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10-15-2012, 05:19 PM   #1
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Different exposition values in Av and Manual

Using a M42 lens in my Kx I got very different exposition with the same diaphragm, lightning conditions and scene, when in Av from Manual mode (using green button)

10-15-2012, 06:02 PM   #2
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What exactly is "very different"? Half a stop? A full stop? Three stops?
10-15-2012, 06:36 PM   #3
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QuoteOriginally posted by elliott Quote
What exactly is "very different"? Half a stop? A full stop? Three stops?
1 – 2 stops are not uncommon (dependent on f). See these posts:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/200075-pentax-rico...ml#post2113449 (follow the external link)
https://www.pentaxforums.com/forums/pentax-dslr-discussion/200075-pentax-rico...ml#post2113755

Last edited by Alfie; 10-16-2012 at 03:39 AM.
10-15-2012, 07:01 PM   #4
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There is definitely a difference on my K100D, using green button seems to be more accurate for me, but usually the difference is only a stop at the most which isn't the biggest deal.

10-15-2012, 10:38 PM   #5
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Opens & closes the diafragm smoothtly?
10-16-2012, 05:42 AM   #6
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I get the same thing on my K100D with all my Takumars, which are the only lenses I use. AV mode is always 1.3 stops under compared to M, so I just leave it set at that value permanently. Can't for the life of me figure out what causes it though, since it's the same metering sensor looking through exactly the same aperture in both modes. One of those little mysteries that makes me think I'm not really built for the digital era. . .
10-16-2012, 08:28 AM   #7
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Very different is from 1 to 2 stops even more sometimes. I could not figure out a fixed pattern in the behavior, but seems to me that it varies according to ligth condition: less light more stops differences. I was wondering if the measuring range are changed by the OS when allowed non A position. I use spot metering always.
10-16-2012, 09:02 AM   #8
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Av mode with non-A or DA/FA lenses is supposed to be used with the aperture wide open. Metering wide open in Av vs M stop-down metering with or with or without the green button yields closer results. Green button in M vs stop-down via DOF preview (on my K10D) usually meter the same. There must be some sort of compensation going on when the situation is assumed not to be wide open.

It's a pain, but if I'm using Av mode with a non-A lens I'll set the exposure compensation in USER mode.

10-16-2012, 11:48 AM   #9
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Yes, a DSLR is designed to work with lenses that identify their maximum apertures using the electrical contacts on the mount, and I know that I'm really not entitled to complain when my DSLR doesn't play nice with a 50 year old Takumar. And I'm not complaining; I'm really not. I just use +1.3 stops compensation in AV mode and snap away quite happily. But it would be satisfying to know quite why the camera's firmware processes the meter reading so differently depending on the mode selected.
10-16-2012, 04:03 PM   #10
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it may have to do with the fact that the M42 lenses usually do not short any lens contacts where as in Manual green button, the camera knows there is a manual lens connected.

I agree it should be smart enough to figure out, but......
10-16-2012, 06:05 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
it may have to do with the fact that the M42 lenses usually do not short any lens contacts where as in Manual green button, the camera knows there is a manual lens connected.

I agree it should be smart enough to figure out, but......
If that is the case, might you get different results if you use a Pentax (or clone) style adapter vs a flange style one? This might be something that could easily be tested.
10-17-2012, 06:46 PM   #12
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I tried shorting the pins out with foil on some of my M42 mounts that are wide enough to touch the pins. On my K10D the only change was that it enabled catch-in-focus. Results may vary with different bodies as some people report this solves the exposure issues.
10-18-2012, 03:48 PM   #13
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I used to pay close attention to these types of issues with my K10d, and I have to say that in its case, the amount of "error" in exposure actually varied with the aperture. I don't remember the details as it has been years, but it would over-expose at one end and under-expose at the other end. I also recall that this was documented quite a bit with M type lenses not just the M42. i.e. if you were manually adjusting the aperture, you were going to see this problem.

I actually never bothered to see if it was corrected with my K5. I don't vary my aperture much anymore and I don't swap lenses much any more especially when I am shooting an old M lens. As a result, I typically set things up once an adjust the exposure compensation as needed, and regardless of the lens I find that I am making some type of adjustment that always seems reasonable based on the lighting.
12-12-2012, 08:09 AM   #14
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Just found this thread and wanted to add my experience.

My K-x differs two full stops between Av and M modes on two of the three M42 lenses I own. I found that my Helios (the one that meters correctly in Av) shorted the contacts on the mount while the other did not. So I tried putting some aluminium foil between lens and mount to short the contacts when I attached the other two lenses and it worked, I now get the same exposure in both Av and M modes with those lenses as well.

So FWIW shorting the contacts seems to work on the K-x at least.
12-12-2012, 11:00 AM   #15
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QuoteOriginally posted by enemix Quote
Just found this thread and wanted to add my experience.

My K-x differs two full stops between Av and M modes on two of the three M42 lenses I own. I found that my Helios (the one that meters correctly in Av) shorted the contacts on the mount while the other did not. So I tried putting some aluminium foil between lens and mount to short the contacts when I attached the other two lenses and it worked, I now get the same exposure in both Av and M modes with those lenses as well.

So FWIW shorting the contacts seems to work on the K-x at least.
Great reply, from a K-x owner like me. I am sorry but you put the foil over all contacts? Because we must use un adapter that I made permanent in my M42 lens, and dig a hole for the lens to lock, so the mount pin lock them. Now I must care for the mount contacts to be shorted some how, that s it? All of them? No mater what information the contact pass! It will work like that...amazing, I am going to try this as soon I get home! thanks a lot!
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