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09-16-2011, 02:42 PM   #1
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Mirror flopping when metering w/ green button and old m42 lens

I've read several topics about various strange mirror actions relating to old lenses, and this is one more but with a very obvious cause. I got an old m42 lens in the mail today, it's an Accura Supertel f/2.8 135mm and it's a lovely old lens, but it's the first to give me issues with the mirror flopping when metering with the green button. It does meter correctly when it does this, but it puts unnecessary wear and tear on the mechanism. It's also somewhat intermittent... but it happens more often than not.

Taking a look at the mounted lens, I was surprised to see that it's only partially covering the contacts. I'm pretty sure this is the issue (what else could it be?). Here's a pic of what's going on:



I tried putting a folded strip of aluminum foil over the contacts, using the lens to hold it in place, and that didn't help. I'm thinking I'll try a non-conductive strip next, as this doesn't happen with my extension tubes, which are painted black where they touch the contacts.

If anyone has any other suggestions, I'd appreciate hearing them. If nothing else, I figure this post and picture will be informative for other people who run into the issue.

09-16-2011, 02:48 PM - 1 Like   #2
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Wait! I think I know! Go to the EXIF info and look for "Lens type". It should be "M42 or No Lens". If not, then this is your problem. When flipping the mirror it usually tells me it is an "A Series Lens".
09-16-2011, 03:52 PM   #3
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It just says "Unknown K lens (Id: 1, 1)" in the EXIF. Glad you found this topic, I'm still going to link to it in your thread. I figured this is a broader issue than just the K-r, so maybe better to put it here.

So, my second thought was correct, a strip of scotch tape on the lens mount completely resolves the issue, by insulating the contacts from the lens mount. Easy fix for anyone else who runs into this issue.

EDIT: I've been trying different ways to insulate the lens from the contacts, and the best I've come up with is to cut a few small rings of paper that act as a kind of gasket between the lens and mount. Nice and easy, keep a few extras in my bag, and it doesn't require putting paint or anything on the lens itself.

Last edited by Philoslothical; 09-18-2011 at 04:41 PM.
10-18-2011, 03:22 PM   #4
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I'm confused. Why are you using the green button at all? I shoot with M42 lenses too, and I can't think of any reason to even touch that button.

10-18-2011, 05:17 PM   #5
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Have a read here. The green button is used for manual metering on the K-x and K-r, possibly others. I think the K-7 and K-5 let you use another button, but I'm not sure on that.

How do you currently meter with old m42s?
10-18-2011, 10:50 PM   #6
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That seems to be a preset lens. You just put the camera in Av, turn the aperture ring, look through the view finder and see what happens. As there are no aperture blade that connect with the camera, as with other k mount lenses, there is no need to use the green button, as you are always in control of the aperture by using the aperture ring
10-18-2011, 11:12 PM   #7
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I wish. It doesn't meter correctly in Av, though I'll slap it on for a bit tomorrow just to confirm that. I do like to use manual lenses in M mode though, so it's not a big deal. I just thought the information about insulating the contacts might prove useful to someone, as it's pretty obviously a preventable malfunction.
10-19-2011, 12:10 AM   #8
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QuoteOriginally posted by Philoslothical Quote
I wish. It doesn't meter correctly in Av, though I'll slap it on for a bit tomorrow just to confirm that. I do like to use manual lenses in M mode though, so it's not a big deal. I just thought the information about insulating the contacts might prove useful to someone, as it's pretty obviously a preventable malfunction.
David, I can't remember my presets doing anything when pressing the green button in M mode (not near them at the moment), and I don't think it should have an influence on the mirror, or can be good in the long run, so I appreciate the info. As I actually need that connection with the contacts as I'm using CIF, I actually had to make some shims to establish that connection. The beauty of the M42 lenses IMO, is that it is all manual, without the problem of using the green button, and should meter correctly. You can actually see the difference between f stops in the viewfinder, where using the green button, you leave it to the camera to decide?

My Cunor 200mm seems to have a similar mount than your lens. The lens itself is actually a 46mm with the silver 42mm adapter attached to it?

Edit: I'm using a K-x, don't know if that would make a difference?


