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06-30-2018, 12:32 AM   #1
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SMC M 50mm f1.7 Infinity Alignment - Or Not...

Don't know if this is the right place for this...

Some time ago my 50mm lens stiffened up and wouldn't reach the infinity mark. I set it aside and got a 50mm f1.4 instead. When I retrieved the f1.7 lens from my cupboard, I can now set 'Infinity' on the barrel, but it corresponds to a distance of fifteen feet when the semicircles align.

With three feet set on the barrel, the semicircles align at 27 inches.

Any idea what's happened? The lens hasn't been roughly handled and the barrel now turns smoothly, but the focus doesn't go all the way. I'm quite prepared to have a go and fix it myself - I had a similar problem on an M42 Carl Zeiss Tessar f2.8 50mm where it was just a case of moving the inner section relative to the outer one. But I suspect the PK SMC M f1.7 is a different animal.


Anyone effected a similar repair? If so, is it a big job? All advice appreciated.


Last edited by Russell W. Barnes; 06-30-2018 at 01:54 AM. Reason: Critical word 'focus' added!
06-30-2018, 03:02 AM   #2
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I cannot provide guidance on how to adjust the infinity focus on a smc Pentax-M 50 1:1.7 but here are 2 links that might give you some ideas:
  • Here is a step by step instruction how to dismantle and clean a Pentax M 50 1.7: Fun with Fungus.! Unfortunately the pictures in this thread are inaccessible now, however I copied the thread with the pictures into a PDF some years ago, so if you need it, send me a PM.
  • Here is a description how to adjust the inifinity focus on a Pentax 50 2.0: Pentax 50mm repair 3 I guess the principle is the same as on the 1.7 version.
Good luck!
06-30-2018, 04:00 AM   #3
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win

I would like to win.
06-30-2018, 06:52 AM   #4
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When you say 'semicircles align', are you referring to the focus screen?

It definitely sounds like the helicoid grease has begun to seize up, so a complete disassembly and re-lube would be in order.
Further, if the lens shows the infinity mark lined up with the red diamond, yet you only measure 15ft to the actual point of focus,
that suggests the distance scale has rotated. It's been quite a while since I opened up a 50/1.7, but I do know that the distance
scale on some lenses can rotate if excessive force is applied. With a very stiff helicoid it is possible you inadvertently rotated
the scale while trying to focus.

06-30-2018, 08:04 AM - 1 Like   #5
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I have pictures in the cloud somewhere but it's quicker to do this than find them.

Start here. You want the filter threads to be free of grit. An old toothbrush might help. You need to unscrew the "name ring" with all the engraved writing with a friction tool. You can improvise here. The link above suggests sticking double sided tape to the ring and using an M42 lens cap to stick to the tape. That should work. The ring is threaded into the filter threads. If the ring doesn't move, you can try very tiny drops of penetrating oil if you can get it just on the filter threads. If you lube the name part, it's hard to get the friction thing working.



When the ring is off, you should see this. You want to remove the three screws that aren't in the little cutouts. They are JIS screws, so ideally you'd use a JIS screwdriver. A Phillips #00 will work OK on these particular screws in my experience. These hold on the front barrel. Lift that off.



Now you can see three more screws just inside the focus ring, in an aluminum ring. Continue to ignore the three screws you ignored previously. The screws you want are holding the focus ring to the focus helix. Don't remove them yet. Put the camera on a tripod and the lens on the camera. Point the camera at a distant object. Focus to the infinity stop on the lens. Loosen the three screws. Now you can turn the focus ring without moving the focus helix and changing the lens focus. Turn the focus ring back a few degrees, maybe to the 1 of the 15m mark. Tighten one screw and you can focus further towards infinity. Repeat those steps until you are happy with infinity focus. Tighten all the screws, put the focus barrel back on, put the name ring back on and you're done.



BTW, the photos show the aperture opening at f22. If your opening looks larger than this, you can adjust it while your lens is apart. Set the focus first. Then set the lens to f22 and loosen the three ignored screws. The lens assembly can rotate now. Rotate it until the blades open a little, then rotate it in the opposite direction until the blades just stop moving. Tighten one screw. Set the aperture to f1.7 and the blades should retract fully. Tighten all the screws when both extremes look good.

Last edited by Just1MoreDave; 06-30-2018 at 08:14 AM.
06-30-2018, 01:27 PM   #6
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Wow!

Many thanks for that! And thanks all for the advice: I'll print it out give it a go. Yes, I was referring to the focussing screen. I have one of those grippy rubber sheets for opening stubborn jars so I may be able to remove the name bezel with that.
06-30-2018, 02:32 PM   #7
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Done it!

What a fantastic resource this forum is: every day is a learning day. Regarding the helicoid grease, I can only think it softened up in the warmth of my cupboard.
06-30-2018, 07:07 PM   #8
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The prime lenses are easy like that, maybe a few exceptions.

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