The focal length in photography is typically set to the length of the diagonal of the image as it is on the negative. The following are the diagonals as used by various formats.

### Formats and Diagonal Length (in mm)

The 35mm has a diagonal length of 43. The 4.5 cm × 6 cm has a diagonal length of 75 mm and the 6 cm × 7 cm, a diagonal length of 92 mm. For the 6 cm × 9 cm, the diagonal length is 108. The 2¼” × 3¼” format uses a diagonal length of 100 mm. The 4” × 5” format employs a 151 mm diagonal length. The 5” × 7” format uses a diagonal length of 207 mm. The 8” × 10” format uses a 311 mm. The 11” × 14” utilizes a 436 mm.

### Using Focal Length in Photography

The type of lens employed in cameras today may vary somewhat from these measurements. Some use 50 mm in 35mm. The 210 mm is used in 4 x 5. There are also wide angle lenses being used. These lenses usually have greater depth of field than the average lens.

Except for a few instances, the focal lengths have the same depth of field when at the same aperture. A telephoto denotes any lens type that has a longer focal length than usual.

### The Angle of View for 35 mm

Anyone studying the focal length of photography should also be aware of the various angles of views used by the different formats. The measurements are the following for the 35 mm.

For 24 mm slides: 14 mm focal length (81 degrees), 15 mm (77 degrees), 18 mm (67 degrees), 20 mm (62 degrees) and for 21 mm (60 degrees). For the 24 mm focal length it is 53 degrees; for 28 mm (46 degrees) and for 35 mm (38 degrees).

Other focal measurements are the following: 50 mm (27 degrees); 85 mm (16 degrees) and 90 mm (15 degrees). For 100 mm (14), 105 mm (13 degrees), 135 mm (10 degrees) and 180 mm (8 degrees). For 200 mm it is 7 degrees and for 300 mm (4.6 degrees). For 400 mm (3.4 degrees) and for 500 (2.9 degrees).

### Other Focal Length in Photography Information

For the angle of view for 6 cm x 6 cm photography, the measurements are the following. For 30 mm focal length: side 87 degrees; diagonal 107 degrees. For 40 mm: side 71, diagonal 91 degrees; 50 mm (59 side, 78 diagonal) and 60 (51 side, 68 diagonal). For 80 mm focal lengths: 39 side, 54 diagonal; 120 mm (27 side, 37 diagonal) and for 135 mm (24 side, 33 diagonal).

For a 150 mm focal length: 22 side, 30 diagonal and for 180 mm (18 side, 25 diagonal). For 350 mm: 9 side, 13 diagonal; and for 500 mm (7 side, 9 diagonal). For the 1000 mm focal length it is 3.2 sides, 4.6 diagonal.

The focal length of photography may seem like a complicated subject. But it’s a must for anyone wishing to take their photography skills to the next level.

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