Is there a formula to "convert" an APS-C/MFT lens focal length when used on a full frame sensor?

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TheAthenA714 New Member • Posts: 19
Is there a formula to "convert" an APS-C/MFT lens focal length when used on a full frame sensor?

Hey everyone !

Up until now I've been shooting with MFT. I've used a few adapted lenses as well as native lenses, and I'm used to do the crop factor conversion for focal length.

So for example I have a 42.5mm MFT lens which turns into an 85mm on an MFT sensor.

But now I'm starting to move to a full frame system and I wonder what would happen if I were to use my 42.5mm MFT lens on a full frame sensor.

Since I bought an A7 III as it happens the flange distance is short enough that I can use the lens with just a simple ring adapter, no glass needed so that's good. But obviously the circle of light of the MFT lens will be much much smaller than the full frame sensor. So if I want to get a clean picture out of it, I'll have to crop it in post to remove all vignetting.

Now the question is: what kind of equivalent focal length would that give me? Is there a formula to calculate that? Would that simply give me an 85mm equivalent?

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