EXPOSURE THEORY for DSLR Cameras

Photographers have spoken the language of stops since the beginning. A stop is a quantity of light. All stops of light are the same amount of light. The two controls determining how much light strikes your digital sensor is the aperture and the shutter speed.

Aperture Values:

Here’s the standard aperture series.

1 1.4 2.0 2.8 4.0 5.6 8.0 11 16 22 32

These numbers are derived by dividing the focal length of the lens by the diameter of the lens effective aperture. If you remember the first two numbers, 1 and 1.4, and then alternately double those, you’ll generate the entire series of numbers.

Most lenses start at f/2.8 or f/4 and go to f/22 or f/32. The aperture in the lens is largest when the f-number is smallest and smallest when the f-number is large. Big f-numbers refer to small apertures so bigger numbers like f/22 mean less light. Remember that as the f-number gets bigger (f/8 to f/11 for example), depth of field increases, but the size of the aperture decreases so less light passes through the lens.

Now let’s talk the language of stops. Each of the numbers in the f-number series varies from its nearest neighbor by 1 stop of light. For example, f/4.0 is 1 stop from f/5.6 and f/2.8. If the aperture is set at f/8 and you want to add 2 stops of light, what aperture do you select? If you say f/16, you have moved the aperture 2 stops, but f/16 is a smaller aperture so you didn’t add 2 stops of light, you subtracted them. The correct answer is f/4.0.

Each 1 stop move on the standard full stop scale is a doubling or halving of the light. Moving the aperture from f/8 to f/11 is halving the light and moving to f/5.6 is doubling the light.

Shutter Speed Values:

Shutter speeds are exposure controls too. The speed of the shutter as it opens and then closes determines how long light can strike the sensor so that photons can be captured by the pixels.

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

Each shutter speed listed varies from it’s nearest neighbor by 1 stop of light. This series doubles or halves the light from 1 stop to the next too.