instantaneous brightness of a leak; the paper reports the leak multiplied by thirty minutes. A leak far too faint to see can put half a stop of density along one edge of a sheet over that time.

So a camera that passed the inspection and fails here has not made a fool of you. It has told you that the inspection found the easy leaks and this test found a hard one, which is exactly the division of labour the two tests were designed for.

Stage 2 — The flare test (30 minutes)

Choose a shaded wall or a doorway with real shadow in it, and set the camera so that the sun is between about 50° and 60° off the optical axis — outside the frame at the 80 mm setting, whose corner angle is 45.5°, but well inside the hemisphere the pinhole can see. Aim by the camera’s shadow. Expose sheet 4 at the computed time.

Then make a flag: a piece of black card held on a stick, or taped to the tripod, positioned so that it shades the pinhole from the sun without appearing in the frame. Expose sheet 5 at the same time, changing nothing else.

🧮Why a wide pinhole camera can have a flag but barely a hood

A hood is a tube in front of the hole. If its mouth is a rectangle of half-width w at distance L in front of the plate, it cuts the field at arctan(w/L), so to clear a corner at field angle θ it must satisfy

L < w / tan θ

The hood condition

Take a generous hood mouth 100 mm across, so w = 50 mm, and put the numbers for the reference camera on 4 × 5 inch paper into it:

Setting	Corner angle θ	tan θ	Longest hood that clears it
f = 50 mm	58.4°	1.63	31 mm
f = 80 mm	45.5°	1.02	49 mm
f = 120 mm	34.1°	0.68	74 mm

A 31 mm hood on a 100 mm mouth is a rim, not a hood, and it will shade almost nothing. This is the price of the angle of view: the wider the camera, the less of the sky there is left to shade. The flag works because it does not have to be symmetrical — it sits on one side only, exactly where the offending source is, and can be much further out.

Where the light that becomes flare actually goes

The two outermost rays land past the edge of the film, on the interior wall. Everything the wall fails to absorb comes back as a veil, and that veil is flare.

Stage 3 — The bracket (40 minutes)

Set the camera to f = 80 mm with the 0.30 mm plate on a static, high-contrast scene: a sunlit white wall with a dark doorway in it, or a building against sky. Static matters because a five-sheet bracket takes half an hour and you need the same light on all five; high contrast matters because you are looking for where the shadows open and where the highlights block, and a flat scene shows neither.

Expose five sheets at half-stop intervals, one stop either side of the computed time: 38, 53, 75, 106 and 150 seconds. Re-meter before the first and after the last, and if the light has moved more than half a stop, note it and treat the series as suspect rather than pretending it did not.

Stage 4 — The pinhole-diameter series (35 minutes)

Set the camera to the long setting, f = 120 mm, and photograph one scene three times, changing only the plate. The long setting is chosen deliberately, and the reason is the plot below: at 120 mm the optimum hole is 0.363 mm, so the 0.20 mm plate is far enough below it to be 18 per cent worse than the best, which is a difference a loupe might actually see. At 50 mm the same three plates would differ by one, three and fifteen per cent, and the middle two would be indistinguishable.

Scale the exposure by N² so that the light reaching the paper is identical in all three: 375 s for the 0.20 mm plate, 167 s for the 0.30, 94 s for the 0.40. If the three come back at visibly different densities, the scaling is what failed, not the optics — and that is itself a useful result, because it means one of the three plate diameters in your register is wrong.

Predicted blur for the three plates at the long setting, f = 120 mm

• Geometric blur, = d
• Diffraction blur, = 2.44 λ f / d
• Sum: what the negative should show

