# How do adjustments to flange focal distance affect front focal distance?

I am using a Nikon Micro-Nikkor F-mount lens with 105mm fixed focal length in a custom optical set-up (F-mount adapter to custom optics). This means that my flange focal distance is only an approximation of the 46.5mm flange focal distance I would achieve if this lens were mounted to an appropriate DSLR camera; it is slightly smaller than 46.5mm which results in the front focal distance being slightly longer than what's indicated on the lens adjustment ring (e.g. if I am at a working distance of 0.5m, I need to adjust the lens to approximately the 0.45m indicator line on the lens body, in order to actually achieve focus).

I know already that smaller flange focal distance causes the front focal distance to increase, and vice-versa, but are there any resources for determining, more quantitatively how changes to the focal flange distance affect changes to the front focal length? Is the response strictly inverse (e.g. halving focal flange distance, doubles front focal length)?

My end goal is to increase the flange focal distance beyond 46.5mm in order to decrease the front focal distance and get closer to 1:1 magnification scale, but I would like to do this in a more quantitative manner.

• Wikipedia has equations you can stare at. – xiota Apr 17 '19 at 21:15
• "(e.g. if I am at a working distance of 0.5m, I need to adjust the lens to approximately the 0.45m indicator line on the lens body, in order to actually achieve focus)." Don't put too much stock in the accuracy of distance scales on lenses. They're no known for being precise, to put it mildly. – Michael C Apr 18 '19 at 7:46

Photographers will normally use extension tubes or bellows to increase the distance between the lens and the sensor (where you talk about flange focal distance, if I understand the question correctly) as a way to focus on things closer to the lens and have a higher magnification (not lower). My suggestion would be to search for `macro photography extension tube calculator`. That should give you either online calculators or the math you need.