The aperture is the hole the light goes through, in the lens. It can be varied. If the hole is very wide it lets a lot of light in so you can use a faster shutter speed (shorter exposure), which is better for ensuring your picture is not blurred because of movement. If the aperture is smaller, less light gets in and so the exposure has to be longer (slower shutter speed).

The aperture is given a number to signify how much light is getting in. This is called an ‘F’ number eg. F4.5, F8. The way the number is calculated is complicated but what it means is that whatever the camera or lens a similar ‘F’ number lets in the same amount of light.

The aperture has a very important effect on your photographs. It affects what is called the ‘Depth of Field’; that is the area in front the camera which will appear sharp. If a camera focuses at, say, 10 feet there will be an area nearer than 10 feet that will still appear to be ‘in-focus’ and an area beyond 10ft that will also be ‘in-focus’.

If you have a wide aperture the band that appears to be ‘in-focus’ is narrower; as you close the aperture (the number gets higher) the band gets wider. This area is called the ‘Depth of Field’ and is different for wide angled lenses than telephotos (zoomed in).

Depth of field

A wide angle lens with a small aperture (say F16) will have almost everything in front of the camera in focus. A zoomed in lens (telephoto) with a wide aperture (F2.4) will have a very narrow ‘depth of field.

If we know these facts we can use them when photographing cats (or other pets or animals). If there is a lot of movement you will need as short as exposure as possible (at least 1/250) so you will need to open the aperture to let in more light. As you do so the ‘depth of field’ decreases so you have to be much more careful about focusing.

If you are close to the cat and want him/her to really stand out from the background you need the background out of focus. You achieve this, again, by opening the aperture to reduce the ‘depth of field’. Be careful about reducing the ’depth of field’  too much otherwise you may find the cat’s nose is in focus but her eyes are not.

Sometimes lighting conditions dictate camera settings. If it is very bright and the camera is already set at its fastest shutter speed you will need to have a smaller aperture and that will increase ‘depth of field’. Most digital cameras allow you to alter the virtual film speed. Please see the section on ‘Sensitivity’

You can overcome, to some degree, the problems of having too much ‘depth of field’ by moving further away and using a telephoto (zoom in), which will have the effect of reducing the amount of light entering and allowing a wider aperture and thus reducing the ‘depth of focus’.