[REPAIRS] 24mm/2.8 AIS - focus issue: Infinity focus is actually at 1m!

[whatdoineed2do]

[ edit subject after finding that when focused at "infinity" objects at around 1m are in focus!!! ]

Having found significant inspiration from this group I (foolishly?) aquired cheap(ish) "spares/repairs" 24mm f/2.8 AI-S, that has scratches/rub marks on the front and rear elements and bump to the front: I intended this for learning / practise dismantaling and if it still worked, use it as a don't-care-walkabout-lens.

Following previous articles from Richard Haw and youtube mikeno62 's specific tear down vid I was successful in tearing down, cleaning the junked up elements/interiors and then reassembling.  However, I in my excitement of ripping this lens apart, I neglected to mount/check the lens before the teardown;  with this reassembled and mounted I find that focus is not sharp at any distance nor does it improve at f/8 etc.  I expect a loss of contrast with the rub marks on the rear element but expected this to be at least in focus.

I've checked the elements and they look straight and true and I havent put the elements back to front - I did not separate the helicolds.

The focus issue may already have been there but it's too late to know.  With that said, is there any pointers for how I could diagnose where the focus problems will be?

thanks

[ will upload sample images later ]

[Matthew Currie]

Is it possible some previous person put it together wrong? 

e.t.a. previous link deleted, as I discover the AI and AIS are different, and the diagram I linked to is therefore no use.  But I think if one looked around one might find a diagram of the optics and compare to what you have.

[whatdoineed2do]

I've checked the lens elements and they all look like they sit in the correct orientation - most of them can only fit into their retaining rings one way or they are set in their spacer rings that have notches for lens spanners - the orientations also match what is shown on mir.com.my



Some images to highlight the results:

The close focus images 6934 (f/8) and 6945 (f/2. are at min focus distance based on me shifting back/forth til I get something in focus: probably 27cm away from sensor plane.


The problem with the lens comes with the later image - If we look at 6942 - that's the 24mm mounted on an FG with a split focus screen and it's almost focused (the vertical is vertical).  However this image was taken with the lens focused at infinity whilst the subject is about 1meter away!  Yikes.

So there's some focus calibration needed.  Anyone have any ideas?  Could this be an example of someone unscrewing the helicoid and mating it incorrectly?

[richardHaw]

that is a CRC lens I think...

good luck, you will have to experiment with the helicoid and the CRC 

[whatdoineed2do]

Ok, I figured this out - This is where the phrase "buyer beware" is perfect even though it was sold as "spares/repairs".

This lens has 2 helicoids - on the rear there is a brass tab on the rear objective that fits into a notch driven by the helicoids - this brass tab was bent and not engaged in the notch;  so when the focus ring is turned the rear elements are not turned.  The other problem was thing that turns the rear lens block was jammed.

So to fix this (and therefore to adjust the near / far focus) is the align the brass tab correctly - theres a bunch of screw mount holes that this tab can be seated and therefore adjusted;  i'll add some photos of this later for everyone's reference.

[richardHaw]

that's the CRC unit 

[whatdoineed2do]

After fixing my original problem, I kinda got carried away and ended up with this:


And for closure, this is the CRC / brass tab I was referring to earlier in the post - it was simply not engaged in the fork so wouldn't drive the CRC when focus was turned.  Images of it removed and attached.



Entire tear down set here for those that are interested:
https://www.flickr.com/photos/whatdoineed2do/albums/72157695913724665

https://whatdoineed2do.blogspot.com/2018/04/nikkor-24mm-f28-ai-s-disassembly.html