Formula: Lens + Ring

Hello all,

I recently bought a nikkor 300 f2.8 mf. By means of a ring, I'd like to reduce the minimal focus distance. Is there some kind of formula to calcuate what the effect of a certain ring is on the focus distance?

All advice is welcome!

Cheers,

b_nikon

I know the minimal focus distance can be calculated when using extension rings, but don't remember the exact formula. Try a google search of +"extension ring" +"focusing distance", or similar. I sure you will get some useful data.