How sensitive a film is to light, written as an ISO number on the box: 100, 200, 400. Each doubling needs half the light for the same exposure, so ISO 400 film is one stop faster than 200 and two stops faster than 100. The maker measures it from the film’s characteristic curve under its standard development.

What does ISO mean on film?

The film’s speed by the ISO standards, which joined the two older scales: the arithmetic ASA number and the logarithmic DIN degree. A film marked ISO 400/27° is ASA 400 and DIN 27. ISO is the International Organization for Standardization, not an abbreviation of anything about light.

Is ASA the same as ISO?

Yes for the number: ASA 400 and ISO 400 are the same speed, and an old camera’s ASA dial is set to the film’s ISO. The DIN degree is the other half of the full ISO speed: ISO 400/27°.

The ISO speed printed on the box: the maker’s measured speed for normal development. Shooting at box speed means setting the meter to it. Rating a film at another speed is shooting it at an exposure index (EI) other than box speed.

Should I shoot film at box speed?

It is the safe default, and the right one for slide film, which has little room either side. Color negative film takes more light well: Kodak gives Gold 200 latitude “from two stops underexposure to three stops overexposure”, and many photographers rate ISO 400 color negative film at 200. Black and white takes a stop faster with normal development, by Kodak’s Tri-X sheet; further needs a push.

Is 200 or 400 film better?

Neither in general. ISO 400 is a stop faster: twice the shutter speed in the same light, or a stop smaller aperture for more depth of field. It suits overcast days, shade, indoors and moving subjects. ISO 200 suits sun. Grain depends on the film more than the number: by Kodak’s own index, Portra 400 is finer-grained than Gold 200.

What film speed should I use?

ISO 100 or 200 for bright sun, 400 for an all-day film that also works in shade and overcast, 800 and up, or a push, for dim light without a flash. At f/8 by the sunny 16 rule, ISO 100 in open shade needs 1/25 s, which blurs hand-held; ISO 400 gets 1/100 s.

Is film ISO the same as digital ISO?

They are set to give the same exposure, so a meter works for both, but they are different things. A film’s ISO is its emulsion, fixed for the roll. A digital camera’s or a phone’s ISO is how much it amplifies the sensor’s signal, chosen per frame; a higher ISO brings noise, not grain.

Can you rate film in Clack?

No. Clack is a camera app; its films are models of real ones, each metered at its box speed. The phone’s ISO follows the light, and the grain grows with it, as film shot in a dim room. Brightness is the EV scale, and Grain and Contrast are their own controls.

Guide · Updated 6 November 2026

What is film speed

Box speed, ISO and ASA, and rating film.

Film speed is how sensitive a film is to light, printed on the box as an ISO number: 100, 200, 400, 800. Each doubling needs half the light, so ISO 400 film is one stop faster than 200 and two stops faster than 100: in the same light it takes a shutter speed twice or four times as fast. The number is the box speed, measured by the maker from the film’s curve under normal development. ASA is the same number by an older name. Shooting a film as if it were faster or slower is rating it, and it changes the exposure, not the film.

Below: the ISO, ASA and DIN scales, how box speed is measured, what rating film does to a negative and to a slide, 200 against 400, which speed for which light, film ISO against a phone’s, and film speed in Clack.

01 · The scale

ISO, ASA and DIN

A film’s full ISO speed has two parts: an arithmetic number, which is the old American ASA number, and a degree, which is the old German DIN one. HP5 Plus’s sheet calls it ISO 400/27°. The arithmetic number doubles with each stop; the degree adds 3. Most boxes and cameras give only the first.

ISO and ASA	DIN	Stops from ISO 100	Films at that speed
50	18°	−1	–
64	19°	−⅔	Kodachrome 64
100	21°	0	Ektar 100
200	24°	+1	Gold 200
400	27°	+2	Portra 400, Tri-X 400, HP5 Plus 400, Superia X-TRA 400
500	28°	+2⅓	Vision3 500T (T: balanced for tungsten light)
800	30°	+3	–
1600	33°	+4	–

DIN is 10 × log10(ISO) + 1, rounded; the stops are log2(ISO / 100) (our arithmetic).

02 · Box speed

How box speed is measured

The maker exposes the film to a range of known exposures, develops it normally and reads the density. For black and white negative film, ISO 6 puts the speed where the film first records: 0.1 of density over the bare film’s fog, at an exposure of 0.8 / S lux-seconds, where S is the speed. A meter is calibrated to put a mid gray at 10 / S, 12.5 times more light, so a box speed leaves 3⅔ stops of shadow under mid gray before the film stops recording. Color negative film (ISO 5800) and slide film (ISO 2240) have their own points on the curve, by the same idea.

Ilford HP5 Plus 400 as Clack’s engine holds it: density on the negative against exposure, 0 at the meter’s mid gray. ISO 6 puts the speed point where the film reaches 0.1 over its fog, at 0.8 / S lux-seconds, under a development that adds 0.80 of density over the next 1.3 log units of exposure. A meter puts mid gray at 10 / S, 3.64 stops higher. On this curve the speed point is 0.27 (fog 0.17) and the next 1.3 log units add 0.77 (our arithmetic on the engine’s data).

