Exposure: Film and Digital

Exposure is the change caused by light when it strikes a photosensitive material. It has two factors—time (or duration of exposure) and intensity (the volume of light striking that material.) In both film and digital systems the time of exposure is indicated by the shutter speed, the amount of time that the gate through which light travels after it leaves the lens remains open. The volume of the light that gets through in that time period depends on the size of the diameter of the opening in the lens, known as the aperture number or f-stop. Exposure is like turning on a faucet and letting water flow through a pipe. If we leave the tap open for a longer period of time more water will flow through. If we enlarge the pipe more water can run through in that same period of time.

Exposure is calibrated with a system of stops, or the modern equivalent, EV (exposure value). If the total amount of light doubles or halves there is a change of one stop, or one exposure value. A 1 stop or 1 EV change can be made by halving or doubling the exposure time (shutter speed, for example changing from 1/15 to 1/30 second, or from 1/30 to 1/15 second). It can also be changed by opening or closing the lens diameter by one f-stop (for example, by going from f/11 to f/8 or from f/8 to f/11).

This system allows us to expose with the same amount of light and alter image effects by balancing the changes we make in aperture and shutter speed. If we use a faster shutter speed we can freeze motion; if we use a narrower lens opening we can increase depth of field. As long as we maintain a balance between the two (change the shutter speed in proportion to how we change the aperture—one going up one stop and the other down one stop) we maintain the same overall exposure. This setup is called “equivalent exposure” and is the basis for many creative techniques used in photography.

This exposure system holds true for both film and digital photographic systems, as do the image effects they create. As mentioned, the physics of photography do not change because we have gone from a film to a digital medium.