I'm not really sure if this is trully the "impossible" since I use this lens quite a bit, but since the register distance for the FD lenses is shorter than Pentax, I suppose this is the right place. Since these Canon lenses are quite a bargain it was worth my time (4-5h) to do this conversion.
Canon FD 50mm f1.2, 1977-1978 vintage (excuse the lazy product shot
).
The following was done in order to make this possible (everything is fully reversible):
- Obtain a 2.5mm thick Pentax-K mount
- I got very lucky here, the mount's diameter was perfect
- Mill the inner side flat
- Drill and bore for the screws
- Fabricate a "finger" from thin steel to mount on the plastic aperture ring with two custom-cut screws. The "finger" telegraphs the aperture ring movement to the internal aperture leaver. This was probably the trickiest part.
- Ground 0.5mm off an internal focus stopper so that the lens could move an extra 0.5mm towards the camera sensor for infinity focus.
- Remove most of the FD mount, shortening the lens considerably (around 6mm)
- Mount the new Pentax mount
- Look for a replacement bearing ball for the aperture ring (totally unplanned)
- Adjust infinity (the lens could originally focus to around 50 meters, I could have milled the mount more but this was easier)
- Shim the filter ring 0.5mm so that it did not catch on the focusing helicoid assembly.
Another "pretty" lens - a few pics attached. All shots but 1st are wide open at f1.2, unprocessed.