Show the numbers behind this plot

Three curves of blur against pinhole diameter from 0.10 to 0.60 millimetres, computed for a focal distance of 120 millimetres and a wavelength of 450 nanometres, the region blue-sensitive paper works in. The geometric blur is a straight line equal to the hole diameter, rising from 0.10 to 0.60. The diffraction blur falls as a hyperbola: 1.318 millimetres at a hole of 0.10, 0.659 at 0.20, 0.439 at 0.30, 0.363 at 0.363, 0.329 at 0.40 and 0.220 at 0.60. The two cross at 0.363 millimetres, where each is 0.363. Their sum is a broad U with its minimum of 0.726 millimetres at that crossing: 1.418 at a hole of 0.10, 0.859 at 0.20, 0.777 at 0.25, 0.739 at 0.30, 0.726 at 0.363, 0.729 at 0.40, 0.764 at 0.50 and 0.820 at 0.60. The three plates used in this experiment sit at 0.20, 0.30 and 0.40 millimetres, giving predicted total blurs of 0.859, 0.739 and 0.729 millimetres, which are 18.3 per cent, 1.8 per cent and 0.5 per cent above the best achievable. The undersized 0.20 millimetre plate is therefore the only one of the three that should be visibly softer, and it should be soft in the diffraction way, with a bright core and a faint far-reaching skirt, rather than in the plain flat-topped way of a hole that is too large.

Series	Pinhole diameter d, mm	Total blur at the film, mm
Geometric blur, = d	0.10	0.10
Geometric blur, = d	0.15	0.15
Geometric blur, = d	0.20	0.20
Geometric blur, = d	0.25	0.25
Geometric blur, = d	0.30	0.30
Geometric blur, = d	0.36	0.36
Geometric blur, = d	0.40	0.40
Geometric blur, = d	0.45	0.45
Geometric blur, = d	0.50	0.50
Geometric blur, = d	0.60	0.60
Diffraction blur, = 2.44 λ f / d	0.10	1.32
Diffraction blur, = 2.44 λ f / d	0.15	0.88
Diffraction blur, = 2.44 λ f / d	0.20	0.66
Diffraction blur, = 2.44 λ f / d	0.25	0.53
Diffraction blur, = 2.44 λ f / d	0.30	0.44
Diffraction blur, = 2.44 λ f / d	0.36	0.36
Diffraction blur, = 2.44 λ f / d	0.40	0.33
Diffraction blur, = 2.44 λ f / d	0.45	0.29
Diffraction blur, = 2.44 λ f / d	0.50	0.26
Diffraction blur, = 2.44 λ f / d	0.60	0.22
Sum: what the negative should show	0.10	1.42
Sum: what the negative should show	0.15	1.03
Sum: what the negative should show	0.20	0.86
Sum: what the negative should show	0.25	0.78
Sum: what the negative should show	0.30	0.74
Sum: what the negative should show	0.36	0.73
Sum: what the negative should show	0.40	0.73
Sum: what the negative should show	0.45	0.74
Sum: what the negative should show	0.50	0.76
Sum: what the negative should show	0.60	0.82

Computed from d + 2.44λf/d for f = 120 mm and λ = 450 nm; nothing here was measured by this course, and the sum is the upper bound of the true convolved blur rather than the blur itself. The experiment is a test of this prediction, not an illustration of it. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Stage 5 — The focal-distance series (15 minutes)

Fit the 0.30 mm plate, take both frames off to reach f = 50 mm, point the camera at the same scene from the same tripod position, and expose one sheet at 29 seconds. That sheet and the 0.30 mm sheet from stage 4 are the pair: one plate, one viewpoint, one scene, two focal distances.

Predicted differences, all computed from the same two numbers:

Quantity	f = 50 mm	f = 120 mm
Effective f-number	f/167	f/400
Exposure	29 s	167 s (2.53 stops more)
Diagonal angle of view	116.8°	68.2°
Corner field angle	58.4°	34.1°
Corner falloff, cos⁴ alone	3.7 stops	1.1 stops

Two and a half stops more exposure buys two and a half stops of corner brightness, near enough, and loses you 49° of angle. That is the whole design trade of the part, on two sheets of paper.

Stage 6 — Process all fourteen identically (45 minutes)

Under the safelight, unload every sheet and process them one at a time, in the same solutions, with the same times, in one session, in the order sheet 1 to sheet 14 so that the control goes through the freshest developer and any drift in the bath works against you rather than for you.

MULTIGRADE developer at 1+9, 1 minute at 20 °C; ILFOSTOP at 1+19, 10 seconds; rapid fixer at 1+4, 30 seconds; wash in fresh running water above 5 °C for 2 minutes. ILFORD publish all four, and they also warn that RC paper should not stay wet longer than 15 minutes because prolonged immersion causes edge penetration and curl. Develop for the full minute by the clock and never by inspection: on a page whose entire method is comparison, a sheet pulled early is a sheet that cannot be compared.

