Accuracy
So how accurate is the lens magnification and depth of field calculator? In the following, results for some popular lenses are compared to the figures published by the manufacturers.
Magnification
Standard Lenses
| lens type | focal length | MFD | extension tube | |||||
|---|---|---|---|---|---|---|---|---|
| none | 12 mm | 25 mm | ||||||
| magnification | ||||||||
| calc. | data sheet | calc. | data sheet | calc. | data sheet | |||
| Canon EF-S 10-22 mm 1:3.5-4.5 USM | 10 mm | 0.24 m | 0.05 | n.a. | n.a. | n.a. | n.a. | |
| Canon EF 17-40 mm 1:4L USM | 17 mm | 0.28 m | 0.07 | 0.78 | 0.83 | n.a. | n.a. | |
| Canon EF-S 10-22 mm 1:3.5-4.5 USM | 22 mm | 0.24 m | 0.11 | 0.17 | 0.66 | 0.77 | 1.25 | 1.51 |
| Voigtländer Nokton 25 mm F0.95 | 25 mm | 0.17 m | 0.22 | 0.26 | 0.70 | 1.22 | ||
| Canon EF 35 mm 1:2 | 35 mm | 0.25 m | 0.20 | 0.23 | 0.55 | 0.58 | 0.92 | 1.00 |
| Canon EF 17-40 mm 1:4L USM | 40 mm | 0.28 m | 0.21 | 0.24 | 0.51 | 0.59 | 0.83 | 1.02 |
| Canon EF 50 mm 1:1.4 USM | 50 mm | 0.45 m | 0.15 | 0.15 | 0.39 | 0.39 | 0.65 | 0.68 |
| Canon EF 85 mm 1:1.8 USM | 85 mm | 0.85 m | 0.13 | 0.13 | 0.27 | 0.27 | 0.42 | 0.44 |
| Canon EF 135 mm 1:2L USM | 135 mm | 0.90 m | 0.23 | 0.19 | 0.31 | 0.29 | 0.41 | 0.41 |
For these standard (i.e. non macro) lenses, the magnification calculator is pretty accurate, with a deviation below 10% (marked in green) or below 20% (marked in yellow) in most cases. The simplifying basic assumption that a photographic lens can be treated as a simple thin lens is thus working surprisingly well.
Macro Lenses
| lens type | focal length | MFD | magnification | comment | |
|---|---|---|---|---|---|
| calc. | data sheet | ||||
| Canon EF 50 mm 1:2.5 Compact Macro | 50 mm | 0.230 m | 0.47 | 0.50 | |
| Leica DG Macro-Elmarit 45mm F2.8 ASPH OIS | 45 mm | 0.15 m | 1.00 | 1.00 | impossible? |
| Nikon AF-S Micro Nikkor 60 mm/2.8G ED | 60 mm | 0.185 m | 1.00 | 1.00 | impossible? |
| Olympus Zuiko Digital ED 50 mm 1:2.0 Macro | 50 mm | 0.240 m | 0.42 | 0.50 | |
| Pentax smc DFA 50 mm f/2.8 Macro | 50 mm | 0.195 m | 1.00 | 1.00 | impossible? |
| Sigma Macro 50 mm F2.8 EX DG | 50 mm | 0.190 m | 1.00 | 1.00 | impossible? |
| Sony 50 mm f/2.8 Macro | 50 mm | 0.200 m | 1.00 | 1.00 | |
| Zeiss ZF Makro-Planar T* 50 mm f/2 | 50 mm | 0.230 m | 0.47 | 0.50 | |
Again, the magnification calculator is pretty accurate. However, some macro lenses such as the models by Leica, Nikon, Pentax and Sigma are seemingly violating the basic limitation that the minimum focusing distance must be at least 4 times the focal length, as derived in equation (18). So what's wrong?
The answer is simply that they don't violate it—these lenses have an internal focusing mechanism that is changing the focal length during close-up, i.e. the focal length is slightly decreased. This effect is also accounted for by the calculator.
Depth of Field
... to be continued ...
July 2, 2011