Normal lens

In photography and cinematography, a normal lens is a lens that reproduces a field of view that appears "natural" to a human observer.

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.

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 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 [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."

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