Log the temperature at the start and at the end. If it has drifted more than a degree, say so.

Stage 7 — Make one working positive (15 minutes)

Pick the best sheet from the bracket and turn it into a positive so that you can see the picture the way a viewer would.

The quick route is to photograph the dry negative on a light box or against a window with a phone and invert it in any editing application. It costs nothing, it is honest as a reading aid, and it tells you nothing about print quality.

The contact-print route is emulsion to emulsion — the negative face down on a fresh sheet of paper, both emulsions touching, glass on top, and the exposure made through the back of the negative. The paper base is in the light path, so it scatters, and the positive is softer than the negative deserves; a resin-coated base transmits less than a fibre base, so the exposure is longer again. This course has not obtained a printing reference giving exposure factors for printing through a paper negative, and quotes none: range-find it with a test strip the way you would range-find anything else. Waxing or oiling the negative to raise its transmission is Talbot’s own remedy and belongs with the contact-printing work of Part XIX.

Expected observations

Sheet 1, the control, should be white with the faintest overall tone. That tone is base fog and it is your zero. If it is grey, the box, the safelight or the developer is at fault and nothing else in the session can be interpreted until you know which.

Sheets 2 and 3 should match sheet 1. Anything darker is a leak, and its shape names it:

• A broad soft band along one edge, densest at the edge and fading inwards, is light entering at that edge and spreading — a seam, a corner, the mouth of the dark-slide slot.
• A sharp streak or wedge is a straight path: a screw hole, a crack, a gap at a rebate.
• A general grey with clean white borders where the paper stops covered the sheet is not a leak at all. It is fog reaching the whole emulsion, which means it happened before or after the camera: the box, the safelight, or handling.
• Sheet 3 dark and sheet 2 clean is unambiguous: the shutter’s light trap is the whole fault.

Sheets 4 and 5 should show the same wall. The unshaded one should have lighter shadows on the negative — remember that a negative is inverted, so shadows in the scene are the thin parts, and flare puts density into the thin parts. Expect the difference to be small on a well-blackened camera and obvious on a bare-wood or foil-lined one.

The bracket should run from a sheet with empty white shadows and readable highlight detail at 38 seconds to a sheet with dense, closed shadows at 150. Somewhere in the middle is the sheet where the shadows have just opened, and that sheet is your answer.

The diameter series should be indistinguishable in density and different in sharpness, with the 0.20 mm plate softest — and softest in a particular way, with a faint glow around every edge rather than a plain even blur. Be prepared for the 0.30 and 0.40 sheets to be indistinguishable, because the prediction says they differ by one per cent.

The focal-distance pair should differ by 49° of angle and by two and a half stops of corner darkness, both obvious at a glance.

Six signatures on a paper negative, and what separates them

1. 1Base fog — uniform, everywhere including under the stops; present on the control sheet
2. 2Edge fog — soft band, darkest at one edge, fading inwards; continues under the stops
3. 3Seam or hole leak — hard-edged wedge from one corner; continues under the stops
4. 4Shutter leak — on the capped-pinhole sheet only; absent from the closed-shutter sheet
5. 5Flare veil — even tone over the image area, stops dead at the paper stops
6. 6Hot spot — bright patch shaped like the wall it came from, opposite the source

Two questions do most of the work: does the mark continue under the paper stops (leak or fog), or stop at them (flare, which came through the hole)? And is it on the control sheet (the box, the safelight or the developer) or not (the camera)?

What is happening chemically

Nothing on this page is new chemistry, and it is not re-derived here. What is worth doing is naming the step at which each of the day’s faults enters the same chain Part IV established.

