Fog and Where It Comes From

Fog is the fault that damages a picture most and looks least like damage. It does not make a negative obviously wrong. It quietly raises the floor that the whole image stands on, so the shadows lose their separation from underneath while the highlights still look fine — and by the time the negative looks too dense, the picture has already lost the part of itself that was hardest to record.

This page separates the half-dozen different things that all get called fog, gives each one its signature, and sets out a sequence of five tests that identifies which of them you have. Every step in that sequence eliminates one class, which is what makes it a diagnosis rather than a list.

Three words that are not synonyms

Base density is the support and whatever is coated on it: the plastic, its grey or blue tint, the antihalation layer if any survived processing, and on paper the baryta and the brightener. It is a property of the material and you did not cause it.

Fog is developed silver where no image exposure was intended. It is metallic silver, chemically identical to the picture, produced by grains that were made developable by something other than the light you meant to record.

Base plus fog is what a densitometer actually reads on a piece of the material processed but never exposed. It is a single number and it contains both. Manufacturers’ curve sheets often express their speed and contrast points relative to it — Kodak’s Tri-X sheet marks its points as densities of 0.3 and 1.0 above gross fog — which tells you that a sensitometrist regards the floor as a moving quantity to be measured on the day rather than a constant to be assumed.

!What fog actually costs, and why it is not simply 'more density'

It is tempting to think of fog as adding a constant to every density, leaving the picture intact underneath and merely printed a bit darker. It does not work that way, and the reason is the shape of the curve.

Near the toe, the curve is nearly flat: a large change in exposure produces a small change in density. That is where the shadows live. Add a tenth of a unit of fog and you have not shifted the shadows up by a tenth — you have raised the floor to a level that the toe took most of a stop of exposure to climb above, and the separations that existed within that stop are gone.

The highlights, sitting on the straight-line section where a change in exposure produces a proportionate change in density, barely notice.

So the signature of fog on a print is not “too dark”. It is flat, muddy shadows and a highlight range that still looks correct — which is why fog is so often misdiagnosed as under-development, and why a rebate reading settles it in ten seconds.

The same film at three fog levels, and the shadow separation lost at each

• Fresh film, well stored: floor about 0.10
• Moderate fog: floor about 0.28
• Heavy fog: floor about 0.52

Drawn to show the geometry rather than measured from a fogged film. The three floors are chosen to illustrate the argument; the numbers that belong here are the ones you measure on your own material against your own record. 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.

Chemical fog

What it is. Development that has gone past the point where only exposed grains are developing, so that unexposed grains begin to develop too.

Why it can happen at all is worth understanding, because it explains why every developer has a fogging limit rather than a fogging switch. Kodak’s 1928 primer states the requirement precisely: a developing agent has to be a strong enough reducer to reduce the exposed silver salt and at the same time must not affect the unexposed one, “so that its affinity for oxygen must be within certain narrow bounds”. Development is a race between two reduction reactions that differ only in the presence of a sensitivity centre to catalyse one of them. It is a difference in rate, not a difference in kind. Give the slow reaction enough time, enough temperature or enough driving force and it happens too.

What pushes it over. All four are ways of raising the driving force or extending the time:

1. Too much alkali. The primer is explicit: “the quantity of alkali governs the energy of a developer, and if too much alkali is present, the developer will tend to produce chemical fog, while if too little alkali is present, it will be slow in its action.” A carbonate developer mixed with an over-dose of carbonate, or a hydroxide developer mis-measured, fogs.
2. No restrainer, or not enough. The primer again: “bromides and iodides are added to a developer to compensate for any chemical fog produced by the developer, or inherent in the emulsion.” The bromide in D-76 is not decoration; it is the antifogging term. Part VIII owns the chemistry and the organic antifoggants that do the same job more selectively.
3. Too long, or too warm. Both extend the slow reaction’s opportunity.
4. A contaminated developer. Two causes are named in the primer’s own mixing chapter and both are easy to hit at home: a trace of alkali added before the developing agent’s crystals have dissolved, so that “each crystal becomes oxidized at the surface and the resulting solution will give fog”; and the metals — “tin, copper, and zinc, or alloys of these metals will usually produce bad fog and stain with photographic developers”. A galvanised bucket is zinc. A brass tap fitting is copper.

