technical: aperture

Aperture, lenses, and depth of field

The aperture of the lens is the opening which lets light in to the camera. This can be adjusted according to the light conditions; made wider to let in more light, or smaller to let in less light. Aperture also has a big effect on the way the picture looks, in particular on depth of field, or how much of the image is in focus.

The controls

The size of the aperture is measured in f-stops, which are relative fractions of the focal length of the lens. The smaller the number, the wider the aperture. Depending on the camera and the focal length of the lens, the f-stop numbers you might find on the lens may be something like this;

diagram: large and small apertures, showing the mechanism used to control the aperture size

f1.4           f2           f2.8           f4           f5.6          f8          f11          f16          f22
wide aperture-------------------------------------------------------------» small aperture

The widest aperture here is f1.4, the smallest is f22. The difference between each aperture is known as a stop, and either halves or doubles the amount of light allowed to reach the film. The widest aperture available on any lens is known as the speed of the lens; it is a useful feature of a lens to have a relatively wide maximum aperture as then the camera is more versatile in lower light conditions.

Depth of field

Depth of field refers to the amount of a photographic image that is in focus. When all of an image, foreground, middle ground and distant ground is in focus it is said to have a wide depth of field. If some parts of an image are out of focus, for example, the background and the foreground, leaving only the middle ground in focus, then the image is said to have a narrow depth of field.

Wide depth of field looks like this:

a drawing showing the effect of a narrow aperture

Narrow depth of field looks like this:

a drawing showing the effect of a large aperture

There are 3 main considerations that control depth of field;

1. Aperture

Aperture controls depth of field as follows:

f1.4     f2     f2.8     f4     f5.6     f8     f11     f16     f22
wide aperture---------------------------» small aperture
narrow depth of field-------------------» wide depth of field

a diagram showing focal length of large apertures

Selecting a small aperture, ie. f11 and smaller, will allow you to have more of the subject in focus.

a diagram showing focal length of small apertures

Selecting a wide aperture, ie. f4 and wider, will restrict the depth of field so the photographer must choose which part of the image to focus on.

2. Type of lens

A diagram showing the depth of field found in telephoto lenses

A telephoto lens restricts depth of field. The longer the lens the more this will be the case. You have to be careful to focus on the part of the subject that is of interest. A tripod can be helpful both to hold the lens steady, and to allow the photographer to use a slow shutter speed and a small aperture if a wide depth of field is necessary. Wide angle lenses can display infinite depth of field, whatever aperture is chosen. See Lenses.

3. Subject to camera distance

A diagram showing the depth of field found in telephoto lenses

The distance of the subject to the camera has an effect on depth of field. The closer the subject is, the more the background will tend to be thrown out of focus. This is even more noticeable when using a telephoto lens, and / or a wide aperture. Only a very wide angle lens will be able to give you a close up subject and a distant subject that is in focus at the same time in the same image.

Tip: You might want to use a narrow depth of field to make your subject stand out more clearly, for example, if you are trying to take informal portraits against a busy background. A wide aperture, getting close to your subject, or selecting a telephoto lens can help to blur backgrounds so that they are not a distraction.