Development is amplification, and fog is amplification of the wrong thing. A photon absorbed in a crystal frees an electron, the electron is trapped, and a silver ion is reduced at the trap:

Ag⁺ + e⁻ → Ag

Repeated a few times per crystal: the whole of the latent image

A crystal that has accumulated enough silver atoms at one site has a developable centre, and the developer then reduces the entire crystal:

AgBr + e⁻ → Ag + Br⁻

Chemical development, per formula unit: hundreds of millions of them per crystal

Fog is that chain running on crystals that carry no image information. The photons that started it came from a leak, from a safelight, from a stray reflection or from nowhere at all — a crystal can develop because of a thermal or chemical accident rather than a photon, which is chemical fog, and Part IV’s latent image lesson owns the distinction. The developer cannot tell the difference. Amplification does not ask where the centre came from. That is the whole of why a small leak matters: the developer multiplies it by the same hundred million it multiplies the picture by.

Why paper punishes leaks more than film does. Photographic paper is a short-scale, high-contrast material by design, because a print has to carry a negative’s whole range in a reflection density of about two. A high-contrast material converts a small exposure difference into a large density difference, which is exactly what makes it useful for printing and exactly what makes it a sensitive leak detector. The same leak on a long-scale film would put a density difference you would need an instrument to see.

Why flare and leaks both cost contrast, and both do it at the same end. Non-image light adds a constant exposure everywhere. Where the picture already delivered a lot of light, an extra small amount changes very little. Where the picture delivered almost nothing — the shadows — the same constant is most of the total, so the shadow densities are lifted much more than the highlight ones. The toe of the curve is compressed, the shadows go flat and muddy, and the whole picture loses contrast without losing highlight detail. Part XIII owns the curve; what belongs here is the consequence: the fault appears in the shadows, so a soft, grey, lifeless shadow is where you look for it first.

Data to record

One exposure-log row per sheet, and the first ten fields written before the shutter opens. The fields are the ones the exposure lesson set out, and the discipline is the one it argued for: a prediction written afterwards is a memoir.

Field	Written	Example, sheet 8
Date and time	before	2026-09-04 14:22
Camera and configuration	before	modular camera, body + frame A
Plate id from the register	before	plate C
Measured f	before	79.4 mm
Effective N	before	f/265
Subject	before	sunlit gable wall with a dark doorway
Material and effective ISO	before	MULTIGRADE RC at ISO 3
Metered light	before	bright sun, distinct shadows; Kodak daylight row
Computed t	before	74 s
Reciprocity correction and its source	before	none published for paper; uncorrected; noted
Actual t given	before	75 s (middle of the bracket)
What moved in the frame	after	a cat, sheet 9 only
Processing	after	MULTIGRADE 1+9, 60 s at 20.2 °C; stop 10 s; fix 30 s; wash 2 min
Result against the step ladder	after	shadows at step 2, highlight just held
What to change	after	start at 53 s next time

Three tables on top of that, because this is a commissioning report rather than a day’s shooting:

1. The leak table. Sheet, test, duration, what appeared, where it was on the sheet, what was done about it, and the result of the re-test. A camera that needed tape in four places is a normal first camera; write down all four.
2. The flare table. Shadow reading on sheet 4 and sheet 5 against the step ladder, and the difference in steps.
3. The prediction-and-outcome table. One row per prediction made in stages 4 and 5 — blur, density, angle of view, corner falloff — with the predicted value, the observed value and a sentence on the gap. The gap is the finding. A table with no gaps in it usually means the predictions were made after the fact.

Analysis

1. Read the sheets in the right order

Control first, then the two leak sheets against it, then the flare pair, then the bracket, then the two series. If the control is fogged, stop and find out why before reading anything else; if the leak sheets fail, the bracket and the series are contaminated and should be repeated after the leaks are sealed. That sounds severe and it is the entire reason the sheets are numbered in that order.

2. Get your first effective paper speed out of the bracket

Pick the sheet whose shadows have just opened while the highlight still holds detail. Then:

ISO_eff = ISO_assumed × (t_computed / t_chosen)

Effective speed from a bracket

Worked. You computed 74 s at an assumed ISO 3 and the 53-second sheet is the one you would print. Then ISO_eff = 3 × (74/53) = 3 × 1.40 = 4.2. Your paper is about half a stop faster than the pessimistic end of ILFORD’s range, and 4.2 sits comfortably inside the ISO 3 to 6 they publish. Use 4.2 in every calculation from now on, write it in the log’s material field as “MULTIGRADE RC at EI 4.2 (own bracket, 2026-09-04)”, and expect to revise it when the light or the developer changes.

