Gene Wright talks about the Normal Lens

 

50mm lenses have an angle of view of about 46°, and produce a view that roughly corresponds to what a human sees in clear focus. For this reason, the pictures produced seem "normal". In addition, many of these lenses are fast (large maximum aperture), of high quality (and reasonable cost), and are compact and light. Multiply 30mm x 1.5 and you get an angle of view of 45° for an AP-C camera, so a 30mm lens  becomes a "Normal Lens" for a DX camera

 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; this angle of view is about 53° diagonally.

Typical normal focal lengths for different formats

Film still

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, 35mm 24 × 36 mm 43.3 mm 45 mm, 50 mm
120/220, 6 × 4.5 (645) 56 × 42 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
120/220, 6 × 12 56 × 112 mm 125.0 mm 120 mm
large format 4 × 5 sheet film 96 × 120 mm (image area) 153.7 mm 150 mm
large format 5 × 7 sheet film 120 × 170 mm (image area) 208.0 mm 210 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.

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.

Sensor type TV-tube diameter * Image dimensions Image diagonal Normal lens focal length **
1/3.6" 7.1 mm 4.0 × 3.0 mm 5.0 mm 5 mm
1/3.2" 7.9 mm 4.5 × 3.4 mm 5.7 mm 5.7 mm
1/3" 8.5 mm 4.8 × 3.6 mm 6.0 mm 6 mm
1/2.7" 9.4 mm 5.4 × 4.0 mm 6.7 mm 6.7 mm
1/2.5" 10.2 mm 5.8 × 4.3 mm 7.2 mm 7 mm
1/2" 12.7 mm 6.4 × 4.8 mm 8.0 mm 8 mm
1/1.8" 14.1 mm 7.2 × 5.3 mm 8.9 mm 9 mm
1/1.7" 14.9 mm 7.6 × 5.7 mm 9.5 mm 9.5 mm
2/3" 16.9 mm 8.8 × 6.6 mm 11.0 mm 11 mm
1" 25.4 mm 12.8 × 9.6 mm 16.0 mm 16 mm
Four Thirds 33.9mm 17.3 × 13 mm 21.63 mm 22 mm - 4/3" 33.9 mm 18.0 × 13.5 mm 22.5 mm 23 mm
APS-C 45.7 mm 22.7 × 15.1 mm 27.3 mm 27 mm
DX n/a 23.7 × 15.8 28.4 mm 28 mm
FF (35 mm film) n/a 36 × 24 mm 43.3 mm 50 mm
(4 × 5 cm) n/a 49.0 × 36.7 mm 61.2 mm  

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.

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

See also