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04-11-2011, 08:54 PM   #1
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m28/2.8 - focus to infinity...and beyond

Something weird. All of a sudden (like all was fine this morning, and then not ok this afternoon), my m28/2.8 focuses at "infinity" between 5-10 ft. Beyond, it gets blurry, as if the focal flange length is off.

I threw the m50/1.4 and 1.7 up on the k7 as soon as I got home and they're good at infinity at infinity.

What do you guys make of this?

04-11-2011, 09:34 PM - 1 Like   #2
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Finding Infinity again...

You have a situation where the three lock screws have come loose. This is due to either age has caused the failure of the "thread-lock" applied to the screws or the lens had been repaired and no new "thread-lock" was reapplied. The trick is to reset the infinity... you will need to open the lens. The procedure is easy...

Spin off the name ring at the front...try wearing rubber gloves to increase friction.
locate and remove the three shroud screws with proper jeweler's driver.
locate the three lock screws that will be under the shroud
adjust infinity first and then loosen screws, but don't remove is the tiny washers are a pain
rotate focus ring to infinity and re-lock one screw
check your results on the camera and try to verify in good light at a distant target
if it all looks good, lock all screws and remount shroud and finally name plate.
04-11-2011, 09:42 PM   #3
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It would have been really interesting if the other M lenses showed a different infinity. You might have been in a Dr. Who episode.

The most likely problem is the three screws holding the focus ring to the lens have loosened and the focus ring has slipped. You shouldn't be able to focus quite as closely as the lens used to. Eventually you'll have to fix this or have it fixed, as the ring will continue to shift. It's not that hard if you can get the "name" ring (where SMC Pentax-M is engraved) unscrewed. If I remember right, you'll see something like this:



except the front element of the M28 is smaller. Remove the filter ring by removing the screws marked with red arrows. Then you can get to the focus ring screws. Since you can focus to infinity now, the adjustment is easy. Set the camera up on a tripod, focus on Kaho'olawe or something like that, loosen the screws, turn the focus ring until it points to infinity again, and tighten.
04-11-2011, 09:46 PM   #4
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Hmm. My DA55-300 goes a litle past infinity. Wonder if it is as easy to adjust?

04-11-2011, 09:50 PM   #5
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OK, maybe you can't see Kaho'olawe from Makawao.
04-11-2011, 09:53 PM   #6
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QuoteOriginally posted by shiner Quote
Hmm. My DA55-300 goes a litle past infinity. Wonder if it is as easy to adjust?
No, and you don't want to. The AF system works better if it is allowed to go too far and back up. Also with some zooms, infinity is different at the long and short ends, so the lens must allow for that.
04-12-2011, 07:25 AM   #7
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QuoteOriginally posted by Just1MoreDave Quote
No, and you don't want to. The AF system works better if it is allowed to go too far and back up. Also with some zooms, infinity is different at the long and short ends, so the lens must allow for that.
Thanks- that explanation actually made me feel better about all my zooms.
I use manual focus a lot.
04-12-2011, 09:47 AM   #8
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Thanks Onion, Thanks Dave for the photo and instruction. Good to know I wasn't going nuts, and glad to know it's a relatively quick fix.

When I get a chance to work on the ring, I'll target Molokai, which is right on the horizon. Right now, I'm seeing Siberia.

04-12-2011, 09:33 PM   #9
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Woohoo

Thanks for the help folks.

Got it fixed up.

Taking off the faceplate got me to this:

(standard/flat head screws...when I bought the lens, there weren't any screws. Stuck some eyeglass screws in there )

Then I had to take off the filter mount to see:


loosed all three screws and mounted the lens on camera & found infinity. Set the screws back in place.

Couldn't have done it without your help.

Thanks!
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