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11-02-2012, 04:31 PM   #1
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Help! Pentax M 50mm 1.7 can't focus to infinity

I just got the Pentax M 50mm 1.7 from Ebay after reading great reviews from the forum. I can't focus the lens to infinity, it will only go to 0.7m . I can't turn the focusing ring past 0.7m as shown in the attachment. Everything else works great, this problem is really bugging me.

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11-02-2012, 04:48 PM   #2
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It could have been taken apart then assembled wrong
11-02-2012, 06:52 PM - 4 Likes   #3
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It looks like the focus ring isn't attached in the right place. The front barrel on your lens is sticking out about the right amount for infinity - or at least, a lot farther away than infinity. If you still can, send it back and get a refund.

Back in my day , the sellers would just laugh at the idea of a refund, so I learned to fix these myself. Turn the focus ring as far as you can towards infinity, without forcing it at all. You need something to unscrew the front trim ring that says SMC Pentax-M 1:1.7 50mm etc. Something rubber will work, like a chair leg protector, large rubber stopper, etc. Just make sure it doesn't touch the front element. The ring unscrews through the filter threads. Then your lens will look like this:

Use a #00 screwdriver to remove the screws marked with red arrows, leaving the ones in cutouts alone. Now you can take off the filter ring/front barrel and see this:

There should be three screws on your lens like the ones marked with red arrows. Loosen all three and the focus ring should turn without focusing the lens at all. It has two mechanical stops to keep it from rotating completely around but it should turn freely. Turn the focus ring until it points at the infinity mark. Then tighten one screw.

Now you need to set infinity more precisely. I think your lens is probably focusing slightly beyond infinity right now. Mount it on a camera and try to focus on something several miles away, the furthest thing you can see. If I'm right, you'll have to turn the focus ring back to maybe the 25 foot mark. When you're happy with the focus, loosen the one tight screw, turn the focus ring so the center of the infinity symbol lines up with the orange diamond, then tighten all three screws.

Optional maintenance: Some of these lenses are dirty inside so I usually have an old toothbrush handy to brush out debris. Then I'd check to see if the aperture is fully open at f1.7. You shouldn't see any of the aperture blades. If you want to adjust this, set the aperture ring to f1.7 and loosen the three screws that were in cutouts in the first photo. Then you can rotate the whole optical assembly. If you turn it one way, the blades disappear. Rotating the other way makes the blades stop down further. Rotate the assembly until the blades just disappear, then tighten the screws. Check to see if they continue to stop down at each position on the aperture ring. Then tighten all the screws.

Now you can reinstall the filter ring and trim ring and start using the lens.
11-02-2012, 07:03 PM   #4
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having taken apart many Pentacon's Ziess,s and other random German lenses.. These look a treat to work on!

11-02-2012, 08:01 PM   #5
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Thanks for the instruction, I tried removing the bezel with the same method shown in this video
, the thing won't budge at all. I am going to Lowes tomorrow to something with more friction. Are there any specific rubber product you recommend? Thanks in advance.
11-02-2012, 08:42 PM   #6
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Or you may simply contact the seller first. If he will not accept a return, ask for a refund of all but the value of the lens for salvage purposes (~$5-$10 USD, assuming the optics are clean and scratch free). If the seller is non-responsive, open a dispute with PayPal and leave negative feedback with eBay.

Attempt to fix the lens yourself as a last resort and then, only after exhausting all avenues with the seller. Bad product on eBay and even here on Pentax Forums is becoming increasingly commonplace and it is up to the consumer to not accept junk.

11-02-2012, 09:16 PM   #7
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QuoteOriginally posted by zainy Quote
I am going to Lowes tomorrow to something with more friction. Are there any specific rubber product you recommend? Thanks in advance.
I use non-slip shelf liner wrapped around whatever I can find that will fit well on the name ring, sometimes I raid the spice shelf and use a cap from the basil or something.
11-03-2012, 01:37 PM   #8
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Something like this would work It may not be exactly the right size but once you find this aisle, look for something rubber and roughly 1.5-2" in diameter. You may have to cut out the center to keep it away from the glass and carve the edge to match the bevel of the trim ring.

11-03-2012, 06:49 PM   #9
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Thanks for the advice everyone. I bought assortments of rubber sink plugs. No matter how much pressure I applied and how I much I tried to twist, the bezel won't budge at all. I had no choice but to take apart the lens from the back. I figured out the problem: the front part of the lens wasn't aligned right. Someone did take the lens apart and failed to put it back together right. I unscrewed the 4 screws that held the 2 copper tabs that prevents the glasses from turning when the focus ring's turned. I removed that group (I hope I am using the right terms) from the barrel. It took me several tries to put the group in right so that the the group is aligned properly in the barrel. This was an enlightening experience and I got a chance to clean some dusts out. Thanks for all the encouragements.

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