Infinity Focus Adjustment

[Tristin]

I will be servicing the 200mm f/4 Ai I purchased recently to deal with it's extremely tight focusing, and decided to see if the infinity focusing needs adjustment while I am at it.  To check it I mounted it on a tripod, pointed at a tower that is at least a few miles away, and checked the focus in magnified LV, wide open.  It seems to fall a bit short of focusing on infinity.  Is this an appropriate method for checking infinity focus?

[Bjørn Rørslett]

Yes, should work.

Even better is using remote city lights or other bright point sources of light. At night time, focus on these pinpoint lights and see how the circle of unsharpness shrink into a point if the lens is working properly.

[Bernard Delley]

Its a very capable lens, far beyond expectations from its unassuming appearance. Unfortunately my sample also has a shortish infinity stop. I would be very interested to learn from the experts here, how to adjust this with minimal disassembly of the lens.

[richardHaw]

what i do for checking infinity focus is to focus on a building around 500m or more away from me
during the day, i just focus on the clouds

you adjust this ring if you want to adjust the focus to infinity

[richardHaw]

and this is how it is for the 200mm ai-s
it devolved from a nicely milled screw thread to scotch tape

[Bjørn Rørslett]

"infinity" is more than 500 m away ... At home, I use a target 1.7 km away (a restaurant on top of a hill on the other side of the valley) and for many lenses there is a considerable movement of the focusing collar from 500 m to 1.7 km.

Earlier when Erik lived near the seaside in Copenhagen, we used city light in the Swedish city Malmö across the Øresund Straits, as infinity targets. The distance is approx. 16 km.

[Bernard Delley]

Thanks for the reply.
But, I could not see from your pictures the actual infinity stop  and how it could be adjusted to really have focus beyond 300m.