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09-10-2016, 05:36 AM   #1
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K-S2 Meter off with M42 Lenses when in OVF

Hello

Noticed this with my new K-S2. Whenever I use a M42 screwmount lens (tested: Super-Tak 35/2, Pentacon 30/3.5, Mir-1B) the meter will give a reading that would overexpose the entire scene by 1/3 to 1 stop when using the OVF, but it is fine when using live view. Didn't see this on my K mount lenses (tested: Ensinor 24/2.8 Macro, Tokina 28/2.8). Any suggestions?

Sincerely

09-10-2016, 05:53 AM   #2
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Mine does the same thing but with all lenses. Kinda like the camera is doing some ETTR on it's own. I've seen it mentioned elsewhere too.
I just dial in -1/2 EV Comp and it works well. Also, IIRC, when you use an M42 lens your metering switches to Center Weighted rather than Multi-Point.
Good Luck.
09-10-2016, 07:10 AM   #3
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I honestly wonder if the center weighting and the meter tuning is the reason. I see this with A series lenses as well. I use exposure compensation of -1 typically with most A series lenses.
09-10-2016, 07:15 AM   #4
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QuoteOriginally posted by UncleVanya Quote
I honestly wonder if the center weighting and the meter tuning is the reason. I see this with A series lenses as well. I use exposure compensation of -1 typically with most A series lenses.
My M and A lenses don't do this, only M42s.

09-10-2016, 07:31 AM   #5
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QuoteOriginally posted by butangmucat Quote
My M and A lenses don't do this, only M42s.
My experience is different... Interesting. I honestly can say that i oversimplified, some A series need adjustment, some don't.
09-10-2016, 08:58 AM   #6
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I can't see that we should expect any better frankly, and it's rather good what we actually get in this respect ... To be able to couple technologies that are 50 years or so apart is fairly extraordinary I feel.


As I see it it's a case of recognising these variations and compensating and being glad if the variation is fairly consistent throughout the aperture range for each lens.
09-10-2016, 09:25 AM - 1 Like   #7
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There could be a bunch of reasons. For example, the lens might have light leaks, might have internal reflections (especially if the inside of the lens and its aperture blades are shiny), might render a too large image circle.. All of these things make light bounce inside of the mirrorbox. You don't necessarily see this as a big optical problem, just lower overall contrast, but I think it could confuse the camera in some way.
Making sure the mount fits nicely and using a tight lens hood (APSC FoV) could help.

But expecting perfect metring from 60 year old lenses might be too much to begin with
09-10-2016, 12:28 PM - 2 Likes   #8
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QuoteOriginally posted by butangmucat Quote
Whenever I use a M42 screwmount lens (tested: Super-Tak 35/2, Pentacon 30/3.5, Mir-1B) the meter will give a reading that would overexpose the entire scene by 1/3 to 1 stop when using the OVF, but it is fine when using live view.
There are long standing issues with stop-down metering that are common to all PDAF dSLR cameras across brands. The full description of the issue and its cause are complex, but it is enough to say that things are much better now than when I first started shooting non-A lenses in the 2007 with the K10D.

The reason things are consistently good in live view is that a different metering system is used (the sensor itself) rather than that used by the optical viewfinder.

QuoteOriginally posted by UncleVanya Quote
I honestly wonder if the center weighting and the meter tuning is the reason.
Undoubtedly using center-weighted rather than matrix may influence the reading for some subjects, but it is "meter tuning" that is responsible when stop-down metering fails.

QuoteOriginally posted by butangmucat Quote
My M and A lenses don't do this, only M42s.
Play with your lenses some more. An A-series lens should work seamlessly with all meter and exposure modes, though not quite as well with P-TTL flash. The meter should have problems with your M-series glass similar to what your are seeing with M42. Note that this is a metering problem, not a lens problem. This is apparent when one considers that we are talking about stop-down metering where the light to the meter is supposed to be the same as the light to the sensor.

QuoteOriginally posted by Na Horuk Quote
But expecting perfect metring from 60 year old lenses might be too much to begin with
Lenses don't meter. A 60 year-old CZJ will stop down the same for metering regardless of whether it mounted to a Spotmatic, an ME Super, a L-3II, or a K-1. That being said, only the first two bodies will provide consistently accurate stop-down readings through the full aperture range when metered against a standardized light source. The same is true for a recent production Z-series Zeiss in M42 mount.

This topic was explored in extreme depth when the K10D and K20D were new and was fairly well characterized at that time, though not with complete understanding. Then, as with now, Hoya/Pentax was not interested in robust performance with legacy glass any more than Ricoh/Pentax is now. Here is the general characterization:
  • Stop-down metering with the optical viewfinder may yield inconsistent results regardless of whether Av or M mode is used. In concise terms, meter response is not linear through the full aperture range.
  • The degree to which this is true varies by lens
  • For a given lens, meter readings may only be linear through part of the aperture range and generally will not be at wider apertures when approaching fully stopped down*
  • Pentax models from the K-7 forward are less variable than the K10D/K20D
  • Exposure using a hand-held meter and M mode will be consistently good
Causes are much more difficult to pin down. Here are the pertinent points, some of which are conjecture :
  • Due to a good portion of the light being siphoned off for use by the PDAF system (partial-silvered main mirror), focus screens on PDAF dSLRs are generally designed to present exaggerated brightness for a given aperture. The degree to which this is true appears to be aperture dependent. (How this works is a mystery to me.)
  • In order for open-aperture metering to work, the exposure system must compensate for the screen brightening by applying a correction. The amount apparently varies according to the maximum aperture communicated by the mount contacts. When the maximum aperture information is missing, a default program is substituted.
  • Unavoidable deficiencies in the default program are the proposed cause of the metering issues.
Note that this is not something that can easily be addressed using exposure compensation. The degree of reliability varies by lens and the degree to which the meter system reading varies from true varies by aperture.

Back when this was a hot issue, I graphed the variance of measured vs. expected for the non-A lenses on my shelf. I also did stop-down vs. wide-open for my A-series and FA-series lenses. After going through the exercise, I came to the conclusion that stop-down accuracy is generally pretty good between f/4 and f/8 and not so good either side of those apertures. In practice:
  • I generally expect essentially accurate readings in that range (f/4 - f/8) and expect wild variation wide open and at narrow apertures
  • I chimp freely and use the histogram as a guide for determining a working EV and adjust shutter speed to match aperture once the EV is determined
  • There is no shame in going full manual with a hand-held meter
  • Live view is generally dependable, Av mode for M42 and M mode for K-mount
I hope all this is clear and helps.


Steve

* At narrower apertures when using stop-down metering, the lower limit of meter sensitivity may become an issue. As that limit is approached, linearity suffers. The manuals for the Pentax Spotmatic and similar stop-down meter cameras often included a table showing the EV values where the meter is linear. The Spotmatic goes even further and provides a visual indicator of invalid film speed/shutter speed combinations.


Last edited by stevebrot; 09-10-2016 at 02:25 PM.
09-10-2016, 12:35 PM   #9
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^^^ Thanks for posting this, Steve - very interesting and useful.

I've become used to the weird metering when using my rolder lenses. Some of the time I remember what amount of compensation I need on a particular lens at a particular aperture, but other times I forget. Like you, I chimp, then if necessary I adjust and re-take the shot - not a problem for me generally, as I rarely use my older manual lenses for anything other than considered shots...
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