The signature, and it is decisive. Chemical fog is even across the whole film, including the rebate and the leader, because the developer reached all of it equally and none of it needed exposing. It has no direction, no edge and no relationship to the picture or to the geometry of the tank.

That is the observation to make first, because it separates chemical fog from every kind of light fog in one look — light has to come from somewhere, and coming from somewhere always leaves a gradient.

Light fog, and the four things its signature tells you

Light fog is exposure you did not intend. Because it is exposure, it obeys the geometry of light, and that geometry is written into the mark.

It has a direction. Light falls off with distance from its source, so a leak produces a gradient: strongest nearest the hole, fading away from it. A perfectly even light fog is nearly impossible to produce accidentally and should make you suspect a chemical cause instead.

It often has an edge. Something cast a shadow. The edge of a camera back, the lip of a tank, the rim of a reel, a hand. An edge is the single most informative feature light fog has, because it is a picture of the object that made it.

It frequently repeats with the geometry of the material. Film on a spiral is a spiral; film in a cassette is a coil; sheets in a box are a stack. A leak that reaches a wound film produces a mark that recurs at the pitch of the winding, growing or shrinking as the coil diameter changes. A leak that reaches a stack of paper fogs the top sheet most.

It either stops at the frame or it does not, and that is the whole in-camera question. A fog that respects the frame edges came through the lens or the shutter and was formed while the frame was exposed. A fog that runs across the frame line into the inter-frame gap and the rebate reached the film some other way — through the back, through the cassette mouth, through the darkroom, through the tank.

Kodak’s Z-133 lists the causes it sees in a laboratory under Light fog — plus density, especially noticeable in low-density areas: incorrect use of the safelight; inspection during development; light leaks in the darkroom or the processor, together with luminescent tape, timers, indicators and fluorescent lamp afterglow; and improper loading or handling, or a camera malfunction. The middle group is the one people forget. A glow-in-the-dark strip on a light switch, the display of a timer and the residual glow of a tube that has just been switched off are all light, and all of them are still emitting when your eyes have decided the room is dark.

Safelight fog

A safelight is not safe. It is safe enough, for a stated material, at a stated distance, for a stated time, and every one of those qualifiers can fail independently.

Kodak’s K-4 publication states the physical reason there is no such thing as an unconditionally safe lamp: the colour sensitivity of most emulsions does not end abruptly at a wavelength, so most papers and films retain some sensitivity to the very colours a recommended filter transmits. A safelight buys you time, not immunity, and Kodak’s own instruction follows from that: minimise the exposure of photographic materials to safelight illumination, rather than treating the lamp as though it did nothing.

Its dependence on distance and time follows from that. Fog accumulates with the total light received, so it grows with time under the lamp and falls off steeply with distance. Both ILFORD and Kodak give the same working geometry for black-and-white paper: a 15 W bulb at not less than 1.2 m (4 ft). ILFORD’s general recommendation for their papers with an SL1 or 902 lamp adds the third term — safe for up to 4 minutes — and MULTIGRADE FB CLASSIC’s own sheet adds the fourth, a filter whose cut-off is no lower than 580 nm.

Its habit of appearing first where paper sat longest is the most useful diagnostic feature it has. Safelight fog is not uniform across a session: the sheet that lay face-up on the bench while you made a test strip has had four times the exposure of the one that went straight into the developer. If your prints are variably fogged and the variation tracks how long each sat out, the safelight is the suspect.

Its absence under anything that covered the sheet is the confirming test, and it costs one sheet. Lay an opaque object — a coin, a strip of card, a lens cap — on a sheet of paper on the bench, leave it under the safelight for the time you actually take to make a print, and develop it. If the shape of the object appears as a light patch on a grey ground, the safelight fogged everything around it. If nothing appears, the safelight did not fog it in that time.

✦It is almost never the safelight's existence

When a safelight fails a test, four things are far likelier than the lamp being the wrong kind.

The filter has faded. ILFORD state that a filter in use for several hours a day gradually fades and should be changed each year, with the date of installation recorded. A dye that has bleached passes shorter wavelengths than it did when new, and it does so invisibly — the lamp looks the same colour to you.

The bulb is the wrong wattage. Somebody replaced a 15 W lamp with the 40 W that was in the drawer.

The distance is wrong. The lamp was moved, or the bench was, or you started working at the dry side under a lamp positioned for the wet side. Falling from 1.2 m to 0.6 m is not a small change.

