F-number

Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one. The actual size of the aperture will depend on the focal length of the lens.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture^[1]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.

Notation

The f-number (f/#) is often notated as and is given by

where is the focal length, and is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or . The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.

The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

Diagram showing why two lenses, with different focal lengths but the same aperture setting, will produce the same illuminance in the focal plane.

A 100 mm lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).

Stops, f-stop conventions, and exposure

A Canon 7 mounted with a 50 mm lens capable of an exceptional f/0.95

A 35 mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring. This lens has an aperture range of f/2.0 to f/22

The term stop is sometimes confusing due to its multiple meanings. A stop can be a physical object: an opaque part of an optical system that blocks certain rays. The aperture stop is the aperture that limits the brightness of the image by restricting the input pupil size, while a field stop is a stop intended to cut out light that would be outside the desired field of view and might cause flare or other problems if not stopped.

In photography, stops are also a unit used to quantify ratios of light or exposure, with one stop meaning a factor of two, or one-half. The one-stop unit is also known as the EV (exposure value) unit. On a camera, the f-number is usually adjusted in discrete steps, known as f-stops. Each "stop" is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. This corresponds to a decrease of the pupil and aperture diameters by a factor of or about 1.414, and hence a halving of the area of the pupil.

Modern lenses use a standard f-stop scale, which is an approximately geometric sequence of numbers that corresponds to the sequence of the powers of the square root 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, f/64, f/90, f/128, etc. The values of the ratios are rounded off to these particular conventional numbers, to make them easier to remember and write down. The sequence above can be obtained as following: f/1 = , f/1.4 = ,f/2 = , f/2.8 = ...

Shutter speeds are arranged in a similar scale, so that one step in the shutter speed scale corresponds to one stop in the aperture 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 at this larger aperture, as at the previous aperture, the shutter speed is set twice as fast (i.e., the shutter is open half as long); the film will usually respond equally to these equal amounts of light, since it has the property known as reciprocity. Alternatively, one could use a film that is half as sensitive to light, with the original shutter speed.

Photographers sometimes express other exposure ratios in terms of 'stops'. Ignoring the f-number markings, the f-stops make a logarithmic scale of exposure intensity. Given this interpretation, one can then think of taking a half-step along this scale, to make an exposure difference of "half a stop".

Fractional stops

Most old cameras had an aperture scale graduated in full stops but the aperture is continuously variable allowing to select any intermediate aperture.

Click-stopped aperture became a common feature in the 1960s; the aperture scale was usually marked in full stops, but many lenses had a click between two marks, allowing a gradation of one half of a stop.

On modern cameras, especially when aperture is set on the camera body, f-number is often divided more finely than steps of one stop. Steps of one-third stop (1/3 EV) are the most common, since this matches the ISO system of film speeds. Half-stop steps are also seen on some cameras. As an 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.

To calculate the steps in a full stop (1 EV) one could use

2^0×0.5, 2^1×0.5, 2^2×0.5, 2^3×0.5, 2^4×0.5 etc.

The steps in a half stop (1/2 EV) series would be

2^0/2×0.5, 2^1/2×0.5, 2^2/2×0.5, 2^3/2×0.5, 2^4/2×0.5 etc.

The steps in a third stop (1/3 EV) series would be

2^0/3×0.5, 2^1/3×0.5, 2^2/3×0.5, 2^3/3×0.5, 2^4/3×0.5 etc.

As in the earlier DIN and ASA film-speed standards, the ISO speed is defined only in one-third stop increments, and shutter speeds of digital cameras are commonly on the same scale in reciprocal seconds. A portion of the ISO range is the sequence

... 16/13°, 20/14°, 25/15°, 32/16°, 40/17°, 50/18°, 64/19°, 80/20°, 100/21°, 125/22°...

while shutter speeds in reciprocal seconds have a few conventional differences in their numbers (1/15, 1/30, and 1/60 second instead of 1/16, 1/32, and 1/64).

