In 1867, Sutton and Dawson defined the “Apertal Ratio” as the ratio of the stop diameter to the focal length (i.e., 1/N), but used it only as an indicator of depth of field. The origins of the concept of the F-number date back to the mid-19th century.
#F stop vs aperture series#
More modern lenses are marked with a slightly different series that has become the international standard, with the numbers rounded off to make them easier to engrave and remember:į/1, f/1.4, f/2, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128, etc. This system became accepted in Europe, and many vintage European lenses are marked according this system. The numbering of the stops is therefore arranged in a geometric sequence based on powers of the square root of 2. For exposure to halve or double between stops, the f number must change by the square root of 2, or 1.414. Thus an exposure of 1/100 second at f/5.6 will be exactly the same no matter which lens or film size one uses. The beauty of the f-stop system is that, at a given F-number, every lens will allow exactly the same amount of light per square centimeter to fall on the film. This gives one some insight into why fast telephoto lenses are large, bulky, and expensive- a modest 200 mm telephoto lens with a maximum aperture of f/2 has an opening of 100 mm, or about 4 inches! Consequently, our 50 mm lens with a 25 mm opening is a nice fast f/2 lens, while our 200 mm telephoto lens with a 25 mm opening is frustratingly slow at f/8.
To resolve this dilemma, the lens aperture is expressed as the “ f-number” (usually written as N), a dimensionless parameter expressed as ratio between the focal length of the lens ( f) and the diameter of the pupil in the iris diaphragm ( D):Īlthough expressed as a ratio (e.g., 1:4.5) in Europe for many years, in English-speaking countries, the diameter of the aperture is usually expressed as a fraction of the focal length, e.g., f/4.5, or 1/N. In other words, if the iris diaphragm opening is 25 mm, it will expose the film only one-sixteenth as much on a 200 mm telephoto lens as it will on a 50 mm lens. Rather, the amount of light admitted by a given opening depends on the focal length of the lens. Having the same size opening does not necessarily mean that the same amount of light falls on the film. Canon 35mm Camera with Unusual f/0.95 Lens
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