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11-21-2012, 02:27 PM   #1
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Repair internal anti-reflexring SMC 500mm f4.5

Repair internal anti-reflexring SMC 500mm f4.5

Hello,

Since a week I am the proud owner of a legendary lens from the old days. It took quite some efforts to get ‘m here, and I must say, it is a beauty. Good preserved, all rings, focus as well as aperture, turning smooth, no eyecatching scratches but only caused by use, clean and untouched frontelement. It is ok. But,… there is a anti reflexring internal getting loose. The lens looks as if it is only used few times, but this ring rattles as you move the lens. The ring also nears the apertureblades, cautious for damage of them, and hits the inner surface of the last frontelement. You can wait for more damage.
I suppose that during shipping to my place, Holland, the lens got hit by strikes and bumps, allthough no damage according to this matter showed on the box. Still, these strikes could have happened when the box hits a hard surface in the full face and the lens was not packed, it is a heavy one, with bubblefoil all around. Or, the lens has been sold with the loose ring allready. I do intend to keep’m, so I am looking for a solution.

I suppose the frontpart of the lens with the sunhood, almost similar to the Takumar 500mm f4.5, can be removed, but I am anxious to do that. I don’t know exactly how the frontpart is assembled to the barrel. A ghosting from the Takumar shows me what the mounting is, thread(?) combined with some locksrews (?), but such a huge lens should have more than only a thread, considering the size and the weight. I suppose also that unthreading the frontpart takes quite some force, not to say a thing about how to hold this part, the barrel too, in your hands, cause it is too big! Such a force can easily destroy a fine construction.

Is there anybody whom has some experience with unmounting the frontpart of this lens ?

I really want to know what and how. Maybe there are some photo’s showing how to handle. The ring is glued, expected to be very lightweigthed, the glue did not hold.


pentaxior

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11-22-2012, 07:14 AM   #2
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I haven't been into this lens, but yes they can be held together with set screws and threads.The threads are extremely fine and from my experience, the threaded area is around an inch long. Makes for quite a good bond.
Mark your lens with some masking tape so you know where all the parts should end up upon reassembly.
Removal of the set screws requires a very good screw driver, a precision tool that fits the slot nearly perfect.
Once the screws are removed the barrel should unscrew without a lot of force. Rubber strap wrenches work if the bond isn't extremely tight.
Make your repair and reassemble. Be cautious with your lens barrel threads and don't cross thread them .

DISCLAIMER. If at any point a screw won't release....stop and find a lens repair person. A frozen screw requires you to drill it out and re-tap the hole.

All this being said, unless your mechanically inclined, send it to a lens repair shop. By education I'm a Mechanical Engineer and some times lens and small cameras are very tedious to work on.
12-10-2017, 10:27 AM   #3
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Bumping up this old thread because I have the same issue with a well-worn late variant SMC Pentax 500mm F4.5. I’ve had this lens for years but have never used it. Now that I have an MX body on the way, I’d like to repair it.

I’m leery of shipping it anywhere because the baffle is bouncing around and can contact what appears to be the rear of the second element. This is definitely not a collectors condition lens, so I’m willing to try repairing it myself first. I am a hobby model maker, so I do have tools and am used to working with precision items - just not lenses.

Can anyone help with specific tear down instructions, or even a photo or diagram showing how and where this lens comes apart?

Thanks!
12-10-2017, 11:45 AM - 1 Like   #4
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This may not be very helpful... BUT this is very close to a telescope, in that there is nothing between the front elements and the rear element, except for the baffles you describe.

The entire front element is threaded onto the body—no set screws. It may be possible to screw off the front lens assembly and reach down the tube to secure the baffle that worries you. It is at least worth a look to remove the front lens assembly, marking its original position, of course.

Good Luck

12-10-2017, 01:00 PM   #5
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Thank you! Are you saying the entire front element pair comes out as a cell? If so, is the cell internal to the barrel or does the barrel simply screw apart?
12-10-2017, 01:43 PM - 1 Like   #6
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Just took my 500mm f4.5 K mount and removed the whole front cell. The set screws are only for the sighting pip and need not be touched. Front element is all in front of the sighting ring.

The threading is very fine and takes a dozen or more turns to remove the cell. When I opened mine, I could see the baffle you're talking about it appears to be plastic and is shaped with several pointed bits around the edge. I did not reach in, since mine hasn't moved. I leave for you to try it.

Please post your results...
12-10-2017, 01:55 PM   #7
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Thank you! I need to find my rubber strap wrenches, and then I’ll give that a try. I’ll report back!

12-17-2017, 12:17 PM - 1 Like   #8
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The entire front end will turn off fairly easily, and the front end comprises of the 3 front elements. Diagrams in link below. I have removed the front end to clean out dust and haze on my Tak-500/4.5 which is identical except for M42 mount.


http://whitemetal.com/pentax/smct_500_45/index.htm
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/266306-rep...0mm-4-5-a.html

Last edited by KevinR; 12-17-2017 at 12:35 PM.
12-17-2017, 07:41 PM   #9
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Kevin, I saw your original post - the setscrews can be left alone, right?
12-18-2017, 11:20 AM - 1 Like   #10
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yes, @squareeyes is correct. They locate the sight ring but nothing else.
12-18-2017, 02:21 PM   #11
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Thanks for the confirmation. My cheap rubber strap wrenches have gone walkabout, so I’ll try to stop at the store tonight and grab a new pair. Light a candle for me! Lol
12-18-2017, 07:53 PM   #12
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Ok, the lens is open. Now I’m looking for my JB Weld...
12-19-2017, 11:29 AM   #13
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I hope no small animals crawl in while you're looking!
12-20-2017, 08:02 AM   #14
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Lol - even mid sized ones!

The baffle is reattached, I’ll clean the inside of the lens up this weekend and reassemble. So far, so good.
12-20-2017, 08:46 AM   #15
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Good job, Ken!
I can rest easy now.
ROn
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