ISO in Film Photography: What the Number on the Box Means

ISO is the film's light sensitivity. Bay Photo Lab puts it simply: the higher the number, the more sensitive the film. On a film box, ISO, ASA and film speed mean the same thing, so an old camera with an ASA dial takes the ISO number straight off a modern box.

ISO is not an acronym. Wikipedia says the short name of the International Organization for Standardization is derived from the Greek word isos, meaning equal, though that explanation was not mentioned at the organization's founding meetings. Its standards (ISO 6, ISO 2240 and ISO 5800) define film speed. Before that, the American Standards Association created the ASA rating in 1943, Europe used DIN, and the two were merged into ISO in 1974.

A full ISO rating has two parts. A roll marked ISO 100/21° is 100 on the old ASA scale and 21 on the DIN degree scale. This page uses the first number, the arithmetic ISO speed.

The number on the box is arithmetic: double the number and the film is twice as sensitive, which photographers call one stop. ISO 400 film needs half the light of ISO 200 film and a quarter of the light of ISO 100 film.

The degree number works differently: add 3° and sensitivity doubles, so ISO 200/24° is twice as fast as ISO 100/21°. The table runs the common speeds through both scales and through the Sunny 16 rule, which says that in bright sun you can shoot at f/16 with a shutter speed of 1 over the ISO. Use the closest shutter speed your camera has.

• Want the exposure math with aperture and shutter speed together? See the exposure triangle for film.
• Shooting without a meter? The Sunny 16 rule covers shade, overcast and backlight adjustments.

Pick the speed for the light you will shoot most of the roll in, because you can't swap it halfway. If you are unsure, Bay Photo Lab suggests ISO 400: it is sensitive enough for many conditions and works at night with a flash.

Stores draw the slow and fast lines in different places. Parallax Photographic calls ISO 20 to 200 low speed and 400 to 3200 fast; Focus Camera calls ISO 400 versatile and ISO 800 to 3200 high speed. Treat the classes as rough. The prices below are single 35mm rolls from two US film sellers, the Film Photography Project store (FPP) and Glazer's Camera in Seattle, checked October 28, 2026.

Generally, yes. Film gets its speed from larger silver halide grains in the emulsion, and larger grains catch light more easily but show up as visible texture. That is why ISO 800 and 3200 films generally look grittier than ISO 50 or 100 films.

For smooth, crisp images, Bay Photo points to low-ISO stocks like Kodak Ektar 100 and Ilford Pan F 50. Kodak says Ektar 100 has the finest, smoothest grain of any color negative film available today.

Exposure changes grain too: underexposed film looks grainier than overexposed film, so a roll shot under its box speed can look grainier than the number on the box suggests.

No. With film, the entire roll is shot at one ISO, because every frame is developed together. Unlike a digital camera, you can't raise ISO for one dark frame and lower it for the next.

Turning the ISO dial on the camera only changes what the light meter assumes. Bay Photo's example: leave ISO 200 film in the camera, set the dial to 800, and you get muddy, underexposed photos, because the film's real sensitivity never changed.

Cameras that read DX codes set the speed for you. Kodak introduced DX encoding on January 3, 1983, a conductive pattern on the cartridge that automatic cameras read, and the first point-and-shoots to set film speed from it came out in 1984. On a camera that doesn't read DX, you set the ISO or ASA dial by hand each time you load a roll.

Portra 400

Set a higher number than the film and you underexpose it. Each doubling is one stop less light: ISO 400 film shot with the dial at 800 is one stop under, at 1600 two stops under. Underexposed color negatives get burnt shadows with no detail and more grain.

Set a lower number and you overexpose it. Color negative film handles that well because it has far more exposure latitude than slide film. Moment's Portra 400 guide says you can overexpose Portra 400 by one to two stops with excellent results, and many shooters rate it at ISO 200 to 320 for denser negatives, then have it developed normally.

Kodak, for its part, rates Portra 400 at true ISO 400 and says that is the speed that delivers its skin tones and saturation, so box speed is a safe default and overexposure is a choice.

Box speed is the ISO printed on the box, the maker's recommended rating. Exposure index (EI) is the speed you choose to rate a film at for a situation, when it differs from box speed. Rating ISO 400 film at EI 800 and push-processing it is one way to get printable negatives in low light.

Each stop of push doubles the rating and each stop of pull halves it. From box speed 400, +1 is 800, +2 is 1600 and +3 is 3200; -1 is 200, -2 is 100 and -3 is 50. Pushing adds contrast and makes grain more noticeable. The table shows the push ratings Kodak states for its films, plus the figure Wikipedia lists for Ilford HP5 Plus. Our guide to pushing and pulling film covers development in detail.

Delta 3200

Not quite. According to the data sheets, Ilford Delta 3200 is actually an ISO 1000 film, and Kodak says T-Max P3200 has a nominal speed of ISO 800; the P means it is designed to be push-processed to EI 3200 or higher.

The DX code on these cartridges carries the marketed speed, 3200, not the measured ISO speed, so that automatic cameras and labs can treat the roll as 3200. If your camera sets 3200 from the code, you are shooting P3200 two stops past its nominal 800 (800 to 1600 to 3200). Writing a push or pull on the canister and the order form, as The Darkroom asks, keeps development matched to how you exposed the roll.

Box numbers have shifted before. When the US ASA standard was revised in 1960, many black-and-white negative film speeds doubled on paper: Ilford HP3 went from 200 ASA to 400 ASA with no change to the emulsion.

Low ISO film (50 to 200) wants bright light. Sunny 16 gives the baseline: in full sun, f/16 at 1/100 s for ISO 100 film. As the light drops, open the aperture (f/11, f/8, f/5.6) or slow the shutter, one stop for each step.

Slow shutter speeds bring camera shake, so Parallax Photographic recommends a tripod when you shoot slow film in less light. Long exposures bring a second catch: film follows normal exposure math from about 1/1000 s to 1/2 s, and outside that range the relationship breaks down (reciprocity failure).

It can, and faster film is more at risk. Parallax Photographic says traditional carry-on X-ray scanners are safe for all but the fastest films, ISO 800 and above, but that newer CT scanners for cabin bags run a high chance of damaging film of any ISO, citing initial testing by Ilford Photo.

TSA recommends packing undeveloped film in your carry-on, or taking it to the checkpoint and asking for a hand inspection. For instant film specifics, see instant film and airport X-ray.