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10-15-2008, 09:50 AM   #31
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QuoteOriginally posted by stevebrot Quote
The thing that puzzles me is why A-mount lenses in the A-setting don't have problems while the same lens using the aperture ring does. Why, for an f/2 lens, would an f/2 aperture setting meter differently when set by the body than at f/2 using the aperture ring (delta = 1-1.5 stops)?
Wow, that's really surprising. I'm almost sorry I didn't choose the K10D over the K200D just to be able to see this sort of phenomenon :-)

Anyhow, sorry for asking the obvious dumb question here, but - you are using the same metering mode (eg, center-weighted) in both cases, right?

10-15-2008, 09:52 AM   #32
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QuoteOriginally posted by Marc Sabatella Quote
Wow, that's really surprising. I'm almost sorry I didn't choose the K10D over the K200D just to be able to see this sort of phenomenon :-)

Anyhow, sorry for asking the obvious dumb question here, but - you are using the same metering mode (eg, center-weighted) in both cases, right?
Marc, the problem is that the camera does not know what the open aperture is unless the lens is set on the A aperture settting. It's in the way the k10d and others figure out the exposure for a particular lens.
10-15-2008, 12:16 PM   #33
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QuoteOriginally posted by Canada_Rockies Quote
Marc, the problem is that the camera does not know what the open aperture is unless the lens is set on the A aperture settting. It's in the way the k10d and others figure out the exposure for a particular lens.
specifically, the camera (K10D specificlly) needs to know the maximum apature of the lens, and it uses this information and (probably a curve like the one I developed) to calculate the amount of light passing through the lens when wide open (metering on A series lenses is done wide open). Apature is then controlled by the camera body apature activating lever and how far it moves.

When you put a lens in manual apature mode, the camera does not know any longer the maximum apature or what you are stopped down to, so that it can calculate the correction necessary. That is why manual metering is so bad on the K10D.
10-15-2008, 01:45 PM   #34
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QuoteOriginally posted by Marc Sabatella Quote
Wow, that's really surprising. I'm almost sorry I didn't choose the K10D over the K200D just to be able to see this sort of phenomenon :-)

Anyhow, sorry for asking the obvious dumb question here, but - you are using the same metering mode (eg, center-weighted) in both cases, right?
Yes, same metering mode. As Rocky and Lowell explained; there appears to be some sort of correction applied to the meter reading based on the selected aperture. When the aperture is unknown, the system apparently defaults to some sort of standard curve that just happens to be wrong for most lenses. Go figure!

The big question is why this is an issue with the K10D (and by report the K20D) and not the Kx00 series cameras. Might it be the penta-prism vs. the penta-mirror? Different meter sensors?

Steve

10-15-2008, 01:58 PM   #35
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QuoteOriginally posted by stevebrot Quote
Yes, same metering mode. As Rocky and Lowell explained; there appears to be some sort of correction applied to the meter reading based on the selected aperture. When the aperture is unknown, the system apparently defaults to some sort of standard curve that just happens to be wrong for most lenses. Go figure!

The big question is why this is an issue with the K10D (and by report the K20D) and not the Kx00 series cameras. Might it be the penta-prism vs. the penta-mirror? Different meter sensors?

Steve
steve, my bet is the focusing screen, because as I reported, changing it to th e*istD screen fixed the problem.

I have been asking everyone with different lenses and camera bodies to bench mark their equipment the same way I did, expecially those with aftermarket focusing screens, to have a good performance criteria for all cameras. so far no one has offered the data. When it is all said and done, the ideal place for things like this would be a camera body review forum, like the lens review forum. I have made notes in my lens reviews about metering performance on the two bodies I own, but have not put the graphs there (yet any way)
10-15-2008, 03:21 PM   #36
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I've just got a K20D and its exposure is messed up with M lenses compared to their use on the K100D Super. M50/F1.7 & M28/F2.8 used in "M" mode . I'll probably stick to using them in the F4-F9.5 range and work out the EV comp for this.

Last edited by dosdan; 10-15-2008 at 03:27 PM.
10-15-2008, 03:45 PM   #37
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QuoteOriginally posted by Lowell Goudge Quote
I have made notes in my lens reviews about metering performance on the two bodies I own, but have not put the graphs there (yet any way)
Adding the exposure compensation information to the lens review is a GREAT idea! It could serve as a ready reference when questions such as this thread come up.

