Originally posted by Adam 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:- Live view
- Av mode
- Lens A/M switch in M position
- Focus with lens wide open
- Adjust aperture ring to desired f-stop or shutter speed
- Take photo
- 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:- Live view
- M mode
- Frame subject and focus with lens wide open
- Stop lens down to desired aperture
- A/M switch in M position, press green button to set shutter speed
- Exit live view and use optical viewfinder
- A/M switch to A position (lens wide open) to frame and focus
- A/M switch to M position to shoot
- 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:- Live view
- M mode
- Select desired aperture
- Frame subject and rough focus
- Press green button to set shutter speed
- Exit live view and use optical viewfinder
- Frame and focus
- Take photo
- 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.