Detail of a camera showing f stop scale. 
In photography the fnumber expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. For example, f/16 represents a diaphragm aperture diameter that is onesixteenth of the focal length.
The higher the fnumber, the less light is admitted through the lens.
f stops are a way of representing a convenient sequence of fnumbers in a geometric progression. Each 'stop' is marked with its corresponding fnumber, and represents a halving of the light intensity from the one before, corresponding to a decrease of the diaphragm aperture diameter by a factor of √2, and hence an halving of the area of the aperture.
Modern lenses use a standard f stops scale that corresponds to the sequence of the powers of √2 : 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 and f/64. (Note that the values of the ratios are rounded off, to make them easy to write down).
Shutter speeds are arranged in a similar scale, so that one step in the shutter speed scale corresponds to one step in the f stop scale.
Photographers sometimes express exposure ratios in terms of 'stops'. If we ignore the fnumber markings, the fstops make a logarithmic scale of exposure intensity. Given this interpretation, you can then think of taking a halfstep along this scale, to make an exposure difference of "half a stop".
Since all lenses absorb some portion of the light passing through them (particularly zoom lenses[?] containing many elements), for exposure purposes a Tstop is sometimes used instead of fstop. The Tnumbers are adjusted so that the amount of light transmitted through the lens at a given Tstop is equal to that going through an ideal nonabsorbing lens set at that fstop.
In practice the maximal aperture of a lens often differs from a power of √2, and is not one of the standard fstops. For example, the sequence of Fstops on the lens depicted in the picture above has fnumbers of f/3.5, f/5.6, f/8, f/11, f/16 and f/22.
Depth of field increases with fstop; for an example of this relationship, visit the depth of field article.
Picture sharpness[?] also varies with fstop. The optimal fstop vary with the lens characteristics. For example, on modern standard lenses having 6 or 7 elements the sharpest image is obtained around f/5.6f/8, while for older standard lenses having only 4 elements (Tessar formula) stopping to f/11 will give the sharpest image. The reason the sharpness is best at medium fnumbers is that the sharpness at high fnumber is constrained by diffraction, whereas at low fnumbers lens faults known as aberrations will dominate.
As an example of the use of fnumbers, an approximately correct exposure will be obtained on a sunny day using ISO 125 film, an aperture of f/16 and a shutter speed of 1/125th of a second. This is called the "sunny f/16 rule".
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