Aperture

The term aperture is used for the size of the diaphragm or ‘hole’ in the lens. Aperture control is achieved by manipulating the diaphragm in the lens to open and let in more light or to close and reduce the amount of light reaching the film. Therefore the aperture can be used to control the brightness of an image and exposure.

Aperture is measured in ‘f’ stops and the range in full stops is as follows:

2.8  -  4  -  5.6  -  8  -  11  -  16  -  22  -   32

When the aperture is at ‘f’ stop 2.8, the diaphragm in the lens is fully open. Conversely, if the ‘f’ stop is 32, the diaphragm in the lens is at its smallest setting. ‘F’ stops can be considered to be factors by which the diaphragm is adjusted and each value represents an amount of light that is allowed to pass through the lens. Therefore when the aperture is reduced from ‘f’ stop 2.8 to ‘f’ stop 4, the amount of light passing through the lens is halved.

In addition to the control of the image brightness, the main use of aperture is the control of the depth of field. By definition the depth of field is the amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph. Therefore by altering the aperture one can increase the range of objects at different distances from the camera that will appear sharp at one focus setting.

The increase in depth of field is illustrated in the sequence of photos of the chess pieces on the following page where the King piece is the only item that really is in sharp focus for the ‘f’ stop 5.6. But comparatively, almost the entire row of chess pieces is in sharp focus when the ‘f’ stop is 32. Hence by ‘stopping down’ the lens, the depth of field has been increased from ‘f’ stop 5.6 to 32. This indicates that the smaller the aperture the greater the depth of field in the image.