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The f in f-stop is the focal length of the lens. The focal length of the lens divided by the diameter of the aperture is the f-stop. It is also called an f-number. When it is written f/8, this means the aperture diameter is 1/8th the focal length. f/8 on a 45mm lens means that the diameter of the aperture is 45/8, or 5.6mm.

These ratios allow us to use the same number series for all lenses. f/8 on a 45mm lens lets in exactly the same amount of light as f/8 on a 100mm lens. However it is important to note that the diameter of the aperture of these two lenses at these settings is very different, therefore aperture and f-stop are not the same thing.

The amount of light let through the aperture

does not depend on the diameter but the area of the aperture.

**Typical f-stop scale:**

__2 2.8 4 5.6 8 11 16 22__

Although these numbers may seem irregular, each of the numbers in this sequence (each stop) represents a halving to the right, or doubling to the left, of the aperture area and therefore the amount of light from its immediate neighbour.

**NB.** The smaller the number the bigger the aperture and vice versa.