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# F-number

In photography the f-number (focal ratio) 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 one-sixteenth of the focal length (or the focal length is 16 times the aperture).

The greater the f-number, the less light per unit area reaches the focal plane of the camera film.

In the case of a telescope instead of a camera, one may use a photovoltaic detector or other photometer device, a charge-coupled device (CCD), or the viewer's own eye as a substitute for film. In all cases, the principle is the same: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).

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 √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/0.7, 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, f/90, f/128, etc. 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.

On modern cameras, especially when aperture is set on the camera body, f-stops are often divided more finely, resulting in half stops or third stops. The latter system is more common, since it matches the ISO system of film speeds. For example, the aperture that is one third stop smaller than f/2.8 is f/3.2, two thirds smaller is f/3.5, and one whole stop smaller is f/4. The next few f-stops in this sequence are f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, etc. In practice the maximum aperture of a lens may not be an integral power of √2, in which case it is usually a half or third stop above or below an integral power of √2.

Depth of field increases with f-stop; for an example of this relationship, visit the depth of field article.

Picture quality also also varies with f-stop. The optimal f-stop vary with the lens characteristics. 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. Light falloff is also sensitive to f-stop. Many wide angle lenses will show a significant light falloff at the edges at large apertures.

Photojournalists have a saying, "f/8 and be there." 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 100 film, an aperture of f/16 and a shutter speed of 1/125th of a second. This is called the "sunny f/16 rule".

03-10-2013 05:06:04
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