The box rounds the number to the standard series, and a maker can choose to print a practical speed instead. Read off the makers’ own curves:

Film	Box speed	The speed its curves give
Kodak Gold 200	200	About 195, from Kodak’s curves
Kodak Tri-X 400	400	About ⅓ stop faster, near 500, from Kodak’s curves at 6 min in T-MAX developer
Ilford HP5 Plus 400	400	Ilford calls its EI “a practical evaluation … not based on foot speed”
Fujifilm Velvia 100	100	100, from Fujifilm’s curves (ISO 2240’s method)
Fujifilm Velvia 50	50	47, the same way

Derived by us from the makers’ published curves, as Clack’s engine reads them; how close they come depends on how the chart was drawn.

03 · Rating

Rating film: shooting at another speed

Rating a film is setting the meter to a speed other than box speed. The number you shoot it at is the exposure index, EI. Rated at 200, an ISO 400 film gets a stop more light than its box speed asks for; rated at 800, a stop less. The film is no faster or slower for it: only the exposure changed. What happens next depends on the film.

Color negative takes more light well. Kodak gives Gold 200 latitude “from two stops underexposure to three stops overexposure”. The lab prints mid gray back to mid gray, so a negative rated slower and printed comes out close to box speed. Rating ISO 400 color negative film at 200 is common advice among photographers; Kodak’s sheets do not give it.
Black and white negative takes a stop less: Kodak, on Tri-X 400, “you can underexpose by one stop and use normal processing times. Prints will show a slight loss in shadow detail.” Further than that, the roll is pushed, developed longer (pushing film).
Slide film is the picture itself, with no print to correct it: rated slower it comes out lighter, faster darker, stop for stop.

-4-3-2-10+1+2+3+4+5

Portra 400 rated 800

Portra 400 at 400

Portra 400 rated 200

Kodachrome rated 125

Kodachrome at 64

Kodachrome rated 32

Clack’s models of Kodak Portra 400, a negative printed back to the same mid gray as a lab does, and Kodak Kodachrome 64, a slide: a gray card from four stops under to five over the meter’s mid gray, the film rated a stop faster, at box speed and a stop slower. Portra barely moves: rated 200 its light tones print a touch brighter, rated 800 its blacks a touch lighter. Kodachrome moves a whole stop each time. The engine’s output, before grain.

04 · How

How to rate film

Choose the rating. Box speed unless you have a reason not to. Slower than box speed gives a negative more light; faster underexposes it.
Set the meter to it. Set the camera’s ISO dial, or a handheld meter, to the speed you rate the film at, not the box speed.
Shoot the whole roll at it. The roll is developed together, so give every frame the same rating.
Mark the roll. Write the rating on the canister, “EI 200” or “EI 1600”, so whoever develops it knows.
Develop for it. A color negative rated slower is developed normally. Kodak says Tri-X can be rated a stop faster and developed normally; further than that, the roll is pushed.

Metering by hand at any rating: the sunny 16 calculator.

05 · Which speed

200 vs 400 film, and which speed to use

A stop of speed is a stop of shutter or aperture. At f/8, by the sunny 16 rule, the shutter each film speed needs:

Light	ISO 100	ISO 200	ISO 400	ISO 800
Sun, sharp shadows	1/400	1/800	1/1600	1/3200
Overcast	1/100	1/200	1/400	1/800
Heavy overcast	1/50	1/100	1/200	1/400
Open shade, sunset	1/25	1/50	1/100	1/200

The rule’s shutter, 1/ISO at its aperture, moved to f/8 (our arithmetic, unrounded: a camera takes the nearest marked speed). Indoors there is less light than open shade; use a meter.

ISO 100 and 200 suit sun; in open shade they leave a hand-held shutter slow. ISO 400 is the film that works all day, sun to shade, and in the sun it needs a faster shutter or a smaller aperture. Past that light, 800 and up, or a push. Grain grows with speed within one generation of film, but the film matters more than the number: by Kodak’s own Print Grain Index, Portra 400 is finer-grained than Gold 200.

Film	Print Grain Index, 4 × 6	8 × 10
Kodak Ektar 100	Under 25	38
Kodak Portra 400	37	59
Kodak Gold 200	44	64
Kodak UltraMax 400	46	–

Kodak’s Print Grain Index for prints from 35 mm: 25 is the threshold of visible grain; lower is finer. Why speed and light make grain: what is film grain.

06 · Digital

Film ISO and a phone’s ISO

A digital camera’s and a phone’s ISO is set so that a meter gives the same exposure as for film of that speed, which is why sunny 16 works for both. The rest differs.