Steve

BTW...Where does one get a *istD focus screen? Can these be purchased aftermarket?
10-15-2008, 03:48 PM   #38
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QuoteOriginally posted by dosdan Quote
I've just got a K20D and its exposure is messed up with M lenses compared to their use on the K100D Super. M50/F1.7 & M28/F2.8 used in "M" mode . I'll probably stick to using them in the F4-F9.5 range and work out the EV comp for this.
Can you do a simple set of test exposures. Set ISO so that wide open the camera has a shutter speed of 1/2000, and then take a series of shots at a uniform surface (block wall or paved road) stopping down one apature click at a time, and post a table of grey scale vs apature for each lens?

Also note that at f4 to F5.6 the readings shouldbe pretty good, but above that they will probably tend to over expose by 1.5 to 2 stops by F11.

10-15-2008, 07:32 PM   #39
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QuoteOriginally posted by dosdan Quote
I've just got a K20D and its exposure is messed up with M lenses compared to their use on the K100D Super. M50/F1.7 & M28/F2.8 used in "M" mode . I'll probably stick to using them in the F4-F9.5 range and work out the EV comp for this.
I just did a quick and dirty test with my k20D and an M50/1.7, same gear as above.

I took a series of shots at 1 stop intervals from f1.7 up to f22.

The histogram only showed minor differences up to f11, at f16 and f22 it exposed at the same shutter speed as at f11.

I also tried the same with an A28/2.8, and stopped down using the diaphragm ring, results were fine up to f11, but underexposed a little at f16 amd f22.
10-15-2008, 07:57 PM   #40
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QuoteOriginally posted by stevebrot Quote
Adding the exposure compensation information to the lens review is a GREAT idea! It could serve as a ready reference when questions such as this thread come up.

Steve

BTW...Where does one get a *istD focus screen? Can these be purchased aftermarket?
I got one of these for my k10d and it seems good to me
at least any error seems more linear and smaller on my camera.

Pentax | LL-60 AF Divided Matte Focusing Screen | 38572 | B&H
10-15-2008, 08:06 PM   #41
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QuoteOriginally posted by borno Quote
I got one of these for my k10d and it seems good to me
at least any error seems more linear and smaller on my camera.

Pentax | LL-60 AF Divided Matte Focusing Screen | 38572 | B&H
Did it affect exposure with your DA / FA / A lenses?
10-15-2008, 08:19 PM   #42
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Hey Albert,
I add a little +ev with a or da lenses, but I think it underexposed a little before also. Mine is a little off since it got back from service last month, or else it's just me after using the k100d super for 6 weeks
10-15-2008, 08:23 PM   #43
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QuoteOriginally posted by borno Quote
Hey Albert,
I add a little +ev with a or da lenses, but I think it underexposed a little before also. Mine is a little off since it got back from service last month, or else it's just me after using the k100d super for 6 weeks
thanks, Tom
10-15-2008, 10:03 PM   #44
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QuoteOriginally posted by Canada_Rockies Quote
Marc, the problem is that the camera does not know what the open aperture is unless the lens is set on the A aperture settting. It's in the way the k10d and others figure out the exposure for a particular lens.
I understand that this is true in theory, but it does make the results wide open surprising, because you'd think the calculation would be such that this much at least came out the same. Which is why I wanted to rule out the obvious here - the fact that metering mdoe changes when moving the perture ring off "A" unless you've specifically set the camera to something other than multi-segment. Steve has now verified he *was* using the same metering method, so I'll just go back to being surprised that the algorithm could be that lame that it doesn't yield the same answer in the wide open case, but accepting that this in fact must be the case.
10-16-2008, 03:42 AM   #45
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QuoteOriginally posted by John Cafarella Quote
I just did a quick and dirty test with my k20D and an M50/1.7, same gear as above.

I took a series of shots at 1 stop intervals from f1.7 up to f22.

The histogram only showed minor differences up to f11, at f16 and f22 it exposed at the same shutter speed as at f11.

I also tried the same with an A28/2.8, and stopped down using the diaphragm ring, results were fine up to f11, but underexposed a little at f16 amd f22.
but what was the average value?

it should be 110 for 18% grey.

did you shoot a uniform surface or a scene?

facts please, just facts.
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