Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
KenBalbari
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Re: Equivalent focal length for MFT lenses
In reply to draleks, Apr 13, 2013

No, you will have very different depth of field in your example, plus the FF sensor will have less noise, better dynamic range, and likely better color depth. In addition, you don't really need to say "twice as soft" for the lens, as a 300mm lens will naturally have twice the power of a 600mm lens of equal quality.  If the glass is equal, the 600mm will be "twice as soft" so to speak, if you are comparing on a lp/mm basis.

But the best way to do equivalence is to realize that the apperture changes if you fix the f-stop and change the focal length.  This is because what the "f" in f/6.7 stands for is "focal length".  Those who deny this simply don't understand what an f-stop is. So if you change the focal length from 300mm to 600mm, in order to keep the aperture (and depth of field) the same, you need to also change f/6.7 to f/13.4.

Now you have the same amount of total light, but it is being spread into 4 times the area on the 135 film format sensor. So the exposure is less.  To adjust for that, you need to increase the sensitivity 4 times (2 stops), so from ISO 200 to ISO 800 for example.

This will give you as near as you are going to get to an "equivalent" image. So a 300mm f/6.7 ISO 200 on mFT produces an image equivalent to a 600mm f/13.4 ISO 800 on 135 film format.

In reality, the 135 film sized sensor in this case will likely show at least a bit more noise (in practice, the noise difference in most cases at equal ISO currently seems to be closer to one stop than the 2 stops the difference in area implies).  On the other hand, as you get to f-stops below f2.8 on mFT, and especially below f2.0, aberrations become a problem which make it difficult to maintain equivalent sharpness, so in the real world "full frame" does maintain an advantage in many cases where narrow depth of field is desired.

It is much simpler, and more common however, to ignore depth of field, sensitivity, noise and exposure differences, and simply talk of equivalent "angle of view".  From that point of view, 300mm on mFT has an equivalent angle of view to 600mm on full 135 film format.  You don't need to mention the f-stop, if you don't want to worry about these other aspects.

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