Film Speed and ISO Explained: Box Speed, Grain and DX Coding

Film speed tells you how sensitive a film is to light. It is the number on the box, ISO 100 or ISO 400, and it is the starting point for every exposure you make. Unlike a digital sensor, a roll of film has one speed, so choosing the film means choosing the light you can shoot in.

What the ISO number means

The ISO scale is arithmetic: doubling the number doubles the sensitivity, which is one stop. ISO 400 film needs half the light of ISO 200 and a quarter of the light of ISO 100. The common steps are 25, 50, 100, 200, 400, 800, 1600 and 3200, with third-stop speeds such as 125, 160 and 320 in between.

The old American ASA numbers are identical to today's arithmetic ISO numbers. European cameras and films sometimes show the logarithmic DIN scale as well, where every 3 degrees is one stop: ISO 100 is 21°, ISO 400 is 27°.

Slow, medium and fast film

Speed	Range	Good for	Trade-off
Slow	ISO 25 to 100	Landscapes, bright sun, tripod work, big enlargements	Needs lots of light or long shutter speeds
Medium	ISO 125 to 400	General use, travel, people outdoors	Moderate grain
Fast	ISO 800 to 3200	Low light, indoors, night, action	Bigger grain, lower sharpness

Speed and grain go together because faster film uses larger crystals, which catch light more readily. See film grain for why, and how developer and format change it.

Box speed and exposure index (EI)

Box speed is the ISO rating the manufacturer prints on the box, measured under standard development. Exposure index, or EI, is the speed you choose to rate the film at in practice. Many photographers rate black and white film a little below box speed, for example HP5 Plus at EI 320, to put more detail into the shadows. Rating it well above box speed and developing longer to compensate is push processing.

Changing the EI doesn't change the film's real sensitivity. Rating ISO 400 film at 1600 simply underexposes it by two stops; extra development then raises the midtones and highlights back up, but shadow detail that was never recorded doesn't come back.

Some films have no single true speed. Ilford Delta 3200 and Kodak T-Max P3200 have an actual speed nearer ISO 1000 and are designed to be pushed.

DX coding

Most 35mm cassettes carry a DX code: a pattern of silver conductive and black insulated squares on the cassette. Electrical contacts in the film chamber of a DX-reading camera read the pattern and set the film speed and number of exposures automatically. The barcode on the cassette, also called DX, is for lab equipment.

If your camera reads DX and you want to shoot at a different EI, you need a camera with a manual override or exposure compensation. Cameras that only read DX fall back to a default speed, often ISO 100, when a cassette has no code, which catches out people using bulk-loaded or hand-rolled film.

Setting speed on a manual camera

On older cameras you set the film speed on a dial, usually around the rewind knob or shutter dial. Forgetting to change it when you switch films is one of the most common causes of a badly exposed roll. A useful habit: tear the end flap off the film box and keep it in the camera's memo holder or tape it to the back.

Choosing a film speed

ISO 100 for bright light, landscapes and the finest grain.
ISO 400 as an all-rounder that handles dull days and handheld shooting.
ISO 800 to 3200 for interiors and night, or push 400 film when you run out of light.

To see how a particular film looks in the real world, browse the film stock pages, where members' photographs are grouped by the film they were shot on.