Aperture f Number

The aperture measurement, the f number, is derived from a ratio. This ratio is the diameter of the hole which lets the light through divided by the focal length of the lens. For example the old lens I use as a magnifying glass has a 25mm diameter and 50mm focal length. Divide the 25 by 50 to get 1/2 (ie half). The f number is the lower part of the fraction, the denominator, f2. This is the maximum aperture of the lens. Each increase in f number represents half the amount of light entering the camera. So changing from f8 to f11 has the same reduction in light into the camera as changing from 1/250th to 1/500th of a second exposure. This also means that 1/500th at f8 lets in the same amount of light as 1/250th at f11 or 1/60th at f22. f22 is a typical minimum aperture. When the aperture gets smaller (bigger number) the effect of the glass lens is reduced and the lens starts to behave as a pinhole camera. A manual lens should have marks either side of the 'focus' mark with f numbers These indicate the manufacturer's suggested range of acceptable focus for the f number. This means the f4 marks will be closer to the 'focus' mark than the f16 marks. This is called 'depth of field'.

When a lens has a longer focal length the diameter of the lens has to be increased to have the same f number. For example a 200mm focal length (telephoto) lens needs a 100mm diameter to have a f2 maximum aperture.