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Infobank

Lenses: All about apertures

Most photographic lenses contain an iris diaphragm. This is made up of a series of interlocking metal blades arranged to leave a hole, or aperture, at the centre. As the blades move, the size of the aperture changes.

This variable aperture gives you control of the film or sensor exposure − a large aperture lets more light pass through the lens than a small aperture.

Hinged metal blades move to alter the size of the lens aperture.

A scale of values is used to indicate the size of the aperture. But it is not simply a measure of the aperture diameter. Strictly speaking, what photographers refer to as an aperture should be called a relative aperture. It is actually a ratio of an aperture measurement to the focal length of the lens. So if the effective aperture is 12.5mm and the focal length is 50mm, the relative aperture is 50/12.5, which is 4. This is known as the f-number, and is usually written as f/4 or f4. Since it is a ratio, it can also be written as 1:4, and you will find this style used on Canon lenses.

Maximum aperture

Aperture numbers can be a little confusing because a large aperture is indicated by a small number. An aperture of f/4, for example, is larger than an aperture of f/8. Very small apertures have even larger numbers, such as f/16 or f/22.

A typical aperture number sequence for a lens is f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Each number is the result of multiplying the preceding number by the square root of 2. Moving from one value to the next smaller value (f/2.8 to f/4, for example) reduces the amount of light passing through the lens by half.