Something else in the room is the actual source. ILFORD’s own instruction is to check for light leaking into the room before testing the filters, and to do it only after the eyes have adapted to the dark, “which can take about 15 minutes”. Most rooms that fail do so because of a door gap, an indicator LED or an enlarger lamphouse, and testing the safelight first only proves that the room is bad.

The full test procedure — with the before-exposure and after-exposure patches, the stepped safelight times, and Kodak’s definition of a safe time as any exposure time less than or equal to one half of the time required to produce a detectable change — belongs to Part XVI and is not repeated here. This page owns safelight fog as a defect; that page owns the test.

One structural point from that procedure does belong here, because it changes what you look for. Paper is more sensitive to safelight fogging after it has been exposed in the enlarger than before. A latent image already present is easier to add to. So the sheet at greatest risk is the one sitting in the developer tray, face-up, waiting for you to decide whether it is right — which is exactly the sheet most people leave out longest.

Age and storage fog

An emulsion does not need light to acquire developable grains. Given time — and much less time if it is warm — silver halide grains develop sensitivity centres by themselves, and a film that has been in a drawer for six years has a floor that a fresh one does not.

Fast films age faster. The same properties that make an emulsion sensitive to light make it sensitive to everything else: larger grains, more silver, and more of the surface chemistry that creates a centre. Nothing here is free.

Fog arrives before speed leaves. This is the practically useful part. A film past its date typically shows a raised floor while still delivering usable, if slightly reduced, speed. So the first sign of an old film is not a thin negative — it is a slightly muddy one, with shadows that will not separate.

What the manufacturers actually ask for is worth quoting rather than paraphrasing, because the folk advice is more extreme than the instruction. ILFORD ask for film to be stored, for immediate use, in a cool place at 10–20 °C, dry, in its original packaging; they permit a fridge or freezer provided plenty of time is allowed for the film to acclimatise before use. Kodak’s Tri-X sheet asks for unexposed film at 24 °C or lower in the original sealed package, notes that high temperatures or high humidity may produce unwanted quality changes, and asks for film to be processed as soon as possible after exposure.

That last clause is a separate fault and deserves its own name. Between the shutter and the developer, the latent image is slowly decaying — most in the shadows, where it is weakest — so a film left in a warm camera for a year and then developed is thin at the bottom end for reasons that have nothing to do with the meter. ILFORD’s phrasing is “process as soon as practical”; Kodak’s is “as soon as possible”. Both mean the same thing and neither publishes a decay rate, so the course does not state one.

Heat, radiation and airport scanners

Heat accelerates everything above. A film left on a car dashboard in summer has had months of ordinary ageing in an afternoon.

Ionising radiation makes grains developable directly. An X-ray photon deposits far more energy than a visible photon, and it deposits it wherever it happens to be absorbed rather than at the surface, so the effect is a raised, generally even fog rather than an image. The dose matters — the effect accumulates with each pass — and fast films are affected first, for the same reason they age first.

What the course can and cannot say about airport equipment. This is a subject where a stale statement would be actively harmful, so here is exactly what the corpus supports and no more. ILFORD’s DELTA 3200 technical sheet, whose page footers are dated June 2025, advises that the film is not subjected to airport scanners, and that if it is, it should always be carried as hand luggage. That is a manufacturer statement, it is recent, and it is about the fastest film they make.

What the course does not have is a sourced statement about the newer computed-tomography scanners now used for cabin baggage at some airports, which work differently from the older cabin-baggage equipment that the hand-luggage advice was written for. The course will not guess. Check the manufacturer’s current guidance for your film and hand-inspection guidance for your route before you travel, treat any published advice as dated, and note that the fog you get from a scanner is not recoverable by anything on this page.

Dichroic fog is not on this page, and here is why

You will meet a fog that is greenish or yellowish by reflected light and reddish-pink or violet by transmitted light. It is called dichroic fog because of those two colours, and it is neither a development event nor a light event: it is finely divided colloidal silver deposited in the emulsion out of a solution — a fixer that is exhausted and carrying dissolved silver, or one that has been contaminated with carried-over developer.

Because its cause is in the fixing bath, it belongs to the fixing, washing and stains page and to the dichroic fog atlas entry, which carries the mechanism, the discriminating test and the historical treatments.

The one thing to carry away here is the discriminator, because it is unusually clean: look at the film, then look through it. A veil that changes colour between reflected and transmitted light is dichroic and nothing else is. Ten seconds, a lamp, no instrument.

