Exposure for Film: Aperture, Shutter Speed and Stops

Exposure is the amount of light that reaches the film. On a film camera you control it with two settings, the aperture and the shutter speed, and the film's speed tells you how much light it needs. The unit that ties all three together is the stop.

What a stop is

A stop is a doubling or halving of light. Give one stop more exposure and twice as much light reaches the film; one stop less and half as much does. Because aperture, shutter speed and film speed are all scaled in stops, you can trade one against another: open the aperture one stop and shorten the shutter one stop, and the exposure is unchanged.

Aperture and f-numbers

The aperture is the opening inside the lens. Its size is given as an f-number, the focal length divided by the diameter of the opening, so f/2 on a 50mm lens is an opening 25mm across. Larger f-numbers mean smaller openings. The standard full-stop sequence is:

f/1.4  f/2  f/2.8  f/4  f/5.6  f/8  f/11  f/16  f/22  f/32

Each number is the previous one multiplied by about 1.4, the square root of 2, because the area of the opening, not its diameter, determines the light. Each step to the right halves the light. Aperture also controls depth of field: smaller apertures give more of the scene in focus.

Shutter speed

The shutter speed is how long the film is exposed. The standard sequence, each step halving the time, is:

1s  1/2  1/4  1/8  1/15  1/30  1/60  1/125  1/250  1/500  1/1000

Fast speeds freeze movement; slow speeds blur it. A common handholding guideline is to use a shutter speed no slower than 1 over the focal length, so 1/60 with a 50mm lens. The B (bulb) setting holds the shutter open as long as you press the release, for exposures longer than the dial allows, where reciprocity failure starts to matter.

On older mechanical cameras, marked speeds can drift with age, especially the slowest and fastest settings. Buying a used film camera explains how to check them.

Film speed

Film speed is fixed for a roll. Faster film needs less light: ISO 400 needs two stops less than ISO 100. On film, choosing the film is choosing the sensitivity, so pick it for the light you expect. See film speed and ISO.

Equivalent exposures

Since each setting moves in stops, many combinations give the same exposure. If the meter says 1/125 at f/8, then all of these expose the film identically:

Which one you choose depends on what matters for the picture: a fast shutter for a moving subject, a wide aperture for a blurred background, a small aperture for depth.

Exposure value (EV)

Exposure value is one number for every equivalent combination above. At ISO 100, EV 0 is one second at f/1, and each EV step is one stop less light. Bright sun is around EV 15 at ISO 100; a typical living room at night is around EV 5 to 7. Many light meters read in EV, and older Hasselblad and Rollei lenses have EV-coupled settings that lock aperture and shutter together.

Exposure modes on film cameras

• Manual: you set both aperture and shutter, guided by the meter.
• Aperture priority (A or Av): you set the aperture, the camera picks the shutter speed.
• Shutter priority (S or Tv): you set the shutter, the camera picks the aperture.
• Program (P): the camera picks both.

Whatever the mode, the camera is only as good as its meter reading. The next guides cover how light meters work, the Sunny 16 rule for shooting without one, and how much error film can tolerate in exposure latitude.

All the guides are on the Learn Film Photography page, and every term is in the glossary. Shot something on film? Share it in the gallery.