Last edited by altopiet; 10-19-2011 at 12:16 AM.
10-19-2011, 02:37 AM   #9
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David, revisiting the thread, I now see the image for the first time. I seems clear that there might be some contact between the lens and contacts, but not proper contact at all times, which might be why the problem is intermittent. I also have a problem with my mount, in that the silver adapter, although conductive, on the inside has a plastic type of step up thread, causing the camera not to "see" a lens at all. I had to remove the silver mount and get proper contact between the lens body and the silver adapter. If you are sure that the lens and adapter actually makes contact in such a way that conductivity is positive, and that the contacts are shorted correctly, might that not solve the problem? I've heard about the mirror flop on the K-5, and if what you experience when using the lens and green button, is the same, i.e. not the normal moving of the mirror to measure the light, I do see that it would be safer to insulate the contacts.
Regarding the metering, I've taken a few shots with an old k mount Vivitar which I converted by removing the aperture lever, and aperture ring ball bearing, making it behave as an old preset M42.
I've fixed the ISO to 100, set the lens to 3.8 (wide open), and got the same shutter speed when shooting the same shot in M,using the green button, as in Av, The results at f5.6 also produced the same shutter speed, so I would be interested to know why your lens do not measure correctly in Av
10-19-2011, 11:38 AM   #10
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OK, so I just had a bit of time to test this. Shooting indoors, near a window, and in M mode it set a shutter speed of 1/30th second with the green button, and the shot was more or less properly exposed. In Av mode, it drifted around 1/125th to 1/160th second on the same subject, and was badly underexposed.

I tried this a couple more times in different spots, with similar results. Av just isn't metering properly with this lens. At one point it tried to use 1/4000th second in Av, and when I switched to M and hit the button, it set 1/640th second, which again, was more or less correct.
10-19-2011, 02:15 PM   #11
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That is odd that some folks have to use some button other than the shutter release to meter with. On my K20D, Minolta Maxxum 7D, and Sony A100 all I have to do is half press the shutter release like normal. That is of course with the body set to Aperture priority mode.

Philo: Are you...
1. ...metering with the lens stopped down to the aperure you want to use to take the shot with...
2. ...opening up the aperture to focus (if necessary)...
3. ...then stopping the lens back down to take the shot?

Have you checked to see that the aperture blades are even working correctly?
10-19-2011, 02:57 PM   #12
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QuoteOriginally posted by zekewhipper Quote
That is odd that some folks have to use some button other than the shutter release to meter with. On my K20D, Minolta Maxxum 7D, and Sony A100 all I have to do is half press the shutter release like normal. That is of course with the body set to Aperture priority mode.
According to this page: Using Older Lenses on Pentax K10D, K200D, K20D | Pentax Support it's normal for you to be using the green button for metering with these kinds of lenses as well, and it warns specifically about exposure issues when using them in Av mode. My understanding of it is that the camera will make the best guess it can in Av, but that it underexposes as often as it gets it right. A couple old threads on the forum here seem to confirm this, with the K20D being off by as much as 2 stops in Av with old lenses. You might want to experiment more with using M mode in this situation, and see if you get better results.


QuoteOriginally posted by zekewhipper Quote
Philo: Are you...
1. ...metering with the lens stopped down to the aperure you want to use to take the shot with...
2. ...opening up the aperture to focus (if necessary)...
3. ...then stopping the lens back down to take the shot?

Have you checked to see that the aperture blades are even working correctly?

That's how it works, yes. I tend to use these lenses when I want a larger aperture anyway, so I don't usually run into problems focusing when stopped down. When needed, the second aperture ring makes it more convenient by acting as a physical stop for the first, so you can easily get back down to the metered aperture without taking your eye from the camera.

The aperture blades in this lens are perfect, all 16 of them. The way this lens handles specular highlights in the bokeh is the main reason I (occasionally) use it.
10-19-2011, 10:56 PM   #13
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what I do with M42 lenses is use them in AV mode.

some lenses, on some bodies do have a 1-2 stop exposure issue, but that is what EV correction is for.

use a paved road or other uniform surface and take a shot . Check the histogram, and adjust the cev correction until it is centered, then you are metering correctly
10-22-2011, 03:59 PM   #14
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I thought that the mirror mechanism doesn't move when metering with M42 or K mount lenses? As the metering isn't done by the sensor.

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/143909-green-button.html

As for metering with green button over Av mode. I've always done green button. It is more accurate then Av as you usually have to do exposure compensation and if your in action sequences you may miss the action.
10-22-2011, 04:24 PM   #15
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QuoteOriginally posted by epqwerty Quote
I thought that the mirror mechanism doesn't move when metering with M42 or K mount lenses?
That's correct, it isn't supposed to. It's a malfunction due to improper contact between lens and mount contacts, in my case, so I posted this topic to help others work around it if they ran into the same thing.
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