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01-28-2018, 10:12 AM   #1
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Over exposure with M42 lenses

Hi there,

I am using a K3 and I got over exposure problem with M42 lenses. I got a Jupiter 37A and a Volna 50mm macro both are uncouple lens, whenever I stop down the lens and re-meter with the green button I will get over exposed couple stops, I tried different modes, P, M, Av, Tv, Green mode, spot and center-weight, auto and manual ISO...etc. all the same result, I can only get the best result when wide open, but I don't have this problem with other coupled lens, for example the Samyang 85/1.4 and Pentax MF 50/2 all worked fine, any idea ?

01-28-2018, 10:37 AM   #2
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Look into the lens, is it actually stopping down when you take the picture ?
01-28-2018, 11:17 AM   #3
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Yes, and the view turned dark, I said over exposed was the image became washout and lower contrast I just don't feel right.
01-28-2018, 11:24 AM   #4
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I have had this problem, especially with the KS-2. The best solution is to shoot in Av mode and use exposure compensation to under expose the photo. Start at EV-1 and then adjust from there. You'll find a good general setting pretty quick.

On the K-S2, with m42 lenses, I was running EV-1.3 or -1.7 pretty consistently.

01-28-2018, 11:27 AM   #5
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QuoteOriginally posted by lotech Quote
Hi there,

I am using a K3 and I got over exposure problem with M42 lenses. I got a Jupiter 37A and a Volna 50mm macro both are uncouple lens, whenever I stop down the lens and re-meter with the green button I will get over exposed couple stops, I tried different modes, P, M, Av, Tv, Green mode, spot and center-weight, auto and manual ISO...etc. all the same result, I can only get the best result when wide open, but I don't have this problem with other coupled lens, for example the Samyang 85/1.4 and Pentax MF 50/2 all worked fine, any idea ?
Inconsistency in exposure with older lenses can vary from lens to lens. I've found that stop-down metering is often more accurate in live view, so you could use that with your more problematic lenses.

Adam
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01-28-2018, 11:35 AM   #6
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Yes I believe this will work.
QuoteOriginally posted by babywriter Quote
I have had this problem, especially with the KS-2. The best solution is to shoot in Av mode and use exposure compensation to under expose the photo. Start at EV-1 and then adjust from there. You'll find a good general setting pretty quick.

On the K-S2, with m42 lenses, I was running EV-1.3 or -1.7 pretty consistently.


---------- Post added 01-29-18 at 02:38 AM ----------

I think without coupling the camera doesn't know it is metering stop down,
and I've not tried LV thanks for the tip !
QuoteOriginally posted by Adam Quote
Inconsistency in exposure with older lenses can vary from lens to lens. I've found that stop-down metering is often more accurate in live view, so you could use that with your more problematic lenses.
01-28-2018, 12:49 PM - 1 Like   #7
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QuoteOriginally posted by Adam Quote
Inconsistency in exposure with older lenses can vary from lens to lens. I've found that stop-down metering is often more accurate in live view, so you could use that with your more problematic lenses.
What he said ^ ^ ^

Stop-down metering consistency and accuracy is better in current generation Pentax dSLRs than it was a decade ago*, but is still a problem. The severity and behavior of the issue varies with the lens mounted and with the chosen aperture. It is not unusual for a given lens to overexpose wide open, yield correct metering at moderate apertures, and underexpose at narrower apertures**. As a result, the exposure compensation is of limited value as a general solution.

The cause is related to how open-aperture metering is implemented to account for quirks introduced by the standard "brightened" focusing screens. Corrections have been added for M-mode in models since the K-7, but it has been unclear whether those have been migrated to Av mode. It is not unusual to get different shutter speeds for a manual aperture in Av vs. green button in M. I tend to avoid Av mode with manual aperture lenses despite the obvious usability advantage.

It occurred to me recently that since the metering inconsistency is related to the focus screen, live view might provide a better result. A few tests later and I was a convert for both non-A K-mount and for manual aperture M42. There are several ways to go:

M42 Battery Hog:
  1. Live view
  2. Av mode
  3. Lens A/M switch in M position
  4. Focus with lens wide open
  5. Adjust aperture ring to desired f-stop or shutter speed
  6. Take photo
  7. Repeat steps 4-6 as needed, staying in live view
Note that camera shake and arm/neck/back fatigue may be worse with this strategy. Ditto for focus accuracy.

