![]() If you want the very best possible results, measure, don't use theoretical formulas that only apply to certain specific lens types.Ie reverse a >50mm lens for magnifications larger than 1:1. So 220mm is the shortest possible distance, going beyond 1:1 will increase the object to sensor distance again, and then it will be a good idea to turn the lens around, as it works best with the shortest distance on the bayonet side. With a 55mm lens this requires 55mm extension, and the lens will sit in the middle between object and sensor, with a total of 220mm from object to sensor (S1+S2, S1=S2=110mm). If extension = focal length (S2= 2*F) S1 will also be 2*F, and magnification will be 1:1. A thin lens has its optical centre at a point this is behind the difficulty with using it as an exact tool for all lenses as an optical centre is not always known, obvious or even calculable. ![]() This is a formula for a thin lens, but still works, although for complex zoom and/or inner focus lenses it is less precise.
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