photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...

and

cinematography Cinematography () is the art of motion picture (and more recently, electronic video camera) photography. Cinematographers use a lens (optics), lens to focus reflected light from objects into a real image that is transferred to some image sen ...

, a normal lens is a

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

that reproduces a

field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...

that appears "natural" to a human observer. In contrast, depth compression and expansion with shorter or longer

focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...

s introduces noticeable, and sometimes disturbing, distortion.

Mimicking human vision

Photographic technology employs different physical methods from the human eye in order to capture images. Thus, manufacturing optics which produce images that appear natural to human vision is problematic. The eye has a nominal focal length of approximately 17 mm, but it varies with accommodation. The nature of human binocular

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

, which uses two lenses instead of a single one, and post-processing by the cortex is very different from the process of making and rendering a photograph, video or film, and then looking at those. The structure of the

human eye The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance. The eye can be considered as a living ...

has a concave

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

, rather than a flat sensor. This produces effects observed by

Abraham Bosse Abraham Bosse ( – 14 February 1676) was a French artist, mainly as a printmaker in etching, but also in watercolor painting, watercolour.barrel distortion, and whether they should be depicted as straight in the picture plane. Hermann von Helmholtz's pin-cushioned chessboard figure demonstrates that straight lines in the world are not always perceived as straight and, conversely, that curved lines in the world can sometimes be seen as straight. Furthermore, the retina has variable sensitivity across its wider-than-180° horizontal field of view, as well as differences in resolution between

peripheral A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...

and foveal vision. Given these differences between human vision and camera lenses, explanations in photography texts to account for this discrepancy nevertheless tend to gloss over or merely restate the problem in terms of the observed phenomena, or claim that using 50 mm lenses "approximates the angle of view and magnification of human vision", or that "the normal focal length for a given format most closely approximates human sight, and projects an image with the least distortion and compression of space from foreground to background", or that "the perspective is correct and we are most comfortable with a picture captured with a 50-mm lens".

What is 'normal'?

A test of what is a normal lens then, is to find one that renders a printed (or otherwise displayed) photograph of a scene that when held at 'normal' viewing distance (usually arm's length) in front of the original scene and viewed with one eye, matches the real-world and the rendered perspective, though Maurice Pirenne (in 1970) and others demonstrate that it is possible to see a scene made with any lens in normal perspective if one adjusts the viewing distance; but that range becomes impractically close for wide angle photographs or too lengthy for a telephoto, and it requires viewing it from a static point at the centre of perspective from which the image was made, supporting the observations of perspective paintings made by

Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 1452 - 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested o ...

The image circle

A normal lens typically has an angle of view that is close to one

radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at ...

(~57.296˚) of the optical system's image circle. For 135 format (24 x 36 mm), with an escribed image circle diameter equal to the diagonal of the frame (43.266 mm), the focal length that has an angle of one radian of the inscribed circle is 39.6 mm; the focal length that has an angle of one radian of the horizontally-bound

inscribed An inscribed triangle of a circle In geometry, an inscribed planar shape or solid is one that is enclosed by and "fits snugly" inside another geometric shape or solid. To say that "figure F is inscribed in figure G" means precisely the same th ...

image circle, is 33 mm; the focal length that has an angle of one radian of the vertically-bound inscribed circle, is 22 mm. This correlates with the popularity of 35 and 24 mm lenses, and the existence of 40 mm lenses, albeit the latter in a more restrained offer. A 50 mm lens has a vertical-bound inscribed circle angle of view of ~0.5 radians. A 70 mm focal length has a horizontally-bound inscribed circle angle of view of ~0.5 radians. An 85 mm lens has an enscribed (frame diagonal) circle angle of view of ~0.5 radians. Effectively, the 24, 35 and 40 mm trio have a 1:2 relation to the 50, 70 and 85 trio of focal lengths. "Normal" lenses, those that cover one radian in at least one of their inscribed or escribed image circles, belong to the first group, with 35 and 40 mm lenses closer to one radian than 50 mm lenses.

Perspective effects of short or long focal-length lenses

Lenses with longer or shorter

s produce an expanded or contracted field of view that appears to distort the perspective when viewed from a normal viewing distance. Lenses of shorter focal length are called ''

wide-angle lens In photography and cinematography, a wide-angle lens is a Photographic lens, lens covering a large angle of view. Conversely, its focal length is substantially smaller than that of a normal lens for a given film plane. This type of lens allows mo ...

es'', while longer-focal-length lenses are referred to as

long-focus lens In photography, a long-focus lens is a camera lens which has a focal length that is longer than the diagonal measure of the film or sensor that receives its image. It is used to make distant objects appear magnified with magnification increasin ...

es (with the most common of that type being the ''

telephoto lens A telephoto lens, also known as telelens, is a specific type of a long-focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens ...

es''). Superimposing a wide-angle image print against the original scene would require holding it closer to the eye, while the telephoto image would need to be placed well into the depth of the photographed scene, or a tiny print to be held at arm's length, to match their perspectives. Such is the extent of distortions of perspective with these lenses that they may not be permitted as legal evidence. The ICP ''Encyclopaedia of Photography'' notes that for legal purposes:

"Judges will not admit a picture that seems to have been tampered with or that distorts any aspect of the scene r does not render a normal perspective..That is, the size relationships of objects in the photograph should be equivalent to what they actually are."

'Normal' lenses vary for different formats

For still photography, a lens with a focal length about equal to the diagonal size of the

film A film, also known as a movie or motion picture, is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, emotions, or atmosphere through the use of moving images that are generally, sinc ...

sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

format is considered to be a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal; this angle of view is about 53° diagonally. For cinematography, where the image is larger relative to viewing distance, a wider lens with a focal length of roughly a quarter of the film or sensor diagonal is considered 'normal'. The term normal lens can also be used as a synonym for

rectilinear lens In photography, a rectilinear lens is a photographic lens that yields images where straight features, such as the edges of walls of buildings, appear with straight lines, as opposed to being curved. In other words, it is a lens with little or n ...

. This is a completely different use of the term.

Typical normal focal lengths for different formats

Film still

Typical normal lenses for various

film format A film format is a technical definition of a set of standard characteristics regarding image capture on photographic film for still images or film stock for filmmaking. It can also apply to projected film, either slides or movies. The primary ...

s for photography are: For a 35 mm

camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...

with a diagonal of 43 mm, the most commonly used ''normal'' lens is 50 mm, but focal lengths between about 40 and 58 mm are also considered ''normal''. The 50 mm focal length was chosen by

Oskar Barnack Oskar Barnack (Nuthe-Urstromtal, Brandenburg, 1 November 1879 – Bad Nauheim, Hesse, 16 January 1936) was a German inventor and photographer who built, in 1913, what would later become the first commercially successful 35mm still-camera, sub ...

, the creator of the Leica camera. Note that the angle of view also depends on the

aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...

. For example, a "normal" lens on 35 mm does not have the same view as a "normal" lens on 645.

Digital still

digital photography Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image is ...

, many smaller sensor sizes are specified in terms such as 1" or 2/3". These measurements do not correspond directly to dimensions of the sensor, but rather to dimensions of an equivalent video camera tube (VCT). The normal lens focal length is roughly 2/3 of the video camera tube diameter.

Cinema

, a

roughly equivalent to twice the diagonal of the image projected within the

is considered normal, since movies are typically viewed from a distance of about twice the screen diagonal.Anton Wilson, ''Anton Wilson's Cinema Workshop'', American Cinematographer, 2004 (Page 100
online

References

{{reflist Photographic lenses Perspective projection Imaging