I had 2 primes, a faulty P 50/2.0 A and a thrift store P 50/1.7 M. Neither had an adequate infinity stop that was sharp. I was able to 'fix' them by simply removing the front bezel of the lens and loosening 3 screws. The 'A' has a plastic ring that you unscrew from the inside thread mount. I used latex gloves to get a good grip on the ring and it unscrewed easily. Once you get it off, there are 3 screws along the perimeter of the focus ring inside the barrel. These are 'cinch' screws that couple the focus ring to the focus yoke. You can loosen these slightly and turn either the focus yoke (hard) or turn the focus ring independently now. I tried to use the 'head to head' method with another SLR with a split screen focus but got better results by just focusing on a distant set of telephone cables. I was able to focus, then loosen the screws, turn the ring to infinity stop and tighten. Check one more time and reassemble. The M was a little different but same procedure. The bezel comes off and there are 6 screws, 3 hold the front element yoke and the other 3 are a cover for the 3 focus screws. I removed the front element, then removed the cover. There you can see 3 more screws on a sliver ring, it looks to be in 2 overlapping pieces. Same procedure, I focused on (>100m) distant telephone wires and focused to sharpness on a split screen/prism of a Super KX SLR. I loosened the screws and moved the outer ring to infinity and tightened, checked again and reassembled after a wipe of the front elements inside. Now both lenses focus sharp at infinity. The M was about 1mm off, true focus between the infinity and the 10M mark. The A was completely decoupled and was spinning over a tight bottomed ocus yoke. I had to tighten the cinch screws to loosen the bottomed focus yoke and then use the focus ring to find infinity and then reset the focus ring to infinity stop.
Both lenses front element were held in by 3 screws, very easy to remove and clean, no recalibration needed. The A rear element was just screwed in by its own threads (mark edge so you can get it back to factory torque/depth) . So if you have dust in there and its within your ability, you can clean these easily with no chance of altering any of the calibrations.
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