If the whole bracket is too dark or too light — if even the end sheet is not right — the answer is outside the range you tested. Do not interpolate off the end. Recompute with the ratio you have and run a second bracket centred on the new number, which is a cheap five sheets.

3. Read the diameter series with a loupe, against the prediction

Put the three dry negatives side by side under an 8× to 10× loupe and look at one high-contrast edge in each. You are testing three specific claims:

• The 0.20 mm sheet should be softest, by about 18 per cent in blur width. Eighteen per cent is not a lot. If you cannot see it, say so; that is a real result about the resolution of the method, and it is exactly what Rayleigh was up against in 1891 when he pierced six apertures in sheet zinc and photographed a test object with each to find which he preferred.
• The 0.20 mm sheet should be soft in a different way. A hole below the optimum is diffraction-dominated, and a diffraction blur is peaked with a faint far-reaching skirt, so edges glow rather than simply spreading. A hole above the optimum is geometry-dominated, and a geometric blur is a flat-topped disc, so edges spread evenly to a definite limit. Two negatives with the same blur width can look quite different, and this is the pair that shows it.
• The three should match in density. They were scaled by N², so if they do not, one of your measured diameters is wrong. A plate that is 10 per cent smaller than the register says gives 21 per cent less light, which is a third of a stop, which you can see.

4. Check the falloff against cos⁴, roughly

On the 50 mm sheet from stage 5, read the density at the centre, halfway to a corner, and at the corner, against the step ladder, and convert the differences to stops. The field angles are arctan(r/f) for each point, and cos⁴ predicts 0, about 1.4 and 3.7 stops for the reference camera. Expect the corner to be worse than cos⁴, because the plate has thickness and the tunnel effect is on top of the geometry. Record the excess in stops with your plate thickness beside it; the geometry lesson notes that this course could find no published measurement of pinhole falloff, so yours is a real contribution to your own notebook rather than a repetition of somebody’s number.

5. Recognise the two failures at the ends of the scale

A bracket is only useful if you can name what is wrong with the sheets that are not the answer, and there are exactly two names, one at each end of the tonal scale. Both are read on the negative, so remember that it is inverted: what was bright in the world is dense here, and what was dark is thin.

A blocked highlight is an area that was bright in the subject and is now so dense that no detail can be seen in it — the white wall and the white window frame in front of it are one flat slab. It means the exposure carried that part of the subject past the top of the paper’s usable range, and it is not recoverable: there is no information in it to bring back, because the material stopped responding before the shutter closed.

An empty shadow is the same failure at the other end: an area that was dark in the subject and is now indistinguishable from the clear paper base, with nothing in it. It means that part of the subject never got enough light to make a developable centre at all, and it is equally unrecoverable.

Both at once means the subject was beyond the material, not that the exposure was wrong, and no single exposure in the bracket will fix it. That is a real and common finding on a first outing with a high-contrast scene, and the response is to change the scene — a subject with less range in it — or to accept the loss deliberately, which is a decision Part VII treats as a picture-making choice rather than a fault.

Which end to protect. With one sheet and no second chance, protect the shadow detail, because a slightly dense highlight can still be printed through and an empty shadow cannot be printed at all. That is the reason the bracket is read by finding the sheet where the shadows have just opened rather than the one where the highlights look best.

6. State what the method cannot do

You have read densities by eye against a ladder calibrated in stops of exposure. That method resolves about half a stop under good conditions and rather less on a glossy surface at an awkward angle, and it is not a density measurement at all. Everything above is therefore correct to roughly half a stop, which is enough to commission a camera and not enough to plot a characteristic curve. Say so in the report, and hand the precise version to Part XIII and Part XV, which own it.

