Philip Wheeler Photography: Measures of Exposure

The basic unit for designating a relative brightness of light in photography is a stop. The three main variables for exposure are aperture, shutter speed, and ISO. Each of these uses a different numbering scheme and yet they can be related to each other in terms of stops.

ISO, the relative sensitivity of the sensor, is the easiest to understand: double the ISO to add 1 stop to the exposure. Halve the ISO to subtract one stop. ISO is most commonly seen as the numbers 50, 100, 200, 400, 800, 1600, 3200, 6400, etc. Thus to add one stop of exposure from ISO 200, you would select ISO 400. To add on-half stop to ISO 200, you would set the ISO at 300. This linear relationship makes the calculation straightforward. Higher ISO numbers mean added sensitivity to light. A word of caution, however: very high ISO settings can increase the noise in the final image. Your available ISO sensitivities depend on the sensor in your camera and on the camera's software (firmware).

Shutter speed is similarly linear. A 1/120th second exposure is one stop less bright than a 1/60th second exposure. A 2 second exposure is one stop brighter than a 1 second exposure. The maximum (fastest) shutter speed is determined by the camera/lens combination. There is a minimum, or longest, speed set within the camera's firmware: with a Nikon DSLR it is usually 30 seconds. You can switch to bulb mode, however, and when using a remote shutter release your maximum shutter open time can be many minutes.

Aperture is the oddball of the lot: full stops in aperture measurement are designated in a progression:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. This full-stop sequence of f/number is worth remembering. There are also sequences of half-stop and third-stop apertures. To double the exposure from f/8, you would set the aperture to f/5.6. Smaller f/numbers mean a larger aperture, or opening, by which light is admitted through the lens.

Higher ISO = more sensitivity to light. Longer shutter speeds = more light. Smaller f/numbers = more light.

In your practice, try combining these. Say you want a wide aperture of f/4 to gain a shallow depth of field. Your camera is set at ISO 200. Your light meter indicates a shutter speed of 1/15th second. But you are holding the camera by hand, and thus you want a shutter speed no slower than 1/60th second (two stops less) to reduce camera shake. Simply change your ISO to 800, thus adding two stops, and you've solved it!

Philip Wheeler Photography