It is calculated by dividing the focal length of the lens by its effective aperture. Lenses will always be quoted with their maximum aperture (smallest F-number) which can be reduced by closing down a diaphragm inside the lens known as an iris. Changing the F-number of a lens will change the focusing qualities (depth of field) The smaller the iris aperture, the higher the F-number and the larger the depth of field.
The adjustable iris inside lenses normally uses standard increments including 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22. Each increment represents a reduction in the amount of light passing through the lens by 50%. The smaller the iris size, the greater the depth of field.
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