APERTURE

Aperture in the photographic sense refers to the opening allowing light though a camera lens. A diaphragm often controls this with six or more blades. This gives the effective aperture . On a camera lens the aperture is often click stopped to give a precise position and value. In this way the light entering the camera is controlled giving exposure control. The depth of field is also controlled which alters the way the image looks. Modern cameras have lenses which have the same relative aperture values, This means that f4 on a 50mm lens allows the same amount of light in as f4 300mm lens.

f2 F2.8	f4 f5.6 f8 f11	f16 f 22
wide aperture		small aperture
more light		less light
small number		larger number

These numbers are referred to as f-stops or f numbers. The numbers above are full f-stops. Other numbers are possible f1, f32, f5.3 and so on but are generally rare for various reasons. Why these numbers? They can be used to calculate the area allowing light into the lens.

Fixed focus lens showing scales and parts. Zoom lens may not show all scales.		Electronic camera showing aperture and shutter speed. Values are dialled in using the command dials.

  Each number doubles or halves the amount of light into the lens with respect to the next.

F4 halves the amount of light from f2.8
F2.8 doubles the light f4

Depth of field

The aperture, magnification of final image and the circle of confusion control the depth of field. Depth of field influences the apparent sharpness of the unfocused areas of the image. The image is sharp at the point of focus. However for 1/3 in front of the point of focus and 2/3 behind a zone of sharpness exists, which may be increased by using a smaller aperture. This zone may be calculated. Depth of field is also influenced by distance, the nearer an object, the smaller the depth of field-the magnification is greater. Focal length of the lens appears to alter depth of field. Wide-angle lenses seem to have more depth of field. Again this is due to magnification of the image.

The wider the aperture (f2.8, f4.) the smaller the depth of field.
The smaller the aperture (f16, f22) the larger the depth of field.

Diagram showing the depth of field for two different apertures for the same point of focus