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02-24-2014, 12:57 PM   #1
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300MM F4 Tak... infinity focus with M42?

I recently acquired a very nice copy of the m42 Tak 300MM F4-22. (the 1962 version) At the time I got it, I had my DS with me and my genuine M42 adapter. Took a few quick sample snaps and was happy and smartly paid the man.

What I did notice apon further experimentation is that I couldn't focus to infinity, and that when screwed on, the center of the DOF scale is not centered to the top of the camera.

I looked at this thread, and while interesting, I didn't think this was my actual problem...

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/69759-500mm...-infinity.html

So, I looked at the screw mount itself on the back of the lens... it's basically a deep cup, with 3 screws holding it over the rear barrel of the lens. I pulled the 3 screws and the took off the cup with the idea of trying the 2 other different orientations of rotating the cup, thinking maybe the lens wasn't setting into the body deep enough...nope, the other 2 orientations actually made it worse...

Then I noticed inside the cup were stacked several layers of metal "shim" rings of various thicknesses that the 3 screws go through when mounting the cup. I removed a couple of thin shims, and screwed the cup back on and I'm closer to infinity now but not perfect yet that. I will wait for a full moon or something and play with it some more, prolly will end up taking out one or two more of the thinner shims.

My question: Is this normal or a known issue to have to re-shim the mount to get infinity on the 300MM? None of my other Takumars needed this done. Is this the correct way to fix this or should I be doing something else?

Just looking for a sanity check here,
Thanks!

02-24-2014, 01:47 PM   #2
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QuoteOriginally posted by Erictator Quote
My question: Is this normal or a known issue to have to re-shim the mount to get infinity on the 300MM? None of my other Takumars needed this done. Is this the correct way to fix this or should I be doing something else?
No, it is not normal to have to shim the mount. Can you post a photo or two of the mount? My guess is that someone has done some "creative" stuff with your lens.


Steve
02-24-2014, 02:26 PM   #3
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QuoteOriginally posted by stevebrot Quote
No, it is not normal to have to shim the mount. Can you post a photo or two of the mount? My guess is that someone has done some "creative" stuff with your lens.


Steve
Hi Steve, thanks for the follow up. Here is a picture (taken just now with my old DS and my M42 Tak 35 F2, heh).

There is nothing that I can detect that has been ground on or messed with. The picture shows the mount screwed back on and the 2 shims I took out laying on my desktop.

With the mount off, I can see where someone has engraved the number "21" into the base of the steel lens barrel. I thought maybe the factory assembly guy notated the amount of shim stock used when he collumnated the lens or something...??

The thin strip of masking tape with the pen mark I made shows the center-line of where the lens lines up with the body when I screw it on. You can see that the tripod foot is off center, I left the collar aligned with the DOF scale so it would be apparent how far off the ink line is on my tape.

Eric
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02-24-2014, 02:28 PM   #4
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QuoteOriginally posted by stevebrot Quote
My guess is that someone has done some "creative" stuff with your lens.
Probably a Nikon shooter.

02-24-2014, 02:39 PM   #5
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QuoteOriginally posted by boriscleto Quote
Probably a Nikon shooter.
The trouble is, I know the guy I got it from... said he didn't think he had an infinity issue, but that he didn't take moon shots either. He's had it for nearly most of it's life, though he did get it used, but that was over 30 years ago he said and he knew the guy he got it from, also used on Pentax, I think he said an H3. He used it on, and still owns, a spotmatic... no Nikons.

The mystery continues...
Eric
02-24-2014, 03:12 PM   #6
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I have some older Takumars that do have shims to adjust infinity focus, so it's not impossible that these shims are original equipment. I was impressed that my lenses were still dead on infinity, but they are only 55mm and 135mm so a lot less material to expand or contract than a 300mm. If the original designers were concerned about this, it makes sense for the shims to be easy to remove or add.

Either way, keep taking shims out until you get to infinity.
02-26-2014, 12:19 PM   #7
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New developement!

OK, so I thought I had it shimmed perfectly then... blur! WTH?

After some more experimentation, I noticed out of the blue, when I pointed the lens straight down I heard a little "clink". Uh oh...

Looked through the lens and focus was total crap, not even close. Then the light bulb went on in my head. I put all the stock shim back, tipped the lens down until I heard the "clink" and checked, yup, infinity was fine.

So... it looks like I have a loose internal element. I checked the front and rear elements and they seem fine. I think this lens only has 4 elements, but it's a preset, with the 18 blades up near the front... so no way I'm tackling this one myself to get at an internal element.

Thanks everyone for the help, but I guess it's going to have to ship out to Tennessee for repair.

Eric
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