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10-16-2008, 08:50 AM   #46
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QuoteOriginally posted by Marc Sabatella Quote

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.
Yes, the exposure calculation apparently is different depending on whether the aperture is set by the body or on the lens. I think that is the basis for the whole exposure issue.

Steve

10-16-2008, 02:51 PM   #47
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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?
Don't forget the possibility of a "fudge factor" that is built in to the lens chip for some lenses. That would explain "anomolies" at say, f2 on "A" and f2 on
"M" . I assume the camera computer ignores all data once it determines the lens is not on "A"
10-16-2008, 05:01 PM   #48
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QuoteOriginally posted by jeffkrol Quote
Don't forget the possibility of a "fudge factor" that is built in to the lens chip for some lenses. That would explain "anomolies" at say, f2 on "A" and f2 on
"M" . I assume the camera computer ignores all data once it determines the lens is not on "A"
Nice try except for one thing. Apature is really apature. nothing to fuge and if it was a fudge factor, how does the lens know to only apply it to the K10D.

Sorry, all the fudge factors are in the body. The only "Fudge factor in the lens is a sliding contact as you zoom out on variable apature lenses to correct automatically within the lens for apature shifts. Sigma did it this way i the 1990's You will still see this today with a variable apature zoom as you zoom out and watch the F number, at some point it will jump 1/2 stop.
10-16-2008, 10:40 PM   #49
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QuoteOriginally posted by Lowell Goudge Quote
but what was the average value?

it should be 110 for 18% grey.

I have no software on this PC (work) to tell. I can only tell you that in M mode, using green button for metering, and using aperture ring to stop down, that the camera's histogram shows good exposure. This is true for all apertures from f1.7 to f11.

Highlights are not clipped, but the highest value is close to the right hand side. The histogram peak is at almost the identical postion at each f-stop.

This is very different behaviour to my previous camera, the K10D which progressively overexposed when increasing the f number.

did you shoot a uniform surface or a scene?

I shot the same scene, not a uniform surface. I took good care to frame the shots identically.

facts please, just facts.
Hope this helps with your fact gathering.

10-17-2008, 03:35 AM   #50
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QuoteOriginally posted by John Cafarella Quote
Hope this helps with your fact gathering.
john

unfortunately no it does not. The only way to accurately assess metering is to shoot a uniform surface at each apature.

it does suggest however some possible improvement but without a uniform scene (i.e. block wall) it cant be judged.

the fact that there is no clipping or the histogram is towards the right is not sufficient.

As I have said before, I would even accept photos and do a check myself, or if people want to upliad a rescaled series of shots I would check them.
10-19-2008, 02:49 AM   #51
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I did two shoots this weekend and I used a M50/F1.7 & M28/F2.8 (as well as A, F & DA lenses) on a K20D. I can't say I noticed any exposure problems when using them between F6.7-F11.

When I get some time I'll look at the M exposure profiles over a wider aperture range.

Dan.
10-19-2008, 08:22 AM   #52
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QuoteOriginally posted by Lowell Goudge Quote
Nice try except for one thing. Apature is really aperture. nothing to fudge and if it was a fudge factor, how does the lens know to only apply it to the K10D.

Sorry, all the fudge factors are in the body. The only "Fudge factor in the lens is a sliding contact as you zoom out on variable apature lenses to correct automatically within the lens for aperture shifts. Sigma did it this way i the 1990's You will still see this today with a variable aperture zoom as you zoom out and watch the F number, at some point it will jump 1/2 stop.
Are you sure of that? There is a data chip in the lens that communicates some info.
As to exposure all the chip would have to tell the body is use +1/2 ev to start at base f stop and would come in handy for the times that the f stop was not truly rendered. I seem to recall that even in the film days lens reviews would print the "actual" f stop vs what the manufacture stated...guess I am getting old. Though Pentax lenses may be primitive to say, Canon's, they are not totally "old school". The one contact is a serial bus to the chip in the lens.
It must have some function
10-19-2008, 02:22 PM   #53
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Off-brand lenses are cheaper becase they have inferior construction. Part of that has to do with the mechanics of the apertures. That's specifically why Vivitar/Tamron/Sigma are cheaper than Pentax/Canon/Nikon etc, even when they're fine optically. Robots make lenses, humans assemble len mechanisms and supervise the quality control you hope you're paying for.

I use an 85/2 M regularly ...it meters perfectly consistently, from aperture to aperture, on K20d with both green button and M.

10-20-2008, 07:44 AM   #54
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QuoteOriginally posted by janosh Quote
Off-brand lenses are cheaper becase they have inferior construction. Part of that has to do with the mechanics of the apertures. That's specifically why Vivitar/Tamron/Sigma are cheaper than Pentax/Canon/Nikon etc, even when they're fine optically. Robots make lenses, humans assemble len mechanisms and supervise the quality control you hope you're paying for.

I use an 85/2 M regularly ...it meters perfectly consistently, from aperture to aperture, on K20d with both green button and M.
Ummm, Janosh...Are you saying that the metering issues are related to use of "off-brand" lenses and is a result of cheap construction? My Pentax-M 5/1.7 suffers from about 1.5 stops underexposure at f/1.7, but is accurate at f/5.6 and narrower.

Steve
10-21-2008, 05:55 AM   #55
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QuoteOriginally posted by stevebrot Quote
Ummm, Janosh...Are you saying that the metering issues are related to use of "off-brand" lenses and is a result of cheap construction? My Pentax-M 5/1.7 suffers from about 1.5 stops underexposure at f/1.7, but is accurate at f/5.6 and narrower.

Steve
Steve, go bnack to my initial posting with the graph of a K10D and an SMC 50mm F1.4.

It is not an off brand or cheap issue, but an issue with any manual lens or lens in manual mode.
10-21-2008, 08:52 AM   #56
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QuoteOriginally posted by Lowell Goudge Quote
Steve, go bnack to my initial posting with the graph of a K10D and an SMC 50mm F1.4.

It is not an off brand or cheap issue, but an issue with any manual lens or lens in manual mode.
Lowell, I know that.

Steve

(Shooting manual most of the time...)
10-21-2008, 11:18 AM   #57
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I am also seeing the strange problem with my K10D. my ist DL works perfectly fine either A or non A mode.
I have FA lens and M42.
the funny thing is at f=22 it gives brighter/over exposed picture than at f=8.0. where as at f=5.6 it gives darker picture.

I have a split screen and I can try to find out whether it is the focus screen or not.

I never had problem with under exposure or over exposure when I use this FA lens other than A in my ist DL.
10-21-2008, 11:35 AM   #58
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QuoteOriginally posted by istDL-K10D Quote
I am also seeing the strange problem with my K10D. my ist DL works perfectly fine either A or non A mode.
I have FA lens and M42.
the funny thing is at f=22 it gives brighter/over exposed picture than at f=8.0. where as at f=5.6 it gives darker picture.

I have a split screen and I can try to find out whether it is the focus screen or not.

I never had problem with under exposure or over exposure when I use this FA lens other than A in my ist DL.
By all reports, the issue is limited to the Kx0D camera series. The Kx00D and earlier models are reported to meter just fine.

Steve
10-21-2008, 05:43 PM   #59
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Here is a test I did a while back with my K10D vs. K110D "green button" metering with a fully manual lens. As you can see, the K110D wins this battle hands down...

https://www.pentaxforums.com/forums/229239-post23.html
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