The test sequence

Five tests, run in this order. Each one eliminates a class, and the order is chosen so that the cheap tests constrain the expensive ones.

Identifying the source of a raised floor, in five steps

1. 11. A strip fixed without developmentCut a piece of unexposed film in the dark and put it straight into fresh fixer. What you get is base alone — the support and its dyes, with no developed silver of any kind. This is your zero.
2. 22. A strip developed without exposureAnother piece of the same film, in the dark, through your normal development and fixing. Its density minus the first strip's is chemical fog plus whatever the film brought with it. If this is high, the cause is in the developer or in the film, and no amount of hunting for light leaks will help.
3. 33. The film's own rebate, as an in-camera controlCompare the rebate of the suspect film with strip 2. If the rebate is higher than strip 2, light reached the film outside the frame — in the camera, the cassette, the changing bag or the tank. If it matches, no light did.
4. 44. The paper packet test, for the safelightOne sheet of paper on the bench under the safelight, with an opaque object on it, for the time a print actually takes. Develop it. The shape of the object appearing on a grey ground convicts the lamp, the filter or the distance.
5. 55. The room test, with everything switched offA sheet on the bench with half of it covered, in the room with all safelights off and the door shut, for five or ten minutes. Give your eyes fifteen minutes to adapt first and look for leaks while you wait. A difference between the halves convicts the room.

Steps 1 and 2 need one film and no darkroom equipment beyond a tank; steps 4 and 5 need one sheet of paper each. The whole sequence costs less than the roll it is diagnosing.

Two notes on running it.

Do steps 1 and 2 before you hunt for leaks. Searching a room for light is slow and the result is never conclusive. Establishing that the developer is producing half a unit of fog on unexposed film is fast and is conclusive, and it makes the room search unnecessary.

Step 3 is free and you have already done it if you read the negative as evidence. The rebate is the control that came with the film.

🕰A published algorithm from a neighbouring process, and how far it carries

Mike Ware’s 2012 workshop handout An Algorithm for the Six Most Probable Causes of Fogging is the best published example of this kind of sequential diagnosis. Its method is to compare a border region of the sensitised area that was coated but masked during exposure with the margin of uncoated paper, and then answer a fixed sequence of questions: is fog apparent on the coating, does it appear during drying, does it appear after exposure, does it appear after wet processing, is there a stain after the wash, is there a stain in the uncoated areas.

Its six causes are specific to the iron-based processes — a sensitiser already decomposed, a hostile chemical in the paper, inadequate safelighting, a negative not dense enough in its maximum value, faulty wet chemistry, and inadequate clearing — and four of those have no silver-gelatin equivalent at all. So the course does not import Ware’s causes, and this page’s five tests are not his.

Two things from it do carry, and are used above with attribution. The first is the structure: build a comparison into the material itself (his masked border and uncoated margin; our unexposed strip and our rebate) and answer questions in an order where each answer eliminates something. The second is a caveat that applies directly to silver-gelatin work and is easy to get wrong: he notes that fogging from a faulty safelight may not be visible until wet processing is complete, especially for development processes. You cannot see safelight fog by looking at the paper. It is a latent image until the developer finds it.

His note distinguishing fog — unwanted residual image substance, reading grey — from stain — unwanted other residual chemicals, reading yellow — is also worth carrying across the boundary, and the fixing and stains page uses that distinction throughout.

The threshold question: when is fog a defect?

Every film has fog. The floor is never zero and it is not supposed to be. So “there is fog on this negative” is not a finding, and a page that treated it as one would have you chasing a property of the material.

What makes fog a defect is a measured departure from a value you have established — either the manufacturer’s published figure for the material, or, far more useful, your own recorded value for your film, your developer and your conditions.

✕A gap the course reports rather than papers over

The manifest for this page asked for published base-plus-fog figures for the films the course recommends, so that “an elevated floor” could be stated as a number.

The datasheets do not carry them. The technical information sheets for HP5 Plus, FP4 Plus, Tri-X and Fomapan 100 in the course’s corpus publish characteristic curves, development tables, spectral sensitivity and storage advice, and none of the four publishes a base-plus-fog value. Kodak’s Tri-X sheet expresses its speed and contrast points relative to gross fog rather than stating what gross fog is.