	Film	A phone or a digital camera
What it is	How much light the emulsion needs, measured once by the maker	How much the sensor’s signal is amplified
When it is set	Once, for the whole roll	Every frame, often by the camera
Higher means	A more sensitive film, with bigger grain in the same generation of film	The same sensor, amplified more: more noise
Shooting at another speed	Rating: the whole roll, then developed for it	Turn the dial

Raising a phone’s ISO is closest to a push: less light, made up afterwards. The noise it brings is not grain; the grain of a film look is drawn, as in how to make photos look like film.

07 · Clack

Film speed in Clack

Clack is a film camera app for Android, with eight films modeled on real ones, each named by its box speed: Portrait 400, Minilab 400, Chroma 64, Golden 200, Vivid 100, Cinema 500T, Press 400, Street 400. Each film is metered at its box speed; the phone’s ISO follows the light.

The meter. On the black and white films, mid gray sits at 10 / S on the film’s own curve, as on the curve above; a color negative is printed back to mid gray, as a lab does. A slide, Chroma 64 (based on Kodak Kodachrome 64), is metered as a film camera’s averaging meter would. Where a sky or a window would clip on a negative, the camera exposes up to two stops under and the print lifts it back: the negative’s latitude, used. Not on a slide, a set dial, a frame at ISO 3200 or more, or a phone without the film base (below).
The phone’s ISO. In Point & shoot the camera sets shutter and ISO. Pro shows both and sets them by hand, in thirds of a stop across the sensor’s range: ISO 100 to 16000 on the tested phone.
The grain follows it. As film shot in a dim room, the grain grows with the ISO the phone needed: about 15 % a stop above ISO 100, up to ×1.74 at ISO 1600 and over.
No rating control. A negative rated slower and printed back is almost the same print with finer grain, so Clack has Grain as its own control; on a slide, rating is brightness, which the EV scale does. Contrast pushes and pulls the development.

The film base: On Android phones whose camera offers a contrast curve, Clack takes the frame flat, with the phone’s local tone mapping and color enhancement off, before the film. On other phones the films run on the phone’s own frame.
Free or Pro: For seven days of shooting every film is open. After that the free tier keeps one film of your choice, with its dials; Clack Pro has them all.

A model of the films’ data on a phone’s photo, not film. Tested mostly on the CMF Phone 2 Pro.

Free to download. More about Clack

Questions

What is film speed?: How sensitive a film is to light, written as an ISO number on the box: 100, 200, 400. Each doubling needs half the light for the same exposure, so ISO 400 film is one stop faster than 200 and two stops faster than 100. The maker measures it from the film’s characteristic curve under its standard development.
What does ISO mean on film?: The film’s speed by the ISO standards, which joined the two older scales: the arithmetic ASA number and the logarithmic DIN degree. A film marked ISO 400/27° is ASA 400 and DIN 27. ISO is the International Organization for Standardization, not an abbreviation of anything about light.
Is ASA the same as ISO?: Yes for the number: ASA 400 and ISO 400 are the same speed, and an old camera’s ASA dial is set to the film’s ISO. The DIN degree is the other half of the full ISO speed: ISO 400/27°.
What is box speed?: The ISO speed printed on the box: the maker’s measured speed for normal development. Shooting at box speed means setting the meter to it. Rating a film at another speed is shooting it at an exposure index (EI) other than box speed.
Should I shoot film at box speed?: It is the safe default, and the right one for slide film, which has little room either side. Color negative film takes more light well: Kodak gives Gold 200 latitude “from two stops underexposure to three stops overexposure”, and many photographers rate ISO 400 color negative film at 200. Black and white takes a stop faster with normal development, by Kodak’s Tri-X sheet; further needs a push.
Is 200 or 400 film better?: Neither in general. ISO 400 is a stop faster: twice the shutter speed in the same light, or a stop smaller aperture for more depth of field. It suits overcast days, shade, indoors and moving subjects. ISO 200 suits sun. Grain depends on the film more than the number: by Kodak’s own index, Portra 400 is finer-grained than Gold 200.
What film speed should I use?: ISO 100 or 200 for bright sun, 400 for an all-day film that also works in shade and overcast, 800 and up, or a push, for dim light without a flash. At f/8 by the sunny 16 rule, ISO 100 in open shade needs 1/25 s, which blurs hand-held; ISO 400 gets 1/100 s.
Is film ISO the same as digital ISO?: They are set to give the same exposure, so a meter works for both, but they are different things. A film’s ISO is its emulsion, fixed for the roll. A digital camera’s or a phone’s ISO is how much it amplifies the sensor’s signal, chosen per frame; a higher ISO brings noise, not grain.
Can you rate film in Clack?: No. Clack is a camera app; its films are models of real ones, each metered at its box speed. The phone’s ISO follows the light, and the grain grows with it, as film shot in a dim room. Brightness is the EV scale, and Grain and Contrast are their own controls.

Sources

Kodak, Tri-X, T-MAX, Portra, Gold, Ektar, UltraMax, Vision3 and Kodachrome are Kodak’s or Kodak Alaris’s; Ilford and HP5 are HARMAN technology’s; Fujifilm, Superia and Velvia are Fujifilm’s. Clack is not affiliated with any of them.