Photography Tech Tip : Aperture
This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.
APERTURE
The aperture of a lens is the size of the opening that allows light onto the sensor when a photograph is taken. It is expressed as a ratio of the focal length of the lens, f, and the size of the opening. The aperture or aperture range is quoted for digital compact cameras (with fixed lenses) and digital SLR lenses, but not for digital SLR cameras, as it is a characteristic of the lens, not camera.
Standard measurements for aperture are:
- f/1.4
- f/2
- f/2.8
- f/4
- f/5.6
- f/8
- f/11
- f/16
- f/22
The smaller the number, the larger the aperture and these particular numbers are used as each allows twice the amount of light through as the next in the list. E.g. an aperture setting of f/2 allows 4x the light of a f/4 setting as there are two 'stops' between them.
What difference does aperture make?
Aperture affects images in two ways. Firstly, all images require a certain amount of light over a certain period of time to form correctly on the image sensor. This combination of light and time is known as exposure (see Tech Tip: Exposure). The more light available, the less time required to form the image and vice versa. Achieving the correct exposure of an image requires a consistent amount of light. Therefore, a larger aperture, allowing more light to pass, requires a faster shutter speed to prevent over exposure. In practical terms, a larger aperture will support a faster shutter speed if, for example, your subject is moving and you want to prevent blurring. Lenses with large maximum apertures are called 'fast' lenses for this reason. A lens with a maximum aperture setting of f/4 lens is said to be 'twice as fast' as one of f/5.6 maximum (one step on the above scale = twice as much light).
The second effect of aperture on your image is in the depth of field created. Depth of field is a reference to the range of distances within an image that are acceptably in focus (see Tech Tip: Depth of field). Because of the optical properties of lenses used in cameras, the wider the aperture, the narrower the depth of field and vice versa. Lenses with large maximum apertures (f/1.4, for example) will have a narrow depth of field.
My lens (or compact camera) has an aperture range from f/2.8 - f/4.6 - what does that mean?
Zoom lenses (those with adjustable focal lengths) and compact cameras with optical zoom will have a range of aperture settings quoted. When using the lens, the aperture range available will vary depending on the focal length or amount of zoom used, with the largest aperture normally only available at the widest angle or shortest focal length (no zoom). For the reasons explained above, aperture range may be a consideration when purchasing a compact camera and should always be a consideration when purchasing an SLR lens. As a rough guide, larger apertures (smaller f-numbers) normally mean better quality lenses.
