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Infinity adjustment of the Rokkor 58/1.2 lens
After the lens is converted to EOS, and mounted on an EOS camera, the lens ID ring should be removed. It unscrews counter clockwise, looking at the front of the lens, and is shown below hanging on the end of the lens barrel.
The all metal focus ring of the earlier models are the same construction, and the front edge will unscrew CCW from the focus grip.
There are several methods of removal. Latex medical gloves will give the hands a better grip. A rubber wrench as shown below will help, also. The nice thing about this lens, is that one is not working with an ID ring so close to the front element glass as to do any harm to the glass.
Once the ID ring is removed, the 4 infinity adjustment screws are accessible around the front element assembly:
Although, there may be several techniques to do this, I find the following works for me, and is very exacting. It requires one to have a computer to view 100% crops on, AND have a tripod setup close enough to view a geometric object several miles away.
The 4 infinity adjustment screws lock the focus ring to the lens helicoid. If you unscrew all 4 screws, the focus ring should move freely and independently from the helicoid.
I find a distant telephone pole or other geometric object several miles away and place it center frame. Itís easiest to use a tripod and set the camera lens wide open, 2 second delay and mirror lockup, aperture priority.
I take a shot with the lens focus ring turned all the way to the infinity stop, Then, take a 2nd shot after turning the focus ring back toward near, a very, very small amount, and a 3rd shot back just a bit more. I then remove the card from the camera & plug into the computer, import & view at 100%, and compare the 3 frames Iíve shot.
There will be 1 of 3 scenarios. Exact, perfect focus. If you think you have perfect focus, then the job is done and lets go take pictures. But, if you want to see if that infinity object can get any sharper, then go to next scenario.
#2 scenario where the 2nd and 3rd frame get progressively more out of focus and your not sure 1st frame is perfect. Turn the lens all the way to the infinity stop and loosen all 4 screws until the focus ring moves freely, and back the ring about half way toward the 30ft mark and tighten just 2 of the screws. Now you can turn the focus ring/helicoid more toward infinity. Continue take a frame and compare, until you find that point the distant object gets no sharper. Once the sharpest frame is found, then loosen the 2 tight screws, move the focus ring to the infinity stop, tighten all 4 screws and replace the lens ID ring.
#3 scenario where the object gets more sharp as you go from frame 1, to 2, to 3. This means you started at past infinity, and should continue to move the lens ring a micro amount, shoot a frame, compare, until you find that point the distant object gets no sharper. Once the sharpest frame is found, then loosen the 4 screws, move the focus ring to the infinity stop, tighten all 4 screws and replace the lens ID ring.
Note: There is a tolerance where the lens can be moved 2 or 3 ďmicro amountsĒ and there will be no difference in center infinity sharpness. If your technique is good enough, its possible to find the short side of this tolerance and bring nearer distances more into focus, thereby increasing depth of field at the infinity setting.
Back to the B&S listing for the Rokkor 58mm Conversion Kit
Edited on Jan 24, 2009 at 02:09 PM · View previous versions