system appears similar to an f-stop scale, and is easily confused with the modern system: At first glance, the numbering for the U.S. System was adopted by the Royal Photographic Society of Britain in 1881, and was one of the first attempts to establish a standard for lens apertures. The second column summarizes the older “U.S.” or Universal System (also called the “Uniform Scale System”) found on many pre-1920 cameras. Even though the numbers are not neatly rounded off in the manner to which we are accustomed (2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, and 64) this column is recognizable as f-stops as we know them. This table does contain the seeds of our modern, streamlined system of f-stops in the first column. Modern day camera collectors need only concern themselves with three aperture systems: The modern f-stop method, the “U.S.” system, and a very simple numerical aperture scheme used by Kodak on their cheapest folding cameras.
Fortunately, the majority of these schemes are largely of historical interest and are rarely encountered. In 1899, a summary of the most common systems looked something like this:Ĭlearly, the situation with respect to lens aperture at the turn of the century was extraordinarily confusing. In 1874, John Henry Dallmeyer stated that “…the rapidity of a lens depends on the relation or ratio of the aperture to the equivalent focus…” However, it was to be many years before there was universal agreement on a mathematical scheme to formalize this principle.īefore today’s f-stop system became the standard, a variety of systems were in use to express lens aperture. 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 NUMBERS 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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