In practice the maximum aperture of a lens is often not an integral power of (i.e. to the power of a whole number), in which case it is usually a half or third stop above or below an integral power of .

Modern electronically-controlled interchangeable lenses, such as those from Canon and Sigma for SLR cameras, have f-stops specified internally in 1/8-stop increments, so the cameras' 1/3-stop settings are approximated by the nearest 1/8-stop setting in the lens.

Standard full-stop f-number scale

Including aperture value AV:

Typical one-half-stop f-number scale

Typical one-third-stop f-number scale

Typical one-quarter-stop f-number scale^[2]

Notice that sometimes a number shows on several scales; for example, f/1.2 may be used in either a half-stop[2] or a one-third-stop system[3]; sometimes f/1.3 and f/3.2 and other differences are used for the one-third stop scale[4].

T-stops

Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly (The T in T-stop stands for transmission),^[3] but modern cinematographic lenses now usually tend to be factory-calibrated in T-stops. T-stops measure the amount of light transmitted through the lens in practice (actually on T-stops the amount of light is measured at the film plane), and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number. In recent years, advances in lens technology and film exposure latitude have reduced the importance of t-stop values. So, F-stops are for focal ratio, T-stops are for transmission.

Sunny 16 rule

An example of the use of f-numbers in photography is the sunny 16 rule: an approximately correct exposure will be obtained on a sunny day by using an aperture of f/16 and a shutter speed close to the reciprocal of the ISO speed of the film; for example, using ISO 200 film, an aperture of f/16 and a shutter speed of 1/200 second. The f-number may then be adjusted downwards for situations with lower light.

Effects on image quality

Comparison of f/32 (top-left corner) and f/5 (bottom-right corner)

Depth of field increases with f-number, as illustrated in the image here. This means that photographs taken with a low f-number will tend to have one subject in focus, with the rest of the image out of focus. This is frequently useful for nature photography, portraiture, and certain special effects. The depth of field of an image produced at a given f-number is dependent on other parameters as well, including the focal length, the subject distance, and the format of the film or sensor used to capture the image. Smaller formats will have a deeper field than larger formats at the same f-number for the same distance of focus and same angle of view. Therefore, reduced–depth-of-field effects, like those shown below, will require smaller f-numbers (and thus larger apertures and so potentially more complex optics) when using small-format cameras than when using larger-format cameras.

Picture sharpness also varies with f-number. The optimal f-stop varies with the lens characteristics. For modern standard lenses having 6 or 7 elements, the sharpest image is often 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-numbers is constrained by diffraction,^[4] whereas at low f-numbers limitations of the lens design known as aberrations will dominate. The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-stops. Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff (vignetting) at the edges for large apertures. To measure the actual resolution of the lens at the different f-numbers it is necessary to use a standardized measurement chart like the 1951 USAF resolution test chart.

Photojournalists have a saying, "f/8 and be there", meaning that being on the scene is more important than worrying about technical details. The aperture of f/8 gives adequate depth of field, assuming a 35 mm or DSLR camera, minimum shutter-speed, and ISO film rating within reasonable limits subject to lighting.^[5]

Human eye

Computing the f-number of the human eye involves computing the physical aperture and focal length of the eye. The pupil can be as large as 6-7mm wide open, which translates into the maximum physical aperture.

The f-number of the human eye varies from about f/8.3 in a very brightly lit place to about f/2.1 in the dark.^[6] The presented maximum f-number has been questioned,^[7] as it seems to only match the focal length that assumes outgoing light rays. According to the incoming rays of light (what we actually see), the focal length of the eye is a bit longer, resulting in maximum f-number of f/3.2.

Note that computing the focal length requires that the light-refracting properties of the liquids in the eye are taken into account. Treating the eye as an ordinary air-filled camera lens may result in a different focal length, thus yielding an incorrect f-number.

Toxic substances and poisons (like Atropine) can significantly reduce the range of aperture. Pharmaceutical products such as eye drops may also cause similar side-effects.

