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Concept
Normal lens
Summary
In photography and cinematography, a normal lens is a lens that reproduces a field of view that appears "natural" to a human observer. In contrast, depth compression and expansion with shorter or longer focal lengths introduces noticeable, and sometimes disturbing, distortion.
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 17mm, but it varies with accommodation. The nature of human binocular vision, 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.
The structure of the human eye has a concave retina, rather than a flat sensor. This produces effects observed by Abraham Bosse, who in his 1665 illustration 'To prove that one can neither define nor paint as the eye sees', demonstrates how the circular projection of the visual cone conflicts with the flat plane of the picture surface, prompting continued debate over whether straight lines in the world are perceived as straight or curved in a form of barrel distortion, and whether they should be depicted as straight in the picture plane.
Helmholtz's (1910) 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.
The retina also has variable sensitivity across its wider-than-180° horizontal field-of-view and ranging in resolution in peripheral or foveal vision.
Given the lack of a clear correlation between human vision and camera lenses, explanations in photography texts to account for this rule's efficacy tend to gloss over or merely restate the phenomenon, claiming that using 50mm 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".
Official source
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