I long ago lost patience with the continual mental conversions between 35mm and digital lens focal lengths.
At the same time, I also don't like the habit of giving a false focal length in order to convey an angle of view (AoV) or magnification in 35mm terms: focal length affects other crucial things (depth of field and focus, for example) as well. Far better to disconnect the focal length figure from the imagined magnification and AoV in favour of a measure that worrks on any format.
For that reason, I started use a system of ratios in percentage form. SO, for example, a standard lens is around 100%, a wide angle is less than 100 percent, a telephoto is more than 100%. Since people keep asking me for paper copies of it, only to get the grumpy answer that it has to be in their head own, here it is for anyone interested. Print it if you wish, or modify to suit your own needs and internalise it as I have done ... or add a small sticker to every lens, marked with its ratio for quick reference.
For convenience, the ratios are in colour coded blocks - very wide angle, moderately wide angle, roughly standard, moderately telephoto and very telephoto. Please don't write and tell me that you would have grouped them differently; it's all subjective, and we all have our own ideas ... if the colours are not helpful, ignore them or change them to suit you.
All ratios are calculated on the basis of the theoretical (not conventional) standard lens for the format concerned. This is a value equal to the diagonal measurement of the negative, slide or digital sensor in use. The exception is 35mm, where I have added a second set of bracketed figures in italics: The main figure relates to the theoretical standard lens of 43mm, the italicised and bracketed one to the conventional 50mm lens.
I can't include every possible focal length, nor every DSLR sensor size. If you want to generate your own table, with more useful examples for your purposes, but are not sure of the process, email me (address in the header of this blog) and I will send you an Excel spreadsheet to do the work for you.
| Crude focal length comparisons | ||||||
| Actual focal length | DSLR ratio (APS/C 1.5) | 35mm FF film ratio | 6x6cm film ratio | 6x7cm film ratio | 5x4inch film ratio | |
| 10 | 36% | 23% | (20%) | |||
| 12 | 43% | 28% | (24%) | |||
| 14 | 50% | 32% | (28%) | |||
| 15 | 54% | 35% | (35%) | 18% | 16% | |
| 17 | 61% | 39% | (34%) | 20% | 18% | |
| 18 | 64% | 42% | (36) | 21% | 20% | |
| 21 | 75% | 48% | (42) | 25% | 23% | 13% |
| 24 | 86% | 55% | (48) | 28% | 26% | 15% |
| 28 | 100% | 65% | (56) | 33% | 30% | 17% |
| 30 | 107% | 69% | (60) | 35% | 33% | 18% |
| 35 | 125% | 81% | (70) | 41% | 38% | 21% |
| 40 | 143% | 92% | (80) | 47% | 43% | 25% |
| 45 | 161% | 104% | (90) | 53% | 49% | 28% |
| 50 | 179% | 115% | (100) | 59% | 54% | 31% |
| 55 | 196% | 127% | (110) | 65% | 60% | 34% |
| 58 | 207% | 134% | (116) | 68% | 63% | 36% |
| 85 | 304% | 196% | (170) | 100% | 92% | 52% |
| 90 | 321% | 208% | (180) | 106% | 98% | 55% |
| 100 | 357% | 231% | (200) | 118% | 108% | 61% |
| 135 | 482% | 312% | (270) | 159% | 146% | 83% |
| 150 | 536% | 346% | (300) | 177% | 163% | 92% |
| 200 | 714% | 462% | (400) | 236% | 217% | 123% |
| 210 | 750% | 485% | (420) | 247% | 228% | 129% |
| 300 | 1071% | 693% | (600) | 354% | 325% | 184% |
| 500 | 1786% | 1155% | (1000) | 589% | 542% | 307% |
| 1000 | 3571% | 2309% | (2000) | 1179% | 1085% | 613% |
| 1600 | 5714% | 3695% | (1600) | 1886% | 1735% | 982% |
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