Focal ratio in telescopes

Diagram of the focal ratio of a simple optical system where is the focal length and is the diameter of the objective

In astronomy, the f-number is commonly referred to as the focal ratio (or f-ratio) notated as . It is still defined as the focal length of an objective divided by its diameter or by the diameter of an aperture stop in the system.

For example if you want to make a 12", f/8 telescope, then the focal length will be 96", which means that light from distant objects must focus 96" behind the lens or 96" in front of the concave mirror.

Even though the principles of focal ratio are always the same, the application to which the principle is put can differ. In photography the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as depth of field. When using an optical telescope in astronomy, there is no depth of field issue, and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of absolute aperture area only, independent of focal length. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD.

For example, the SOAR 4m telescope has a small field of view (~f/16) which is useful for stellar studies, whereas the LSST 8.4m telescope, which will cover the entire sky every 3 days has a very large field of view (f/1.2), due to a special optical design.

Working f-number

The f-number accurately describes the light-gathering ability of a lens only for objects an infinite distance away.^[8] This limitation is typically ignored in photography, where objects are usually not extremely close to the camera, relative to the distance between the lens and the film. In optical design, an alternative is often needed for systems where the object is not far from the lens. In these cases the working f-number is used. The working f-number N_w is given by

,

where N is the uncorrected f-number, "NA" is the numerical aperture of the lens, and is the lens's magnification for an object a particular distance away.^[8] (Note that the magnification m here is negative for the common case where the image is inverted.) In photography, the working f-number is described as the f-number corrected for lens extensions by a "bellows factor". This is of particular importance in macro photography.

History

The system of f-numbers for specifying relative apertures evolved in the late nineteenth century, in competition with several other systems of aperture notation.

Origins of relative aperture

In 1867, Sutton and Dawson defined "apertal ratio" as essentially the reciprocal of the modern f-number:^[9]

In every lens there is, corresponding to a given apertal ratio (that is, the ratio of the diameter of the stop to the focal length), a certain distance of a near object from it, between which and infinity all objects are in equally good focus. For instance, in a single view lens of 6 inch focus, with a 1/4 in. stop (apertal ratio one-twenty-fourth), all objects situated at distances lying between 20 feet from the lens and an infinite distance from it (a fixed star, for instance) are in equally good focus. Twenty feet is therefore called the 'focal range' of the lens when this stop is used. The focal range is consequently the distance of the nearest object, which will be in good focus when the ground glass is adjusted for an extremely distant object. In the same lens, the focal range will depend upon the size of the diaphragm used, while in different lenses having the same apertal ratio the focal ranges will be greater as the focal length of the lens is increased. The terms 'apertal ratio' and 'focal range' have not come into general use, but it is very desirable that they should, in order to prevent ambiguity and circumlocution when treating of the properties of photographic lenses.

In 1874, John Henry Dallmeyer called the ratio the "intensity ratio" of a lens:^[10]

The rapidity of a lens depends upon the relation or ratio of the aperture to the equivalent focus. To ascertain this, divide the equivalent focus by the diameter of the actual working aperture of the lens in question; and note down the quotient as the denominator with 1, or unity, for the numerator. Thus to find the ratio of a lens of 2 inches diameter and 6 inches focus, divide the focus by the aperture, or 6 divided by 2 equals 3; i.e., 1/3 is the intensity ratio.

