Welcome to Photo.net: A Community of Photographers
by Ed Scott; created 1996
Photographers usually are most familiar with one camera format, typically 35 mm. They have a good feel for the angle of coverage of lenses in this familiar format but find that their instincts fail them when they switch to a different size film. The following table shows which lenses provide equivalent angles of coverage among film formats.
Determining the 35 mm focal length equivalent for a 2 1/4 Square format camera
lens is an apples to oranges comparison unless you set some specifications for
framing and print aspect ratio. Once you do that, the principle of similar
triangles makes it quite easy to calculate. Here is a table of 35 mm lens
equivalences for various Hasselblad lens listed in terms of equivalent angle of
coverage for the specified print aspect ratio conditions:
| 6x6 cm Lens | Largest Square | Largest 8 X 10 | Largest 11 X 14 | Full 35 mm |
|---|---|---|---|---|
| 40 mm | 17.3 mm | 21.6 mm | 22.0 mm | 26.0mm |
| 50 mm | 21.6 mm | 27.1 mm | 27.6 mm | 32.6mm |
| 60 mm | 26.0 mm | 32.5 mm | 33.1 mm | 39.1mm |
| 80 mm | 34.6 mm | 43.3 mm | 44.1 mm | 52.1mm |
| 120 mm | 51.9 mm | 64.9 mm | 66.1 mm | 78.1mm |
| 150 mm | 64.9 mm | 81.2 mm | 82.7 mm | 97.7mm |
| 180 mm | 77.9 mm | 97.4 mm | 99.2 mm | 117 mm |
| 250 mm | 108 mm | 135 mm | 138 mm | 163 mm |
| 350 mm | 151 mm | 189 mm | 193 mm | 228 mm |
| 500 mm | 216 mm | 271 mm | 276 mm | 326 mm |
| FACTOR | 0.433 | 0.541 | 0.551 | 0.651 |
These calculations are based upon stock Hasselblad 120 mm and Nikon 35 mm film apertures. The Hasselblad aperture measures 2.15 inches square. The Nikon aperture measures 0.93 inches high by 1.40 inches wide.
These lens conversion factors can be used to find the 35 mm equivalent focal length of unlisted 2 1/4 Square format lenses. Just multiply the 2 1/4 Square format lens focal length by the appropriote factor to get its 35 mm equivalent focal lenth.
The same method can be used to compare 4x5 view camera lenses with 35 mm focal lengths.
| 4x5 Lens | Largest 8x10 | Largest 11x14 | Full 35 mm |
|---|---|---|---|
| 90 mm | 22.4 mm | 22.8 mm | 26.9 mm |
| 127 mm | 31.62 mm | 32.1 mm | 38.0 mm |
| 135 mm | 33.6 mm | 34.2 mm | 40.4 mm |
| 150 mm | 37.4 mm | 38.0 mm | 44.9 mm |
| 210 mm | 52.3 mm | 53.1 mm | 62.8 mm |
| 240 mm | 59.8 mm | 60.7 mm | 71.8 mm |
| 300 mm | 74.7 mm | 75.9 mm | 89.7 mm |
| 500 mm | 125 mm | 127 mm | 150 mm |
| 800 mm | 199 mm | 202 mm | 239 mm |
| 1000 mm | 249 mm | 253 mm | 299 mm |
| 2000 mm | 498 mm | 506 mm | 598 mm |
| FACTOR | 0.249 | 0.253 | 0.299 |
Note: To use this table with an 8x10 view camera either divide the 35 mm lens focal lengths by 2 or multiply the 4x5 lens focal lengths by 2.
The 4x5 lens calculations are based upon a film holder aperture of 3.74 inches by 4.68 inches.
The traditional definition of a normal lens is that its focal length is equivalent to the film diagonal length. In the case of a 2 1/4 Square format it might be useful to modify this definition to specify the largest 8x10 print area of the 2 1/4 film aperture (which coincidentally is closer to 80 millimeters than the square's diagonal). A normal lens typically has an angle of coverage of about 50 to 55 degrees and is roughly comparable to the useful or working angle of coverage of human vision (the peripheral vision areas serving mostly as a motion detection system).
Comparing lens focal lengths between formats in this way assures equivalent angles of coverage for ultimate print aspect ratio. It is easy to see that 80 mm and 150 mm lenses are normal focal lengths for 2 1/4 Square and 4x5 formats respectively. You can also see that 180 mm and 300 mm lenses are ideal portrait focal lengths for 2 1/4 Square and 4x5 view camera formats.
Copyright 1996 Ed Scott; images copyright 1991-2000 Philip Greenspun.
