There are two camera controls that allow the amount of light reaching the sensor to be altered. First is the shutter speed which regulates the duration of light reaching the sensor. The numbers on the shutter speed dial are fractions of a second. They show how long the sensor will be exposed to light. While a 1/100 of a second may seem like a very short time, it really is not. Something that is either moving quickly or is very close can move quite a lot in that brief time, leaving behind a blurred image..

The shutter speed has two functions, one as half of the exposure equation and the other allowing either the freezing of moving things or the blurring of them. Some blurring is necessary if the idea of speed is to be communicated. A car traveling at 100 miles an hour, frozen sharply at an exposure of a ¹/₁₀₀₀ of a second, might as well be parked at the side of the road. The same car photographed at a ¹/₄ of a second will appear as a blur that is suggestive of speed and movement. If the camera is panned with the moving object, the background will be blurred but the subject will be sharp against it. This is particularly effective for communicating the idea of speed.

The other control that regulates light is the aperture or opening in the lens itself, which can be varied in diameter. This changes the quantity of light that reaches the sensor. Doubling the area of the opening will allow double the amount of light through. Because the diameter of this opening is expressed as a function of the focal length of the lens, the sequence of numbers used to describe the aperture, the f-numbers, may seem quite odd.

The important thing to remember is that a change in setting from a smaller number to the next larger halves the area of the opening and so halves the amount of light entering the camera (e.g. going from f5.6 to f8 halves the light). Conversely, opening the lens setting from a larger number to the next smaller will double the amount of light (e.g. going from f16 to f11). Remember that smaller numbers like f2 mean a bigger opening and larger numbers like f16 mean a smaller opening. Not all lenses will have all the numbers; in fact most 50mm standard lenses used on 35mm cameras will only have f-stops from f2 to f16.

In addition to controlling the amount of light reaching the film, the aperture (or f-stop) also regulates the depth of field. This term is used to describe how much of the subject that is nearer or further than the point upon which the camera is focused will also be in focus. Small openings like f16 allow much greater depth of field, large ones like f2 result in very shallow depth of field. Each has its advantages. Shallow depth of field will make a sharply focused object stand out clearly against a fuzzy background, while greater depth of field will allow all of a three dimensional object to appear sharp, in spite of parts of it being nearer or farther from the camera. Similarly it will allow both foreground and background elements to be sharp at the same time.

Many newspaper editors prefer subjects that are set against out of focus backgrounds and for that reason many photojournalists use large f-stops with 200mm or 300mm lenses since they give very shallow depth of field.

Each of the controls of shutter and aperture has two functions. In every photographic situation it is important to chose a combination of a shutter speed and an aperture that will result, firstly in correct exposure and secondly, in the correct freezing or blurring and the right depth of field for the subject involved. This means a constant trade off between the two unrelated functions and we will have to make frequent compromises. Remember also that it is not safe to handhold your camera at shutter speeds that are longer than a 1/30 of a second. You will almost inevitably move the camera during the exposure, blurring the entire frame. Always use a tripod or other support for such longer exposures.