Although he did not yet have access to Ernst Abbe's theory of stops and pupils,^[11] which was made widely available by Siegfried Czapski in 1893,^[12] Dallmeyer knew that his working aperture was not the same as the physical diameter of the aperture stop:^[10]

It must be observed, however, that in order to find the real intensity ratio, the diameter of the actual working aperture must be ascertained. This is easily accomplished in the case of single lenses, or for double combination lenses used with the full opening, these merely requiring the application of a pair of compasses or rule; but when double or triple-combination lenses are used, with stops inserted between the combinations, it is somewhat more troublesome; for it is obvious that in this case the diameter of the stop employed is not the measure of the actual pencil of light transmitted by the front combination. To ascertain this, focus for a distant object, remove the focusing screen and replace it by the collodion slide, having previously inserted a piece of cardboard in place of the prepared plate. Make a small round hole in the centre of the cardboard with a piercer, and now remove to a darkened room; apply a candle close to the hole, and observe the illuminated patch visible upon the front combination; the diameter of this circle, carefully measured, is the actual working aperture of the lens in question for the particular stop employed.

This point is further emphasized by Czapski in 1893.^[12] According to an English review of his book, in 1894, "The necessity of clearly distinguishing between effective aperture and diameter of physical stop is strongly insisted upon."^[13]

J. H. Dallmeyer's son, Thomas Rudolphus Dallmeyer, inventor of the telephoto lens, followed the intensity ratio terminology in 1899.^[14]

Aperture numbering systems

At the same time, there were a number of aperture numbering systems designed with the goal of making exposure times vary in direct or inverse proportion with the aperture, rather than with the square of the f-number or inverse square of the apertal ratio or intensity ratio. But these systems all involved some arbitrary constant, as opposed to the simple ratio of focal length and diameter.

For example, the Uniform System (U.S.) of apertures was adopted as a standard by the Photographic Society of Great Britain in the 1880s. Bothamley in 1891 said "The stops of all the best makers are now arranged according to this system." ^[15] U.S. 16 is the same aperture as f/16, but apertures that are larger or smaller by a full stop use doubling or halving of the U.S. number, for example f/11 is U.S. 8 and f/8 is U.S. 4. The exposure time required is directly proportional to the U.S. number. Eastman Kodak used U.S. stops on many of their cameras at least in the 1920s.

By 1895, Hodges contradicts Bothamley, saying that the f-number system has taken over: "This is called the f/x system, and the diaphragms of all modern lenses of good construction are so marked." ^[16]

Here is the situation as seen in 1899:

Piper in 1901^[17] discusses five different systems of aperture marking: the old and new Zeiss systems based on actual intensity (proportional to reciprocal square of the f-number); and the U.S., C.I., and Dallmeyer systems based on exposure (proportional to square of the f-number). He calls the f-number the "ratio number," "aperture ratio number," and "ratio aperture." He calls expressions like f/8 the "fractional diameter" of the aperture, even though it is literally equal to the "absolute diameter" which he distinguishes as a different term. He also sometimes uses expressions like "an aperture of f 8" without the division indicated by the slash.

Beck and Andrews in 1902 talk about the Royal Photographic Society standard of f/4, f/5.6, f/8, f/11.3, etc.^[18] The R.P.S. had changed their name and moved off of the U.S. system some time between 1895 and 1902.

Typographical standardization

By 1920, the term f-number appeared in books both as F number and f/number. In modern publications, the forms f-number and f number are more common, though the earlier forms, as well as F-number are still found in a few books; not uncommonly, the initial lower-case f in f-number or f/number is set in a hooked italic form: f, or f.^[19] Notations for f-numbers were also quite variable in the early part of the twentieth century. They were sometimes written with a capital F,^[20] sometimes with a dot (period) instead of a slash,^[21] and sometimes set as a vertical fraction.^[22]

The 1961 ASA standard PH2.12-1961 American Standard General-Purpose Photographic Exposure Meters (Photoelectric Type) specifies that "The symbol for relative apertures shall be f/ or f : followed by the effective f-number." Note that they show the hooked italic f not only in the symbol, but also in the term f-number, which today is more commonly set in an ordinary non-italic face.

See also

• Circle of confusion
• Depth of field
• Group f/64
• Exposure value
• Optical telescope
• Photographic lens design
• Pinhole camera
• Printer points
• Telescope

References

External links

• f Number Arithmetic
• Large format photography—how to select the f-stop