Troubleshooting

What you find	Likely cause	What to do
Control sheet is grey all over	Fogged box, unsafe safelight, contaminated or oxidised developer	Process a second control from a fresh packet; if it is clean, the box was fogged. If it is grey too, run the safelight test, then mix fresh developer
Both leak sheets clean, all picture sheets grey in the same place	Not a leak: the fault is downstream of the camera	Look at the dark slide, the light-tight box and the loading routine, in that order
Sheet 2 fogged along one long edge, sheet 3 clean	A body seam or the dark-slide slot, not the shutter	Tape the found edge, re-run sheet 2 only, and record which pass found it
Sheet 3 fogged, sheet 2 clean	The shutter’s light trap alone	The blade is lying flat rather than running in a rebated channel; tape on the outside will not fix a gap that points at the hole
Both leak sheets fogged, edges under the paper stops clean	Light arriving through the pinhole: the cap or the shutter was not actually closed	Repeat with the cap taped on all four sides and the shutter checked by eye in daylight
Flare sheets identical	Either the interior is already good, or the sun was not where you thought	Check the camera’s shadow in a photograph of the setup; a source at 70° off axis produces much less flare than one at 50°
Whole bracket too dark, even the shortest sheet	Effective speed higher than ISO 3, or the light brighter than the table row you used	Recompute the ratio from the shortest sheet and run a second bracket centred there
Whole bracket too light	Effective speed lower than assumed; or, indoors, the paper’s blue sensitivity meeting a red light source	Move outdoors for the calibration; the exposure lesson’s tungsten example shows why an indoor number does not transfer
Diameter series differs in density as well as sharpness	A measured diameter in the register is wrong	Re-measure that plate by a second method; the pinhole lab gives three
All three diameter sheets look identical under the loupe	Entirely possible and worth recording	The prediction was 18, 2 and 1 per cent; only the first is near the limit of the method. Repeat at f = 50 mm with a deliberately oversized hole to see the effect at all
One sheet much softer than its neighbours, in one direction only	Movement, not optics	Look at edges in two directions: optical blur is the same in both, motion smear is not. The break/fix page at the end of this part separates them properly

Clean-up

Pour used developer and used stop away according to your local rules; collect the used fixer and the first change of wash water into the labelled silver waste container. Rinse the trays, the graduate and the tongs in running water and stand them to dry. Wipe the bench. Return the paper to its bag inside its box, folded closed, before the room light goes on — the commonest way to fog a box is to leave it open while tidying up.

Wipe the camera down, check that any tape you added is still where you put it, and note in the build record which leaks you sealed and when.

Storage

Dry negatives go into sleeves or a folder, numbered to match the log rows, kept flat, dark and cool. A paper negative fades and yellows like any print if it was under-fixed or under-washed, so the two minutes of wash are not optional even on a test sheet you think you will throw away — you will want to compare it with something in six months.

Working solutions: ILFORD give working-strength stop and fixer a life measured in days to weeks depending on how they are kept, and a developer once diluted is a one-session solution. Date every bottle you keep, and write the number of sheets it has processed on the label; that number is the beginning of a capacity record Part XI will make you keep properly.

Disposal considerations

The chemistry, briefly, because Part II owns it. Used fixer contains dissolved silver as the thiosulfate complex:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Why the fixer, and not the developer, is the stream that is collected

Silver is toxic to aquatic organisms, and a fixer bath concentrates it: ILFORD note that an RC paper fixer can be worked to 4 to 6 g of silver per litre. That is the whole reason for the labelled container. The developer and the stop bath carry no silver; the developer is alkaline and contains a sensitiser, and the stop is a weak organic acid.

Local regulation governs disposal and it differs between authorities, even within the United Kingdom. ILFORD’s guidance for UK domestic users is that local authorities usually accept small quantities of chemical waste at Household Waste and Recycling Centres, bottled separately and clearly labelled, and that different wastes should never be combined for disposal. Find out what your authority accepts before the session rather than afterwards.

Questions

1. Your control sheet is clean, sheet 2 is clean, and sheet 3 shows a 15 mm grey band along the edge nearest the shutter. Name the fault, say what the two clean sheets rule out, and give the fix.
2. A classmate runs the diameter series at f = 50 mm instead of 120 mm and reports that all three plates look identical. Compute the predicted total blur for 0.20, 0.30 and 0.40 mm at 50 mm and 450 nm, and say whether their result contradicts the theory or confirms it.
3. You computed 74 s and the sheet you would print is the 106-second one. What is your effective paper speed, and is it inside the range ILFORD publish?
4. Explain, without using the word “contrast”, why flare shows up in the shadows of the print rather than the highlights.
5. Your three diameter sheets differ in density by about a third of a stop, with the 0.40 mm sheet darkest. Give two explanations and say what single measurement separates them.
6. Why does this page process the control sheet first rather than last?