So this page cannot tell you what your floor should read, and it will not invent a figure. What it can tell you is what to do instead, which is better anyway: measure your own, once, on a good session, and write it down. A floor of 0.14 means nothing in the abstract and everything when your last four films read 0.11, 0.12, 0.11 and 0.12.

This is where a densitometer earns its cost. Without one, the substitute is a strip of known-good film kept for the purpose and compared edge to edge on a light box — a ranking rather than a measurement, but a ranking against a fixed reference is enough to detect a departure.

Prevention, across the whole system

Fog is a system property and the preventions are system-level.

Storage. Cool, dry, original packaging, and buy what you will use. ILFORD’s 10–20 °C and Kodak’s 24 °C are both easily met by a cupboard on an inside wall and both are missed by a shelf above a radiator. Process exposed film promptly.

A restrainer in the developer, and the mixing discipline that keeps it working. The bromide is part of the formula. So is dissolving the developing agent before the alkali, for the reason Kodak’s primer gives: an agent crystal in an alkaline solution oxidises at its surface and the resulting solution gives fog. And so is keeping tin, copper, zinc and their alloys out of the process altogether.

Time and temperature discipline. Chemical fog rises with both. The controls are the same ones that control contrast and are already in your processing table.

A safelight tested at the distance and duration you actually use — not at the distance in the brochure and not for one minute when your sessions take six. Test it, label the lamp with the result, and change the filter on a schedule rather than on suspicion.

A room tested with the lights off and your eyes given fifteen minutes to adapt. ILFORD’s fifteen minutes is not a courtesy figure; it is roughly how long it takes for the eye to become sensitive enough to see the leaks that matter. A room checked after thirty seconds has not been checked.

And a test whenever something changes. New film stock, new bath, new bulb, new bench position, a filter another year older.

Alternative route

Three of the five tests need no darkroom. Steps 1 and 2 — a strip fixed without development and a strip developed without exposure — need a changing bag, a tank and the chemistry you already have; they are done with the room lights on except for the few seconds of loading. Step 3 is a reading taken from a negative on a light box. Between them those three settle the commonest question this page is asked, which is whether a raised floor came from the chemistry, the film’s own history, or light.

Steps 4 and 5 test a darkroom, so without one there is nothing to test — and that is the honest answer rather than a substitution. What replaces them is a room test, which is the same experiment asking a smaller question: whichever space you load film in, and whatever light it has, can be tested by putting a sheet of photographic paper on a surface in it with half of it covered by an opaque card, leaving it for the time you actually spend loading, and developing it later or having it developed. A difference between the two halves tells you that your loading space is fogging material, which is a real fault with a real fix and is worth knowing whether or not you ever build a darkroom.

Safelight fog itself is not reachable without a safelight, and this page says so rather than offering a substitute. If you print by sending files or negatives out, the safelight section is read as diagnosis — it is what lets you recognise the fault in prints that come back — rather than as a test you run.

Atlas entries this page owns

Seven entries in the troubleshooting atlas belong here. Six of them are written: base fog, chemical fog, safelight fog, light-leak fog, age fog and heat fog.

The seventh, radiation fog, is not, and it is unlinked above because there is no page to link to. The atlas publishes an entry when its mechanism, its discriminating evidence and its corrective action are supported; this page has the practical advice and the appearance, and — as the airport section says — no source for the mechanism or the dose dependence. So the row stays in src/data/defects-planned.json and is counted as outstanding, which is the atlas holding itself to a number rather than filling a gap with a plausible account.

Safelight fog is a shared entry with a stated split: this page and its atlas entry own the defect — what it looks like, what distinguishes it, what to do — while Part XVI owns the test procedure and the calibration record. Dichroic fog is named here and owned by the fixing page, because its cause is a solution rather than a light or a development event.

Recap

Base density is the material, fog is unintended developed silver, and base plus fog is what the instrument reads. Fog costs shadow separation before it costs anything else, because the toe is nearly flat and a raised floor swallows it. Chemical fog is even across the whole film including the rebate, and comes from too much alkali, too little restrainer, too long, too warm or a contaminated bath. Light fog has a direction, usually an edge, and often a period that matches the material’s geometry — and whether it stops at the frame line says whether it happened in the camera. Safelight fog depends on the lamp, the filter, the distance and the time together, appears first where paper sat longest, and is invisible until the developer finds it. Age and heat raise the floor before they cost speed, and radiation raises it evenly and irreversibly. Five tests, run in order, identify which of these you have; and what makes any of it a defect is a departure from a floor you measured, not the presence of a floor at all.

