Need help evaluating repair on Rokkor glass

[Laviolette]

I purchased a Rokkor 35mm 2.8 lens last month from a eBay seller from Europe. The seller was describing the lens as being «CLA'ed» : the glass is clean and the apertures blades are snappy. But...the lens won't focus to infinity. As you can see in the picture below, the barrel stops before the infinity mark and according to my tests, I have to stop down to F8 to ensure that my pictures are sharp beyond a certain point.

I contacted the buyer about it, and he said to send it back. But I felt it wasn't worth it when I compared the shipping cost with the price I paid for the lens, and the seller agreed to refund me 25% of the cost of the lens.

I have done some repairs on camera bodies and the like before : changing the film take-up spool on a Minolta XD11, changing the nylon stop in a Hasselblad A12 back, etc. But the only experience I have with lens repair is disassembly and cleaning of a Jupiter 8.

Where should I start with this repair? I have already checked some videos of disassembly of Rokkors lenses, and I have the impression that I could do the repair by dismounting only the front of the lens. If anyone here has some experience with Rokkor glass, I would greatly appreciate a tip or two before attempting the repair. Thanks!

 

[Greg_E]

There are two issues at work here.

#1 is where the lens focuses at infinity.

#2 is where the barrel indicates infinity.

If the lens actually focuses at infinity then you can probably just loosen the set screws and rotate the focus collar to indicate properly. Since it sounds like your lens doesn't hit focus, then you'll probably need to move down to the focus helicoid, it may be as simple as being one tooth off in the helicoid.

 

[Laviolette]

There are two issues at work here.

#1 is where the lens focuses at infinity.

#2 is where the barrel indicates infinity.

If the lens actually focuses at infinity then you can probably just loosen the set screws and rotate the focus collar to indicate properly. Since it sounds like your lens doesn't hit focus, then you'll probably need to move down to the focus helicoid, it may be as simple as being one tooth off in the helicoid.

Great advice! Thank you.

 

[Ranchu]

I don't think it's the helical, focus would be way off. Go through the front and look for three screws under the trim ring, they generally overlap the crack between the actual helical and the focus ring.
http://www.pentaxforums.com/forums/...480-infinity-focus-adjustment-m42-lenses.html

http://k10dpentax.blogspot.com/2008/05/fixing-infinite-focus-on-super-takumar.html

 

[colyn]

If I recall there are 3 grub screws below the rubber grip. Remove this grip, loosen the screws and rotate the helicoid till you find infinity at a distance lock the screws after rotating the focus ring to infinity. Then go inside set the camera on a tripod and point it at a flat sheet of paper with horizontal and vertical line then set the cameras focal point to 4 ft from this sheet. Carefully focus then check to see if the ring is aligned with 4 ft

Repeat the above steps till you get both right..

 

[Ranchu]

I set the infinity stop on the moon, I haven't found the markings to ever be accurate on an slr lens. I think the film curl is ignored for them but the screen is accurate to it.