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05-02-2008, 12:18 PM   #1
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Help me diagnose a lens problem


Your advice is requested!

I recently purchased a SMC M 85mm f2 from an on-line store. The lens is in fair to good shape... it's seen better days overall, but the optics are clear and brassing is minimal.

I am using it on a K10D in M mode. Center Weighted metering. I am not using a hood.

My problem: I've noted on f stops ranging from 22 - 5.6 that every few images will suddenly be waaay overexposed. I might get one or two okay exposures (looking at the histogram) and then a couple of shots that are completely blown out. I see no pattern nor can I tie it to a certain kind of image composition.

Now ... Shots taken at faster stops (f2 - 5.6) the problem seems reversed... shots will come out underexposed a couple of stops. If I reshoot the same composition quickly with no changes to the camera, I'll suddenly get a good one. This underexposure problem is not as frequent as the overexposure problem

I have other manual Pentax lenses and while every so often I might get a weird exposure, it's no where near as frequent as with this 85mm f2.

I've worked the iris while the lens is off the camera. It's not as snappy as my other manual lenses but my finger doesn't sense any obvious binding.

Any ideas? Is there any thing short of sending the lens to the camera shop that I can test out? Other than this pesky issue with weird exposures, the lens is lovely. I'd hate to send it back.

Thanks in advance for your comments.


Austin, TX

05-02-2008, 12:35 PM   #2
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see if this helps answer your question:

However, it's seem strange that you dont see it on all your lens in manual mode.
05-02-2008, 12:37 PM   #3
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Have you cleaned all your lens contacts
That would be where I'd start with any problems like that.
05-02-2008, 02:55 PM   #4
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Firmware fix?

Thanks for the response guys!

I read in one of the links provided that the K10D overexposure issue may have been addressed in a K10D firmware update?


I'm still at 1.0... is there any truth to this firmware rumor? The Pentax site does not list this as a fix in any of the builds.

Can anyone confirm this?



05-02-2008, 03:17 PM   #5
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QuoteOriginally posted by germar Quote
I read in one of the links provided that the K10D overexposure issue may have been addressed in a K10D firmware update?
It hasn't been.

1.3 is worth having, but it doesn't fix the stop-down metering issue.
05-02-2008, 03:52 PM   #6
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maybe just me, but i'd also check the aperture arm for any snags.. make sure it moves freely by hand, and snaps back open with ease, as well..
05-02-2008, 09:59 PM   #7
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I agree with OniFactor on checking your aperture for proper operation. Make sure the aperture blades open and close as the aperture ring is rotated. The blades should not move slowly to the next position. Set the aperture ring to highest f stop and open the aperture blades fully by actuating the aperture lever. The aperture blades should move smoothly.

I have two Sears brand lens that occasionaly overexpose and traced the problem to the aperture blades becoming stuck in the fully open position. I didn't notice any underexposure problems as I stopped using both lens. The lens are not worth the time or expense to fix them due to their relative worth. Your lens on the other hand should be worth it.

It sounds like your over and underexposure problems are caused by sticky aperture blades. The aperture blades of any lens are fully open when mounted on your camera until a picture is taken. The camera will stop the lens down to the aperture selected by the aperture ring, snap a picture and immediately open back up.

05-20-2008, 01:12 AM   #8
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i have exactly the same problem with my M85, how can i verify that its a problem with slow aperture blades?
thank you for comments!
05-20-2008, 06:40 PM   #9
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- take the lens off of your camera
- stop it down all the way
- with your finger use the switch on the back of the lens that the camera body uses to stop the lens down while shooting to open the aperature all the way
- release the switch quickly, and see if the aperature snaps into its stopped down position the way it is supposed to

If the aperature blades close slowly, then you just confirmed the source of your problem. I have an M42 lens with a slow aperature, but it doesn't matter because I just stop the lens down to the desired aperature manually and leave it there.
05-20-2008, 07:25 PM   #10
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This is a KNOWN problem with the K10D and M lenses using the "green" button to stop down meter. on mine, F4 is the sweet spot, it meters very well there, 5.6 isn't bad either, but a tiny bit over exposed. Under F4, it underexposes, above F5.6 it overexposes. See this post where i did a comparison of my K110D and K10D with green button metering on "M" lenses
05-21-2008, 12:16 AM   #11
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thank you!

@steinback: my aperture blade are working good!

@stratman: hmm, what we can do? the only way i found is to set the aperture time manualy, but its very difficult to find the right time.

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