Aperture is the size of the opening that allows light to enter the lens. The wider this opening, the more light can get in and the brighter the image. This is analogous to the iris in a human eye.
Aperture is specified as a ratio or f-number. This is the ratio of the diameter of the opening to the focal length of the lens. For example on a 50mm lens, when the Aperture is f/2 (pronounced Eff-Two) the opening is 25mm.
No matter what the focal length of a lens, if the f-number is the same, the amount of light let in by the lens will be the same. So a 28mm lens at f/2 lets in the same amount of light as a 300mm lens at f/2, but on the 28mm lens the Aperture is only 14mm across, whereas on the 300mm lens it will be 150mm.
An increase in the f-number means a smaller aperture and therefore less light can enter the lens. f/4 on a 50mm lens would be an opening of only 12.5mm. When the f-number doubles, the amount of light entering the lens is quartered. So if an image can be exposed at f/4 correctly, it will need twice as much light to expose at f/5.6, and twice as much again to expose at f/8.
Effect on Image
As well as affecting the exposure of an image, the aperture affects the look of the image by changing the Depth of Field. Depth of Field is related to the actual size of the aperture – rather than the f-number. So a 100mm lens set to f/4 has an opening of 25mm. This will give you less depth of field than a 50mm lens at f/4 which would have an opening of 12.5mm. The larger the aperture (smaller the f-number) the less the amount of depth of field. So shooting with a lens ‘wide open’ (say, f/2.8) will give you much less depth of field than ‘stopping down’ to f/8 or more. Of course, doing so will affect your exposure and you will need to compensate by adjusting another exposure variable such as Shutter Speed.
