 | ||
In photography and cinematography, a normal lens is a lens that reproduces a field of view that appears "natural" to a human observer.
Contents
- Perspective effects of short or long focal length lenses
- Normal lenses vary for different formats
- Film still
- Digital still
- Cinema
- References
However, to find a photographic lens equivalent to the human eye, which has an effective focal length of approximately 17mm, is problematic due to the nature of human binocular vision, being mediated and processed by the cortex, and because of the structure of the human eye which has a concave retina, rather than a flat sensor, with variable sensitivity and resolution across its wider-than-180° horizontal field-of-view.
A normal lens then, is one that renders a printed (or otherwise displayed) photograph of a scene that when held at 'normal' viewing distance (usually arms-length) in front of the original scene and viewed with one eye, matches the real-world and the rendered perspective.
Perspective effects of short or long focal-length lenses
Lenses with longer or shorter focal lengths 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 lenses, while longer-focal-length lenses are referred to as long-focus lenses (with the most common of that type being the telephoto lenses). 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 arms-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 reports that; "Judges will not admit a picture that seems to have been tampered with or that distorts any aspect of the scene" or 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 or sensor 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 43° 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. This is a completely different use of the term.
Film still
Typical normal lenses for various film formats for photography are:
For a 35 mm camera 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, the creator of the Leica camera.
Note that the angle of view depends on the aspect ratio as well; a "normal" lens on 35mm does not have the same view as a "normal" lens on 645, for example.
Digital still
In digital photography, the sensor "type" is not the sensor diameter:
(*) refers to TV tube diameters that were standards in the 50s. The normal lens focal length is roughly 2/3 of the TV tube diameter. (**) this is a mathematical calculation because most of the cameras are equipped with zoom lenses.Cinema
In cinematography, a focal length roughly equivalent to twice the diagonal of the image projected within the camera is considered normal, since movies are typically viewed from a distance of about twice the screen diagonal.