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02-03-2010, 05:12 PM   #1
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Pentax-A 50mm 1:2 lens focussing ring problem/ disassembly help

I bought this lens off fleabay and it ended up coming in with the focussing ring rotating but not moving any of the lens elements. I'm attempting to open up the front elements. Posts around suggested that the name plate [serial number written] can be screwed off; however, I can't seem to get that out for the life of me. Would appreciate any help on this

thanks much...

02-03-2010, 08:13 PM   #2
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The lens was probably dropped. This problem is pretty common with the M50/2 and A50/2. I don't know whether it's a design flaw or just many more lenses produced. Or it might just have some loose screws. The steps to repair it are similar; I'll point out where they differ.

The fix is not that hard. It'll take longer for me to type this than do it. There's a plastic beveled trim ring around the front element. It's supposed to unscrew through the filter threads, but dropped lenses often have damaged or not-round threads. It's not that hard to pry the trim ring out in this case. There are even two slots at its edges for a really thin tool. (This is the ring you've read about in other posts, except on a Pentax-A lens, it doesn't have anything written on it.)

The trim ring hides three screws for the filter ring. I have a Phillips #00 that fits these screws well enough, and they aren't that tight. Remove them and the filter ring comes off. This is a good time to try and repair dents if you want.

Now there are three more screws which attach the focus ring to an aluminum track. More obvious screws hold down a brass washer that keeps the front lens group from falling out; don't bother with those. The screws you want are deeper, just inside the lettering SMC PENTAX-A 1:2 50mm. With luck they won't be hidden behind the ears for the lens mount. I was able to get my screwdriver in there anyway. The screws look similar to the ones marked with red arrows here, although this is a slightly different lens:

On your lens, these screws may just be loose. Try tightening them and turning the focus ring. If it turns and the lens focuses, skip the next paragraph and go on to adjusting infinity focus.

Your lens is jammed. Remove the screws you just tightened and the focus ring comes off. Now you see the aluminum ring. Careful inspection will reveal that it is not parallel with the plane of the lens mount. Look at the gap between its lower edge and the ring with the DOF scale. One side probably has no gap. You can use a knife blade or flat screwdriver to pop this aluminum ring back into place. Some care is required to avoid damaging the ring or DOF scale, but it shouldn't require much force. When it pops back, the aluminum ring should turn smoothly and the lens should focus again.

Move the aluminum ring so that the lens elements are as far back as they go, back towards the mount. Put the focus ring back on with the infinity symbol lined up with the orange diamond mark, and only one screw tight. Mount the lens on your camera and focus on a distant object, something that should be a few miles away. The distance scale probably reads 10 meters or so. The lens can focus "beyond infinity" now; you need to set infinity correctly or the space-time continuum will be damaged. Loosen the tight screw and move the focus ring without changing focus, until the infinity symbol again lines up with the diamond. When you're happy with that setting, tighten all the screws and continue reassembly.
02-04-2010, 11:01 AM   #3
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Hello Dave,

Thanks to your informative post, I was able to get quite far. I had to take out the aperture assembly to get the aluminium ring to sit correctly. Now the focus rings turns ok, but after assembling, I realized I screwed up the 'A' setting on the lens. What makes the camera see the 'A' setting for the lens?

Thanks again!
02-04-2010, 11:18 AM   #4
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I just got to your other tutorial:

I don't see a spring contact -- not sure I saw that when I opened the casing either. I did save the ball bearing.

Any suggestions to salvage the item would be appreciated...


Last edited by K7er; 02-04-2010 at 02:22 PM.
05-20-2011, 11:37 AM   #5
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Join Date: Jan 2010
Location: Twin Cities, MN
Posts: 41

I was using my identical lens this morning when it accidentally got bumped - not all that hard but evidently enough to jam the focus. I figured it was totalled (not worth enough to get it repaired) but thanks to google and this extremely informative post above, I was able to get it working perfectly.

I know this is an old thread but just wanted to say "thanks"!

04-06-2013, 04:27 AM   #6
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Join Date: Jul 2012
Location: Wiltshire
Posts: 14
Bought a 50mm SMC Pentax-A from ebay, got it home to find exactly this problem. Was going to rturn it but fixed in 5 mins with your info...thanks a million!

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