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11-27-2012, 01:23 PM   #1
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Modify old 500mm f8 to infinity focus?

Hi,

I just bought an old 500mm f8 lens (make is "Dufay" but think these are re-branded Paragon etc.) for taking pictures of the moon but it doesn't quite seem to focus to infinity. I've looked online and can't quite work out how to do this? I removed the M42 mount and attached a bayonet as the adaptor was increasing the distance between the back of the lens and mirror, this helped a bit but still not quite there.

Does anyone know how I can correct this? It looks like the front element can be moved onthe threads but haven't messed about with it yet.

Took it apart to remove some fungus out the back elements which cleared it a bit but think the fungus might have etched it a bit as still a bit hazy

Thanks in advance

11-27-2012, 01:38 PM   #2
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Read this.......

https://www.pentaxforums.com/forums/general-technical-troubleshooting/173922-...ml#post1807361


Not the same as yours but may give you some ideas on how to adjust infinity.
11-28-2012, 05:48 AM   #3
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Thanks, I had a look at the link you provided and then noticed I could remove three screws that hold on the distance ring thingy, I took this off and there was a screw that when I removed meant the helicoil could go down further which I think shold give infinity focus? Hopefully the moon will b eout again tonight and I'll give it a go.

Also found this link which I think is a similar thing for anyone else with the same problem:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/69759-500mm-f4...-infinity.html

Thanks,
Andrew
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