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06-21-2009, 06:26 PM   #1
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K20D metering with older lenses (again)

I know that there are several old threads about this topic, but I have a question for which I couldn't find an answer.

The K20D (actually GX20) I'm using seems to meter accurately in Av mode for M42 lenses using manual stop down. In M mode, metering for older K mount lenses is inaccurate (using the green button for stop down) and I understand that a focussing screen swap (to the LL-60) should largely solve this issue. My question is, will the screen swap affect the metering of M42 lenses in aperture priority? I will probably be using more M42 lenses than anything else, so I don't want to mess that up if it's working fine at the moment. I would, however, also like to use old K mount lenses from time to time as well. If the LL-60 screen allows accurate metering with both then I will give it a try.

By the way, I don't really understand why metering should be accurate when manually stopping down an M42, but not accurate with the K mount lenses using the green button. Perhaps the lens isn't stopped down long enough to get an accurate reading? Thanks in advance for any advice.

06-21-2009, 09:28 PM - 1 Like   #2
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QuoteOriginally posted by Wayno Quote
...By the way, I don't really understand why metering should be accurate when manually stopping down an M42, but not accurate with the K mount lenses using the green button. Perhaps the lens isn't stopped down long enough to get an accurate reading? Thanks in advance for any advice.
There is a general problem with meter accuracy for all non-A lenses on the K10D and K20D and to a lesser extent on the KxxxD (mirror prism) bodies. Strangely, the meter behavior is somewhat lens-specific, is aperture dependent, and is worse (by my observation) for lenses with a maximum aperture wider than about f/4 regardless of mount. My Jupiter-9 85/2 is by far the worst offender with about two stops underexposure wide open.

The general pattern is:
  • Underexposure wide open
  • Essentially correct exposure at f/3.5-f/8
  • Mild overexposure at f/11 and smaller assuming adequate light for shooting at those apertures

A short note regarding stop-down metering at narrower apertures in moderate or low light. The published sensitivity for the meter on the K10D is 1 EV (ISO 100). My experience has been that sensitivity is somewhat less with linearity falling off at about 2 EV (ISO 100). Now you might think that 1 or 2 EV is pretty dark and it is when metering wide open. However, when metering stopped down in moderately low light (say f/2 @ 1/30s, ISO 100), at f/11 using stop-down metering, the amount of light actually striking the sensor is only EV 1 (ISO 100) and is at the limit of sensitivity for the meter.

The result is that the meter gives the same reading for all apertures below a certain point at a given light intensity resulting in severe underexposure. This loss of meter linearity in low light and narrow apertures is true for all stop-down metering cameras:
  • Many M42 film cameras
  • All K-mount cameras (film and digital) when mounted to M42 lenses
  • All crippled KA mount cameras (film and digital) when mounted to non-A lenses

Steve

(Much more than a simple answer I know...I guess I am in a verbose mood...)

Last edited by stevebrot; 06-21-2009 at 09:53 PM.
06-21-2009, 10:21 PM   #3
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Thanks for the detailed response.

What puzzles me is that metering seems accurate for M42 lenses (compared with the kit lens) in Av mode, and the reading drops in a linear fashion as I stop down the lens. However, in M mode with an old K lens, it's obviously off.

I've only done limited testing, so I'll have to do some more to confirm those observations.
06-21-2009, 10:37 PM   #4
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QuoteOriginally posted by Wayno Quote
Thanks for the detailed response.

What puzzles me is that metering seems accurate for M42 lenses (compared with the kit lens) in Av mode, and the reading drops in a linear fashion as I stop down the lens. However, in M mode with an old K lens, it's obviously off.

I've only done limited testing, so I'll have to do some more to confirm those observations.
Depends on the lens. My Tamron 70-150/3.5 has essentially accurate metering with the PK adapter while my K-mount Zenitar 16/2.8 is inaccurate at all apertures. There is also some variability depending on focus. My Pentax-M 50/1.7 meters 1/2 stop different between close-focus and infinity for the same blank-wall target. Go figure... I have done some detailed testing with multiple lenses using both the stock and KatzEye screens, but have not taken the time to finish collating the data.

My general approach has been to determine the appropriate exposure for a given scene using a combination of meter reading and "chimping" the histogram to determine how much compensation to dial in. It is highly tempting to invest in a hand-held meter and just be done with it all .

Steve

(Not happy with the stopped down metering situation...but knows that the Nikon and Canon camps have the same issues...)


Last edited by stevebrot; 06-21-2009 at 10:42 PM.
06-22-2009, 06:57 AM   #5
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check trhis link

https://www.pentaxforums.com/forums/241716-post69.html
06-22-2009, 06:33 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
Matches my not so scientific results. Well done, LG.
06-22-2009, 11:23 PM   #7
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Thanks for the replies.

Well I guess I spoke too soon. After some further testing the screwmount metering in Av mode isn't as accurate as I thought... It looks like I'll be trying to get an LL-60 screen some time soon. Mind you, coming from a Spotmatic it's really quite easy with the digi to check the exposure after the shot & correct if necessary. However, it'd be nice to not have to worry about that, so I'll try swapping out the screen.
06-23-2009, 06:41 PM   #8
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It probably does the same thing as it does with K and M lenses - overexposed at one end of the aperture range, underexposed at the other. PITA - and I am not referring to bread. There are a stop or three in the middle where the metering is close enough, and once you find them, you can work around it for a while. "Let me see, it meters ok at f/8 so it metered 1/500 at f/8, but I want f/16 for depth of field, so that's two stops less light, I need 1/125 second exposure." Absolutely brilliantly accurate. But it is still PITA.

06-23-2009, 08:57 PM   #9
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QuoteOriginally posted by Canada_Rockies Quote
It probably does the same thing as it does with K and M lenses - overexposed at one end of the aperture range, underexposed at the other. PITA - and I am not referring to bread. There are a stop or three in the middle where the metering is close enough, and once you find them, you can work around it for a while. "Let me see, it meters ok at f/8 so it metered 1/500 at f/8, but I want f/16 for depth of field, so that's two stops less light, I need 1/125 second exposure." Absolutely brilliantly accurate. But it is still PITA.
You discovered my approach. Meter at f/5.6 - f/8 and calculate the shutter speed for the aperture you really want.

Steve

(Either that or just shamelessly chimp, adjust, and reshoot...)
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