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05-01-2013, 06:19 AM   #1
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Pentax smc 50mm f1.7 , unable to turn to f1.7

Hi All

I have inherited a Pentax SMC M 50mm f1.7 lens on a old ME Super and the aperture ring doesn't turn to 1.7, it only turns to about 4. Looking thru the lens the blades never seem to open completely. I assume this is not normal behaviour? The blades open completely when I move the (stop down?) lever on the back of the lens. Any ideas what could cause this and if it's easily fixed? Needless to say I'd like to get this lens working correctly if I can so I can use it on my recently purchased (but not yet delivered ) K-x.

05-01-2013, 06:40 AM   #2
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Too bad, thats a great lens in terms of IQ. So the aperture actually does open when you move the lever? Maybe a camera will work with it normally? You can try disassembling it and making sure the aperture ring is all right. Or send it to a repair store and get it fixed for 50 bucks. Or buy another one on the used market for $70 plus shipping (they were pretty cheap a couple years ago, but they are getting more popular and expensive now)
05-01-2013, 06:54 AM   #3
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QuoteOriginally posted by Rob Dean Quote
The blades open completely when I move the (stop down?) lever on the back of the lens
Does it look like the lens was dismantled at any stage, marks on the screws on the back mount, etc?
05-01-2013, 06:59 AM   #4
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No, doesn't appear to have been disassembled at any stage (and yes, the aperture opens completely with the lever) . I actually used this lens on the ME super to shoot some film about 5 years ago but can't remember if it had this problem or not.

05-01-2013, 07:20 AM   #5
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I think you should ask @Just1MoreDave, looking at this thread https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/91560-pentax-m...-assembly.html, he has knowledge about stripping the 1.7, and might know what is causing your problem...

EDIT: I've stripped mine a long time ago, but didn't keep a visual record of the process. All I can think of is that if the aperture comes to a dead stop at a certain point, something must be in the way causing the stop at that point. I had another lens that a "friend" borrowed, turning the aperture to far, causing damage to some of the blades, which resulted in something similar to what you describe, I hope that you're problem is not that serious...

Last edited by altopiet; 05-01-2013 at 07:39 AM.
05-01-2013, 09:19 AM   #6
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Take the lens off camera. Does it exhibit the same behaviour? Because bodies like the ME Super also tend to accumulate dirt in the aperture levers, which make the aperture ring impossible to turn.
05-01-2013, 08:01 PM   #7
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No, problem is still there with the lens off the camera. I'm guessing there must be something jamming the aperture ring......
05-02-2013, 12:16 PM   #8
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QuoteOriginally posted by Rob Dean Quote
No, problem is still there with the lens off the camera. I'm guessing there must be something jamming the aperture ring......
Perhaps the lens was dropped at some point?


Steve

05-02-2013, 05:07 PM   #9
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The only thing I can think of to do (without disassembly) is use an old toothbrush to brush the upper part of the aperture ring. There's a space there, maybe 1mm, which might have some grit jamming the ring.

Other than that, find a decent screwdriver, preferably JIS, and unscrew the mount screws. Then you should see this:



There's a spacer/washer that I always set aside and forget later. The photo helps figure out how to put it back on. You can slide off the aperture ring now but be careful of the 2mm bearing trapped under the orange diamond:



The problem should be obvious here. The aperture ring is metal so if you don't see any grit, check Steve's theory. It might not be round any more.
05-03-2013, 08:29 PM   #10
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QuoteOriginally posted by Just1MoreDave Quote
Then you should see this:
Dave's thumbprint? You must get around!


Steve
05-03-2013, 08:32 PM   #11
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QuoteOriginally posted by Just1MoreDave Quote
It might not be round any more.
Or the internal working are jammed due to shock. I have seen lenses where the main body looked just fine, but the internals had fractured and the various pieces were no longer able to slide past each other as they should.


Steve
05-05-2013, 03:22 AM   #12
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Thanks everyone, the pictures and advice above where exactly what I was after. Now to get some time to have a proper look at the lens :-P
05-07-2013, 03:12 AM   #13
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I have a somewhat usable Pentax-A 50mm 1:1.7 with aperture problems. In general the aperture ring is a bit 'sticky' and if I turn it to 1.7 the blades won't open completely and will stay at around 2.8. If I turn the aperture ring to 1.7 very fast, the blades open completely. From the outside the lens is perfect, looks like new. It's hard to believe it was dropped at any point. I also have a 50mm M lens with a very obvious dent from a drop that works mechanically perfectly (glass is damaged).



I dropped this into a lake with a *istDS atteched to it. I'm planning to take it apart and document it properly.
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