Check your understanding

Question 1. A film shows raised density in the rebate as well as in the image areas. Which fog sources remain possible, which are excluded, and what is the next test?

• Only a light leak remains; run the room test
• Only chemical fog remains; discard the developer
• Everything that acts on the whole film at once remains — chemical fog, age or heat fog, radiation, and light reaching the film in the cassette, changing bag or tank — while anything working through the lens is excluded; the next test is a strip developed without exposure, because it separates the chemical causes from the light ones in one step
• Nothing can be concluded until the film is printed

Show the answer and why

Answer: Everything that acts on the whole film at once remains — chemical fog, age or heat fog, radiation, and light reaching the film in the cassette, changing bag or tank — while anything working through the lens is excluded; the next test is a strip developed without exposure, because it separates the chemical causes from the light ones in one step

The rebate sits behind the frame mask, so lens-formed light cannot reach it and the shutter, aperture and metering are all eliminated. That still leaves two families — chemistry and film history on one side, stray light outside the camera on the other — and step 2 of the sequence separates them, because a strip that never saw light but went through your developer isolates the chemical contribution exactly.

Question 2. Why does fog damage shadow separation before it changes how the highlights look?

• Because fog is deposited preferentially in the shadow areas
• Because the toe of the curve is nearly flat, so the shadows occupy a small range of density; raising the floor into that range destroys the differences within it, while the highlights sit on the straight-line section where the same added density is a small fraction of a large difference
• Because shadows are printed with more exposure
• Because fog is only visible against a dark background

Show the answer and why

Answer: Because the toe of the curve is nearly flat, so the shadows occupy a small range of density; raising the floor into that range destroys the differences within it, while the highlights sit on the straight-line section where the same added density is a small fraction of a large difference

Fog is deposited everywhere, not preferentially. The asymmetry is geometric: near the toe, a whole stop of exposure may produce only a tenth of a unit of density, so a tenth of fog erases a stop of shadow information. On the straight line, the same tenth is a small addition to differences that are already large. This is why a fogged negative reads as flat and muddy in the shadows while the highlights still look right, and why it is so often blamed on under-development.

Question 3. A printer's prints are variably grey in the borders — some sheets much worse than others in the same session. What does that variation itself suggest, and what is the confirming test?

• A bad box of paper; test a second box
• Safelight fog, because it accumulates with time under the lamp and the sheets that sat out longest have received the most; confirm it with one sheet left on the bench for a realistic working time with an opaque object on it, developed afterwards
• Developer exhaustion, because later sheets in a session get a weaker bath
• A light leak in the enlarger, which would affect every sheet identically

Show the answer and why

Answer: Safelight fog, because it accumulates with time under the lamp and the sheets that sat out longest have received the most; confirm it with one sheet left on the bench for a realistic working time with an opaque object on it, developed afterwards

The variation is the evidence. A fault that is identical on every sheet points at something every sheet met equally — the paper, the developer, a constant leak. A fault whose severity tracks how long each sheet was out points at a dose that accumulates with time, which is what a safelight delivers. The coin test converts that inference into a demonstration, and it is worth one sheet. Note that the fog will not be visible on the paper before development: it is a latent image until the developer finds it.

Question 4. Why does this page refuse to state what your film's base plus fog should read?

• Because base plus fog cannot be measured accurately
• Because the four film datasheets in the course's corpus do not publish the figure — Kodak's Tri-X sheet even expresses its own density points relative to gross fog rather than stating it — so any number here would be invented; the page asks you to establish and record your own value instead
• Because it varies too much between rolls to be useful
• Because only a laboratory densitometer can read it

Show the answer and why

Answer: Because the four film datasheets in the course's corpus do not publish the figure — Kodak's Tri-X sheet even expresses its own density points relative to gross fog rather than stating it — so any number here would be invented; the page asks you to establish and record your own value instead

This is Rule 1 in operation and it is worth noticing that the honest answer is also the more useful one. A published figure would have been measured with the maker's developer, time, temperature and agitation, none of which are yours. Your own recorded floor, from a session you were happy with, is a reference against which a departure is meaningful — and detecting a departure, not matching a published number, is what the diagnosis actually needs.

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