Tutorial: Lenses

Lens aperture

The amount of light reaching the sensor of your EOS depends partly on the brightness of the scene, but is also controlled by the lens aperture.

The aperture is a hole in the centre of a lens diaphragm positioned inside or behind the lens. The diaphragm is usually made up of a series of metal blades that move to alter the size of the hole.

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An aperture is the hole in a lens diaphragm. A series of interlocking metal blades move to change the size of the aperture.

Depth of field
The size of the aperture is given as an 'f-number'. This is the focal length of the lens divided by the diameter of the aperture. Large apertures have small f-numbers, such as f/2.0 or f/2.8. Small apertures have large f-numbers, such as f/16 or f/22.

The maximum aperture of a lens is particularly important when you use the viewfinder of your EOS. The image seen through the viewfinder is usually created at the maximum aperture - the larger the aperture, the brighter the image.

In addition to controlling the brightness of the light reaching the sensor, the aperture affects the depth-of-field in the image. Depth-of-field is the area of the scene that appears sharp. A large lens aperture gives a narrow depth-of-field - only a small area in front of and behind the focused distance appears sharp (f/1.8 in the animation below).


Shot on Canon EOS 5D Mark III, EF50mm f/1.8

Decreasing the aperture down (increasing the f-number)increases the depth-of-field - much more of the scene in front of and behind the focused distance appears sharp (f/22 in the animation above).

Changing the aperture offers a great deal of creative control. For example, the background in a portrait can be thrown out-of-focus by using a wide aperture (smaller f-number), while everything in a landscape can be made to appear in focus by setting a small aperture. Depth-of-field is particularly important with macro photos; the smaller the focusing distance, the shorter the depth-of-field.

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