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Which factor needs to be applied to the focal length to get equivalent 6Mpixel large crops with different cameras? The question is interesting especially for actual cropping for larger amplification. So not so much for wide angles but more for the telephoto range.
The crop factor (a focal length multiplier) for different-sized sensors indicates the ratio of the size of the sensor compared to the 24x36mm format (often called full format). The factor is 2 for a 4/3sensor and 1.5 for APS-C. In a similar way we can also take into account the number of pixels for a 1:1 pixel representation. Ifd increasing the Mpixel by a factor X this additional crop factor is the square root of X.
It is assumed that the lens is good enough to resolve the pixels. For 6Mpixel in full format the lens, regardless of the focal length, has to have a resolution of about 40 lines per mm (0.5 x 2000/24; 0.5 because you need 2 pixels to resolve a line, 2000 since 2000 x 3000 are 6Mpixel and 24 mm as the short side of the full format). With 16Mpixel the lens has to resolve approximately 67 lines per mm (~ 40 times 1.6).
According to lens reviews on the Internet (E.g. on Lenstip or
dxomark), 50 lines per mm are already very good. The P-Mpixel introduced in 2012 on Dxmark go up to only (date 5.1.2013) 21 Mpixel for full format, to 14 for APS-c and 10 for 4/3, and the lenses with these high values are of course not cheap. In essence this means that for the same lens, even if you increase the Mpixel and/or reduce the sensor size you will not necessarily get more information as would be expected from an higher factor.
Extension factors over a 6Mpixel KB format. The formula is Cropfaktor*(Mpixel/6)^0.5.
| Pixel | 4/3 | APS-C | 24x36mm |
| 6 M | 2.0 | 1.5 | 1.0 |
| 10 M | 2.6 | 1.9 | 1.3 |
| 12 M | 2.8 | 2.1 | 1.4 |
| 16 M | 3.3 | 2.4 | 1.6 |
| 24 M | 4.0 | 3.0 | 2.0 |
| 48 M | 5.6 | 4.2 | 2.8 |
What 6Mpixel focal length has a 500 mm lens on different sensors?
| Pixel | 4/3 | APS-C | 24x36mm |
| 6 M | 1000 | 750 | 500 |
| 10 M | 1290 | 968 | 646 |
| 12 M | 1414 | 1061 | 707 |
| 16 M | 1633 | 1225 | 816 |
| 24 M | 2000 | 1500 | 1000 |
| 48 M | 2828 | 2121 | 1414 |
What focal length are needed with different sensors to have a 24x36mm 6Mpixel 500 mm lens?
| Pixel | 4/3 | APS-C | 24x36mm |
| 6 M | 250 | 333 | 500 |
| 10 M | 194 | 258 | 387 |
| 12 M | 177 | 235 | 353 |
| 16 M | 153 | 203 | 306 |
| 24 M | 125 | 167 | 250 |
| 48 M | 88 | 118 | 177 |