Focal length is one of main parameters of any lens. It determines useful angle of view for specific frame size in vertical and horizontal planes. Shorter focal length means wider angle of view both in the objects plane and in the image plane.

View angle (2β) of a lens is formed by the lines connecting center of the lens front element with border points of the space pictured by the lens with acceptable sharpness and brightness. The lens can be used for shooting on the frame size with diagonal that is less or equal to diameter of the image circle of the lens. Many cine lenses have larger image circle than size of the acceptable image quality circle, so actual view angle of most lenses is greater than view angle determined by the frame size.

We'll deal with angle of view determined bu the lens focal length for a specific cine frame format. Vertical angle of view is determined by the frame height, horizontal angle of view determined by the frame width.

In all cases we'll specify angle values for infinity focused lens (when the view angle reaches its maximum value).

Table 1 lists view angles of different lenses for 35mm film cine frame format (16x22mm).

Table 1

Focal length, mm
Horizontal
view angle
Vertical
view angle
Focal length, mm Horizontal
view angle
Vertical view angle

16

18

22

28

35

40

50

69°

62°50'

53°10'

42°50'

34°50'

30°50'

25°

53°10'

47°50'

39°50'

31°50'

25°40'

22°40'

18°10'

75

80

100

150

200

300

 

16°40'

15°40'

12°30'

8°20'

6°20'

4°10'

 

12°10'

11°30'

9°10'

6°10'

4°40'

 

When focused to a specific distance, the lens moves away from the film plane, and view angle decreases for the same frame size (see picture below).

Changing angle of view at different focusing distances

Changing angle of view at different focusing distances:
α - view angle for infinity,
β - view angle for a specific distance.

This change of view angle becomes significant only at very short focusing distances. Table 2 shows dependence of the view angle on focusing distance for different focal length lenses.

Focal length, mm
Focusing distance, m
0.5 1.0 
2.0 
3.0 
5.0

18

28

50

75

100

61°04'

40°48'

22°38'

14°32'

10°28'

61°58'

41°48'

23°40'

15°32'

11°26'

62°24'

42°20'

24°14'

16°06'

11°58'

62°34'

42°32'

24°26'

16°16'

12°10'

62°40'

42°40'

24°34'

16°26'

12°18'

62°50'

42°55'

24°50'

16°40'

12°30'

Focal length affects image scale factor as well. It is also relates with distance to the object:

R = L2/L1 - F/(U-F)

where R - linear magnification (scale factor), L1 - object size, L2 - object image size, F - focal length of the lens, U - distance to the object. Distance U should be expressed in same measure units as F.

Lenses made in USSR have following focal lengths:

for 16x22 frame on 35mm film - 10, 14, (16), 18, 22, 28, (32), 35, 40, 50, 75, (80), 100, 125, 150, 200, 300, 500, 750 and 1000mm;

for 52.5x23mm frame on 70mm film - (15), 22, 28, 40, 56, 75, 100, 125, 150 and 200mm.

Values in parentheses are not recommended for new lenses to be designed.

Deviation of the actual focal lengths from the values listed above should not exceed ±2% for lenses with F<=22mm, and ±3% for all other lenses.

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