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The higher the f-number, the less light is admitted through the lens.
f-stops are a way of representing a convenient sequence of f-numbers in a geometric progression. Each 'stop' is marked with its corresponding f-number, 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 , 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 : 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. Also note that the slash indicates division. For example, f/16 means that the aperture is equal to the focal length divided by sixteen; that is, if the camera has an 80 mm lens, the light that reaches the film arrives through an opening that is 5 mm (80 mm/16) in diameter.
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. Opening up a lens by one stop allows twice as much light to fall on the film in a given period of time, therefore to have the same exposure, you must have a shutter speed twice as fast (shutter open half as long). Alternatively, you could use a film which is half as sensitive to light. This fundamental principle of photographic technique is known as reciprocity.Photographers sometimes express exposure ratios in terms of 'stops'. If we ignore the f-number markings, the f-stops make a logarithmic scale of exposure intensity. Given this interpretation, you can then think of taking a half-step 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 t-stop is sometimes used instead of f-stop. The t-numbers are adjusted so that the amount of light transmitted through the lens at a given t-stop is equal to that going through an ideal non-absorbing lens set at that f-stop.
In practice the maximal aperture of a lens often differs from a power of , and is not one of the standard f-stops. For example, the sequence of f-stops on the lens depicted in the picture above has f-numbers of f/3.5, f/5.6, f/8, f/11, f/16 and f/22.
Depth of field increases with f-stop; for an example of this relationship, visit the depth of field article.
Picture sharpness also varies with f-stop. The optimal f-stop vary with the lens characteristics. For example, on modern standard lenses having 6 or 7 elements the sharpest image is obtained around f/5.6-f/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 f-numbers is that the sharpness at high f-number is constrained by diffraction, whereas at low f-numbers lens faults known as aberrations will dominate.
Photojournalists have a saying, "f/8 and be there." Many people interpret the expression differently, but one meaning is that f/8 will give a good picture, and being on the scene is more important than worrying excessively about technical details.
As an example of the use of f-numbers, 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".