Further experiments

Photograph nothing but a blank evenly lit wall, filling the frame. The negative is then a map of your camera’s illumination alone, because the subject has no structure of its own. Read it at nine points and plot the falloff against field angle. Compare with cos⁴ and with cos⁴ plus your plate’s tunnel effect, and you have measured a quantity this course could find no published figure for.

Repeat the thirty-minute leak test at 60 and 120 minutes. If a leak appears at 120 that did not at 30, you have found the faintest leak your camera has and can put a number on it: it needs a certain number of stops more exposure to record, which is a measurement of the leak.

Run the bracket twice, once in sun and once in open shade, on the same afternoon. The exposure lesson predicts that reciprocity failure lengthens the shade exposure and the paper’s blue sensitivity shortens it, pulling in opposite directions, and it has no number for either. Two brackets give you the combined answer for your paper, which is a number nobody has published.

Make a fourth plate, deliberately 0.60 mm, and add it to the diameter series. At 120 mm that is 13 per cent worse than the best, and geometry-dominated rather than diffraction-dominated, so it gives you the other end of the U-curve and a soft image of a completely different character. Keep it for the aesthetic reference set in the next experiment.

Photograph the same scene on paper and on a sheet of film in a holder at the same setting. The diffraction lesson predicts that one hole cannot be optimal for both, because paper works at about 450 nm and panchromatic film at about 550 nm, so the same camera should render film slightly softer. The difference is small and the prediction is testable.

Check your understanding

Question 1. Your camera has the 0.30 mm plate at a measured f = 79.4 mm, your paper is taken at ISO 3, and the light is bright sun with distinct shadows, for which Kodak give 1/500 s at f/16 for an ISO 400 film. What exposure should you compute, and what is the bracket?

• About 9 seconds; bracket 4.5, 9 and 18 s
• About 74 seconds; bracket 38, 53, 75, 106 and 150 s
• About 265 seconds; bracket 132 and 530 s
• About 1.3 seconds; bracket 0.7 and 2.6 s

Show the answer and why

Answer: About 74 seconds; bracket 38, 53, 75, 106 and 150 s

Three multiplications, in the order the exposure lesson fixes. First the effective aperture: N = f/d = 79.4/0.30 = 265. Second the speed: the table row is for ISO 400 and the paper is ISO 3, a factor of 400/3 = 133.3, so t = (1/500) × 133.3 = 0.267 s at f/16. Third the aperture: from f/16 to f/265 is a factor of (265/16)² = 274, so t = 0.267 × 274 = 73 s. No extension correction applies, because N was computed from the real pinhole-to-emulsion distance and there is no focal length hiding inside it. No reciprocity correction is applied because no manufacturer publishes one for paper used as a camera negative — which is exactly why the bracket is a full stop either side in half-stop steps rather than the third of a stop a film would need. Option 1 forgets the speed factor, option 3 mistakes the f-number for a time, and option 4 forgets the aperture conversion.

Question 2. Sheet 2 (shutter closed, thirty minutes in full sun) comes back with a soft grey band along one long edge that continues right under the area the paper stops were covering. Sheet 3 (shutter open, pinhole capped) shows the same band. The control is clean. What is it, and what is it not?

• Flare, because it is soft-edged
• A light leak at that edge of the body, because the band crosses under the paper stops and appears on both tests, which share every path except the shutter
• Safelight fog, because the control would also show it
• Chemical fog from an exhausted developer

Show the answer and why

Answer: A light leak at that edge of the body, because the band crosses under the paper stops and appears on both tests, which share every path except the shutter

Two pieces of evidence do the work and each rules out a different thing. That the band continues under the paper stops means the light did not come through the pinhole: image-forming light and flare both stop at the edge of the image circle, and the strip of emulsion that was covered by a 3 mm stop is the cleanest possible control for that. That it appears on both sheets means it took a path the two tests share, and the only path they do not share is the shutter — so this is not a shutter leak either. That the control is clean rules out the box, the safelight and the developer, because the control saw all three and nothing else. What is left is a leak into the body at that edge: a seam, a corner, or the mouth of the dark-slide slot. Tape it, re-run sheet 2 alone, and record which pass found it.

