Blog: Lens Conversions, Easier Said Than Done

By Gary Fong
Photographer: Roy Niswanger,
Figure 1
On a dark and stormy day, I asked Roy Niswanger what he did to his optical formula when converting a Minolta lens to a digital Canon body.  His “photographic lens over exuberance” answer follows:
I retrofit a Minolta MD lens to fit Canon, it’s a bit tricky and the hurdle is trying to match the registry distance of Minolta MD to Canon EOS, 43.72 and 44mm respectfully.  This being said, let’s say the Minolta magically changed to EOS bayonet on the lens, when fitted to a Canon EOS body the lens would work from minimum focus distance (MFD) to some point approaching infinity, but it would never reach infinity focus.  Even with the lens focus ring pulled all the way to infinity the critical focus point would be close, maybe 10’ away.  So with this setup you could use the lens normally between MFD and 10’.
So what I do is first convert the MD bayonet mount of the Minolta lens (actually call RS mount, as MD actually stood for minimum diaphragm, an enhancement Minolta introduced back around ‘78, prior versions were called MC, which I forgot what that meant).  I determined that a cheap M42 to EOS adapter might do the trick and it did.  
So in a nutshell, I remove the MD mount from the Minolta lens, use it for a mounting screw template, then I machine the inner M42 threads and the back seat of the M42 to EOS adapter. Then I drill holes so I can mount this machined bayonet mount to the Minolta.  I then remove the rear element group of the Minolta lens and turn off about 3mm of stock at the end (this is for mirror clearance in the Canon’s mirror box).  I explain it all here:

Here’s my favorite conversion,  Not many of these 800/8 Minolta RF’s around.  The Rokkor Files site reviewed this lens:

I only have 4 converted Minolta lenses and want to do two more but the two left are a little pricey:  Converted:  Minolta MD Rokkor-X 50mm f/1.4, Minolta MD Rokkor-X 40mm f/2, Minolta MC PG 58mm f/1.4 and the Minolta RF 800mm f/8 Catadioptric.  I would love to get my hands on the Minolta MD Rokkor-X 50mm f/1.2 and the Minolta MD Rokkor-X 24mm f/2.8.     -Roy

With that said…I would think buying a Canon lens for a Canon body is a bit more my speed…which opens up a bit more time to shoot a few more pictures.  Roy, if I ever have an urge to convert a Minolta lens(es) to a Canon body…you’ll be the first person I call.