M42 Battery Saver:
  1. Live view
  2. M mode
  3. Frame subject and focus with lens wide open
  4. Stop lens down to desired aperture
  5. A/M switch in M position, press green button to set shutter speed
  6. Exit live view and use optical viewfinder
  7. A/M switch to A position (lens wide open) to frame and focus
  8. A/M switch to M position to shoot
  9. Repeat 7-8 until either light or subject changes
This flow is essentially similar to using a pre-set lens with a stop-down metering camera back in the late 1960s when I started learning the craft. After a few shots, it is easy to see why lenses with auto aperture actuation became dominant. Note: with full manual aperture lenses (no A/M switch), one may "count clicks" as a means to stop down to the desired aperture.

K-mount with aperture ring:
  1. Live view
  2. M mode
  3. Select desired aperture
  4. Frame subject and rough focus
  5. Press green button to set shutter speed
  6. Exit live view and use optical viewfinder
  7. Frame and focus
  8. Take photo
  9. Repeat 7-8 until either light or subject changes
Easy peasy, eh?

All of the above assume center-average metering. Spot-metering works too, but care should be taken to place exposure where you actually want it. A common objection is that one gives up real-time exposure automation with the 2nd and 3rd flows, but my experience has been that real-time auto exposure is overrated for most subjects and may actually work against the photographer in difficult lighting conditions. There is a reason why I tend to carry a hand-held meter in the bag even with the K-3 and modern lenses.


Steve

* Both the K10D and K20D as well contemporary consumer bodies would regularly be off as much as 3-stops plus or minus, oftentimes with the same lens at different apertures.

** I would be remiss if I did not mention that stop-down metering at narrow apertures and/or low light is particularly limited by the meter sensitivity. As the threshold (EV -3 on my K-3) is approached, the meter will increasingly err towards underexposure. This happens much sooner with the lens stopped down than when it is wide open.

Last edited by stevebrot; 01-28-2018 at 01:31 PM.
01-28-2018, 12:52 PM   #8
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QuoteOriginally posted by lotech Quote
I think without coupling the camera doesn't know it is metering stop down
Yep...that is how it works. With a non-crippled mount, the adapter actually enforces the rule by engaging the coupling tab in the body to the max aperture position.


Steve

01-28-2018, 04:03 PM   #9
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Are either or both of these lenses "narrow" barrelled and thus don't cover the openings on the adapter? Light entering from the openings on the adapter would throw the exposure off. If this is the case you need to cover the openings. Some solutions are to use a large enough O-ring, tape over the openings or a gasket/collar of some opaque material like a cut up aluminum beverage can.
01-28-2018, 05:01 PM   #10
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QuoteOriginally posted by Not a Number Quote
Are either or both of these lenses "narrow" barrelled and thus don't cover the openings on the adapter? Light entering from the openings on the adapter would throw the exposure off. If this is the case you need to cover the openings. Some solutions are to use a large enough O-ring, tape over the openings or a gasket/collar of some opaque material like a cut up aluminum beverage can.
Good call. The Jupiter 37A is a long-running model with the possibility of several variants. That being said, the review on this site plainly shows the mount contacts not being covered for a least that version. Whether there is light leak at the adapter is anyone's guess, though it may be possible. Photos I was able to find of the Volna 50mm Macro seem to show a broad base with a generous mount face for that lens.


Steve

Last edited by stevebrot; 01-28-2018 at 05:11 PM.
01-28-2018, 11:43 PM   #11
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I just tried the LV and dead on exposure everytime, just little trouble to use and wasting
battery. I believe all AF/DSLR uses full matte screen and the metering mode is determined by the sensors only, I could be wrong, and I better off to purchase converted lenses instead of M42 to eliminate the problem completely.

QuoteOriginally posted by Adam Quote
Inconsistency in exposure with older lenses can vary from lens to lens. I've found that stop-down metering is often more accurate in live view, so you could use that with your more problematic lenses.
01-28-2018, 11:51 PM   #12
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QuoteOriginally posted by lotech Quote
I just tried the LV and dead on exposure everytime, just little trouble to use and wasting
battery. I believe all AF/DSLR uses full matte screen and the metering mode is determined by the sensors only, I could be wrong, and I better off to purchase converted lenses instead of M42 to eliminate the problem completely.
You might want to try M mode and green button with your M42 too with the optical viewfinder. I have found that it works better than Av.


Steve
01-29-2018, 12:07 AM   #13
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Right, that worked too, tnx !
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