Editing Normal lens (section)

== 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&nbsp;mm,<ref>Pocock, Gillian, Richards, Christopher D., and Richards, Dave A. (2013). ''Human physiology'' (4th ed). Oxford University Press, Oxford p214</ref> but it varies with [[Accommodation (eye)|accommodation]]. The nature of human binocular [[Visual perception|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, and then looking at those.

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'', demonstrated how the circular projection of the visual cone conflicts with the flat plane of the picture surface,<ref>{{Citation|author1=Abraham Bosse|title=Traité des pratiques geometrales et perspectives enseignées dans l'Academie royale de la peinture et sculpture|url=https://trove.nla.gov.au/work/35298327|date=1665|publisher=A Paris Chez l'auteur|access-date=28 July 2018|author2=Cellier, Antoine, 17th cent|author3=Académie royale de peinture et de sculpture (France)}}</ref> prompting continued debate over whether straight lines in the world are perceived as straight or curved in a form of [[Distortion (optics)|barrel distortion]], and whether they should be depicted as straight in the picture plane.<ref>Tyler, C.W. (2009) Editorial Essay: Straightness and the sphere of vision. ''Perception'' 38, pp. 1423–1427</ref>

[[Hermann von Helmholtz|Hermann von Helmholtz's]] [[Distortion (optics)|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.<ref>Rogers, B., & Brecher, K. (2007). Straight Lines, 'Uncurved Lines', and Helmholtz's 'Great Circles on the Celestial Sphere'. ''Perception'', 36(9), 1275-1289.</ref> Furthermore, the retina has variable sensitivity across its wider-than-180° horizontal field of view, as well as differences in resolution between [[Peripheral vision|peripheral]] and [[foveal]] vision.<ref>Cooper, E., Piazza, E., and Banks, M. (2012). The perceptual basis of common photographic practice. ''Journal of Vision'', 12(5), 8.</ref>

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&nbsp;mm lenses "approximates the angle of view and magnification of human vision",<ref>Modrak, R., and Anthes, B. (2011) ''Reframing photography'', New York:Routledge</ref> 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",<ref>Belt, A. F. (2008) ''The elements of photography: Understanding and creating sophisticated images''. 2nd ed Burlington, MA:Focal Press</ref> or that "the perspective is correct and we are most comfortable with a picture captured with a 50-mm lens".<ref>Current, I. (1990) Best viewing distance for photographers. ''PSA Journal'', Sept, 16</ref>