Calculating Exposure

Components of Exposure

Exposure is controlled by the aperture, shutter speed, and film sensitivity used to take an image. All the components of getting a proper exposure are related, so there are many different combinations of elements that let you get a proper exposure.

Aperture

An aperture is an opening which light travels through. In photography, you can change the size of the aperture to control the amount of light that gets to the film.

Apertures on a typical lens

The aperture number, or f-stop, is defined as f/N, where f is the focal length of the lens, and N is the size of the aperture opening. The f-numbers on most lenses follow a geometric series of f-numbers, where each stop allows half the light as the previous: f/2.8, f/4, f/5.6, f/8, f/11, and so on.

Shutter Speed

The shutter speed controls how long the film is exposed to light. The shutter on modern cameras is usually mounted directly in front of the film, preventing all light from hitting the film when not in use. Shutter speeds correspond with apertures; each speed allows twice or half the light as the next.

You can get interesting effects by manipulating the shutter speed. For example, you can freeze motion by using a fast shutter speed like 1/500 of a second, or a slow shutter speed, like 15 seconds, to blur motion.

Film Sensitivity

Film sensitivity is a function of the number of silver grains on the film. It indicates how quickly the image is exposed on the film. The most common measurement of this sensitivity is ISO, which is simply an arithmetic scale that represents the speed. For example, an ISO 100 film captures half the amount of light as ISO 200 film.

The Bucket Metaphor

Properly exposing a picture is kind of like filling up a bucket. You can control the size of the hose (aperture), how long you leave the hose on (shutter speed), and the water pressure (ISO). The bucket, which is a metaphor for the amount of light needed, isn't always the same size either; sunny days will almost always be brighter than indoors in absolute terms.