f Stops

What exactly are they?

A note for the technically minded


The f number on the lens barrel indicates the size of aperture (iris) relative to the focal length of the lens.

A number (N) is defined as the equivalent focal length (f) of the lens divided by the diameter (d) of the 'entrance pupil'. This can be expressed mathematically as N=f/d.

Thus a lens with an effective aperture 25mm in diameter and a focal length of 50mm has a relative aperture of 50 divided by 25 (50/25) i.e. 2. The numerical value of relative aperture is usually prefixed by the letter f so in this instance it would be f2.

Some lenses express the lens aperture as a ratio without the letter f. In this case an aperture of f2 be written as 1:2. The relative aperture of a lens is therefore commonly referred to as its 'aperture'.

All lenses adjusted (or 'stopped down') to the same aperture should transmit the same amount of light. Each change of number indicates a doubling or halving of the size of the aperture, with the exception of the maximum aperture in some cases.

To express this concept a little more technically, the amount of light passed by a lens in inversely proportional to the square of the f number, the numbers in the series are made to increase by a factor of root 2, that is approx. 1.4.

The current standard series of f numbers is:

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, etc.

The maximum aperture of a lens may and frequently does, fall between two of the standard f numbers. The lens is therefore marked with a non-standard f number, e.g. f1.9 .
When a lens opening (or aperture) is made smaller - and the f number value increases, this operation is called 'stopping down'. Making the opening (or aperture) wider with the f number value decreasing, is called 'opening up'.

The word 'stop' is an echo from the early days of camera and lens design, when a set size hole was inserted into the lens barrel to 'stop' (or reduce) a proportion of the light. The varying sizes of holes that could, in those early days, be selected along a brass strip, were known as 'Waterhouse Stops'. The system of drilled holes in a metal strip being devised by Mr. Waterhouse in 1858.

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