Normal lens

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In photography and cinematography a normal lens is a lens that generates images that generally look "natural" to a human observer under normal viewing conditions as compared with lenses with longer or shorter focal lengths. Lenses of shorter focal length are called wide-angle lenses, while longer focal length lenses are often referred to as telephoto lenses.

A lens with a focal length about equal to the diagonal size of the film or sensor format is known as 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;^[1] this angle of view is about 53 degrees diagonally.

[edit] Typical normal focal lengths for different formats

Standard normal lenses for various film formats for photography are:

Film format	Image dimensions	Image diagonal	Normal lens focal length
9.5 mm Minox	8 × 11 mm	13.6 mm	15 mm
APS C	16.7 × 25.1 mm	30.1 mm	28 mm, 35 mm
135	24 × 36 mm	43.3 mm	45 mm, 50 mm
120/220, 6 × 4.5 (645)	56 × 45 mm	71.8 mm	75 mm
120/220, 6 × 6	56 × 56 mm	79.2 mm	80 mm
120/220, 6 × 7	56 × 68 mm	88.1 mm	90 mm
120/220, 6 × 9	56 × 84 mm	101.0 mm	105 mm
large format 4 × 5 sheet film	96 × 120 mm (image area)	153.7 mm	150 mm
large format 8 × 10 sheet film	194 × 245 mm (image area)	312.5 mm	300 mm

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, as a compromise between the theoretical value and good sharpness, as lens technology at the time was such that slightly longer focal lengths were able to achieve optimum sharpness.

In Digital photography, the sensor "type" is not the sensor diameter:

(*) refers to tv-tube diameters that were standards in the 50's. 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.

Sensor type	TV-tube diameter *	Image dimensions	Image diagonal	Normal lens focal length **
1/3.6"	7.1 mm	4.0 x 3.0 mm	5.0 mm	5 mm
1/3.2"	7.9 mm	4.5 x 3.4 mm	5.7 mm	5.7 mm
1/3"	8.5 mm	4.8 x 3.6 mm	6.0 mm	6 mm
1/2.7"	9.4 mm	5.4 x 4.0 mm	6.7 mm	6.7 mm
1/2.5"	10.2 mm	5.8 x 4.3 mm	7.2 mm	7 mm
1/2"	12.7 mm	6.4 x 4.8 mm	8.0 mm	8 mm
1/1.8"	14.1 mm	7.2 x 5.3 mm	8.9 mm	9 mm
1/1.7"	14.9 mm	7.6 x 5.7 mm	9.5 mm	9.5 mm
2/3"	16.9 mm	8.8 x 6.6 mm	11.0 mm	11 mm
1"	25.4 mm	12.8 x 9.6 mm	16.0 mm	16 mm
4/3"	33.9 mm	18.0 x 13.5 mm	22.5 mm	23 mm
(APS-C)	45.7 mm	22.7 x 15.1 mm	27.3 mm	27 mm
DX	n/a	23.7 x 15.8	28.4 mm	28 mm
FF (35 mm film)	n/a	36 x 24 mm	43.3 mm	50 mm
(4 x 5 cm)	n/a	49.0 x 36.7 mm	61.2 mm

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.

Film format	Image dimensions	Image diagonal	Normal lens focal length
N-8	3.7 x 4.9 mm	6.11 mm	12 - 15 mm
Single - 8 (FUJI)	4.2 x 6.2 mm	7.5 mm	15 - 17 mm
Super - 8	4.2 x 6.2 mm	7.5 mm	15 - 17 mm
9.5 mm	6.5 x 8.5 mm	10.7 mm	20 mm
16 mm, magnetical sound	7.5 x 10.3 mm	12.7 mm	25 mm
16 mm, optical sound
35 mm	18.0 x 24.0 mm	30.0 mm	60 mm
35 mm, sound	16.0 x 22.0 mm	40.5 mm
65 mm
70 mm	52.6 x 23.0 mm	57.4 mm