Question 3. Why does this page run the pinhole-diameter series at the long focal setting rather than the wide one?

• Because long exposures are easier to time accurately
• Because the optimum hole grows as the square root of the focal distance, so at 120 mm the fixed set of plates straddles the optimum more widely and the predicted difference in blur is 18 per cent rather than 3 per cent
• Because diffraction does not occur at short focal distances
• Because the corner falloff is smaller and does not interfere

Show the answer and why

Answer: Because the optimum hole grows as the square root of the focal distance, so at 120 mm the fixed set of plates straddles the optimum more widely and the predicted difference in blur is 18 per cent rather than 3 per cent

The optimum diameter is k√(λf), so it moves with the focal distance while your plates do not. At f = 50 mm and 450 nm the equal-blur optimum is 0.234 mm, so the 0.20, 0.30 and 0.40 mm plates give total blurs of 0.475, 0.483 and 0.537 mm — 1, 3 and 15 per cent above the best of 0.469. At f = 120 mm the optimum has moved out to 0.363 mm, and the same three plates give 0.859, 0.739 and 0.729 mm, which are 18, 2 and 0.5 per cent above the best of 0.726. The undersized plate is now four times further from the optimum in relative terms, and 18 per cent is at least arguably visible under a loupe where 3 per cent is not. Option 4 has a real effect backwards: less falloff at the long setting is convenient, but it is not why the series goes there. Note what the choice costs — 375 seconds for one sheet — and that it is worth paying, because a series designed to show a difference too small to see is not an experiment.

Question 4. The bracket sheets are all correct in density but every one of them has flat, grey, lifeless shadows, while the highlights are clean. The leak sheets passed. What is the most likely cause and what confirms it?

• Underexposure; confirmed by a longer exposure
• Flare from the camera interior; confirmed by comparing the flare pair, and by checking whether the veil stops at the paper stops
• Exhausted fixer; confirmed by the clearing time
• A safelight leak; confirmed by the control sheet

Show the answer and why

Answer: Flare from the camera interior; confirmed by comparing the flare pair, and by checking whether the veil stops at the paper stops

The evidence points at non-image light that entered through the hole. Flare adds a constant exposure everywhere, and a constant is negligible where the picture already delivered a lot of light and dominant where it delivered almost none, so it lifts the shadow densities of a negative and barely touches the highlights: the toe compresses, and the result is exactly this — correct overall density, dead shadows, clean highlights. The two confirming tests are the two the session already built. First, sheets 4 and 5: if the flagged sheet has cleaner shadows than the unflagged one, the camera is adding flare. Second, the paper stops: flare came in through the pinhole, so it stops dead at the edge of the image circle, whereas a leak or safelight fog carries on under the stops. The remedy is mechanical rather than chemical — a blacker, more matt interior, a blackened pinhole plate, and a flag when the sun is near the frame.

Question 5. You computed 167 s for the 0.30 mm plate at f = 120 mm. What exposure does the 0.20 mm plate need at the same focal distance, and why is scaling by the square of the f-number the right thing to do here rather than simply guessing?

• 250 s, because the hole is 1.5 times smaller
• 375 s, because the illuminance at the film goes as (d/f)², so the exposure goes as N² and (600/400)² = 2.25
• 111 s, because a smaller hole is sharper and needs less
• 167 s, because the focal distance did not change

Show the answer and why

Answer: 375 s, because the illuminance at the film goes as (d/f)², so the exposure goes as N² and (600/400)² = 2.25

N = f/d, so 120/0.20 = 600 and 120/0.30 = 400. Illuminance at the film goes as the square of the aperture ratio, so the exposure ratio is (600/400)² = 2.25, and 167 × 2.25 = 375 s. The reason this matters is experimental rather than arithmetical: the series is supposed to change one thing, sharpness, and if the three sheets also differ in density then any difference you see could be a density difference misread as a sharpness difference — dark edges look sharper. Scaling by N² is what holds the second variable still. It also gives you a free check on your own register: if the three sheets come back at visibly different densities after correct scaling, then one of the three measured diameters is wrong, because everything else in the calculation is a measured focal distance shared by all three.

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