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I'm considering switching to the Micro Four Thirds system. Finding lenses which match my style is a key part of that, and "normalizing" via crop factor is a convenient tool.

The crop factor for Four Thirds is usually given as , but there's a problem: crop factor is really only meaningful when comparing equal-sized prints, and since Four Thirds uses a 4:3 aspect ratio rather than the 3:2 ratio common to SLRs and 35mm rangefinders.

The 2× multiplier is based on comparing the diagonal, which is good enough for ballpark (especially considering that there's often imprecision anyway), but what are the real numbers when considering common aspect ratios?

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Rather than calculating the crop factor from the diagonal regardless of format, this chart is based on the largest-possible cropped print from the respective sensor. For example, for 3:2 aspect ratio (as in 4×6 prints), the Four Thirds image is cropped along the long edges, while for 4:3 aspect ratio, Four Thirds is uncropped but APS-C or "full-frame" 35mm are cropped along the short edges. For other aspect ratios, both formats are cropped, of course. 5:7 is particularly of interest, as it requires different edges to be cropped from either format.

Aspect | Crop Factor | Equivalent to traditional 50mm "normal"
 4:3   |    1.85×    | 27mm (true normal 23.4mm)
 3:2   |    2.08×    | 24mm (true normal 20.8mm)
 1:1   |    1.85×    | as 4:3
 5:4   |    1.85×    | as 4:3
 5:7   |    1.94×    | 26mm (true normal 22.3mm)
 5:8   |    2.08×    | as 3:2
16:9   |    2.08×    | as 3:2

In short, if you're using a more-square format, use 1.85×. If you're targeting a wider format, use 2.08×, and if you happen to be using 5:7, use 1.94×. And, if you tend to crop arbitrarily or differently based on the subject, 2× is reasonable enough as a quick and easy rule of thumb.

The effect isn't huge, but the variation is enough that it doesn't hurt to be aware. (The difference in extremes is about twice the difference as that between Canon's APS-C and everyone else's APS-C.)

Also note that this doesn't apply to a few Panasonic Micro Four Thirds cameras which have a multi-aspect ratio sensor — for those, the crop factors for wider formats are slightly smaller.

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