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01-25-2009, 02:22 PM   #1
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K10D/K20D - Exposure Variations with M Lenses

At the introduction of the K10D, a number of us experienced, and commented upon, exposure variation with M-Lenses: with the camera in manual mode and aperature-ring enabled, the exposure brightens (histo curve moves to the right) as the lens is stopped down, the green button pushed and the camera selects a shutter speed - i.e. the shutter speed decreases non-linearly as aperature is changed.

I assumed this was a bug in my very early model, and since I almost never use my various K or M lenses, I thought little more about it.

Out of curiosity, I just tried my K28/2 on my K20D and observed identical behavior. While it is possible that these two bodies manufactured over two years apart happen to be afflicted with identical defects, I suspect that I'm either doing something wrong or this characteristic is in the nature of the K10D/K20D platform.

Frankly, while this nonlinear exposure behavior is basically academic in my case, I'm still curious. Anybody know the explination?

Thanks,

Jer

01-25-2009, 02:45 PM   #2
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You are not doing anything wrong.
This is so from the beginning.
It has to do with a non-linear behaviour of the screen.



Btw. this is not only the case with Pentax.
The same happens with Canon and Nikon.
01-25-2009, 03:53 PM   #3
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Wieland pointed out the cause of the problem, so here is my solution which works for both my lenses M 400 and M 100 macro. I use the optical depth of field preview to set my exposures rather than the green button. I find that both my lenses meter quite well at f/8 using the stop down. I then pretend I am back in manual camera land, and change the exposure to equal the good metered stop. I also use "sunny 16" and it works just as well.
01-25-2009, 04:10 PM   #4
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Well, there is both a simple explanation tothis choice by Pentax and (fortunately!) an easy fix...

Pentax simply tried to create an A-lens-friendly focus screen : the original focus screen does increase the perceived brightness of lenses between f/1.2 and f/4.5 (approx.), thus inducing an underexposure with these apertures (but then, everybody moons over the Highlight-friendliness of the Pentax camera, so...).
What they didn't a) check or b) cared about, was that below f/4.5, this screen will induce an overexposure, making the whole lot quite useless with old lenses...

Anyway, thank God Pentax is so user-friendly and provides us with such customizable cameras, because the solution is simply to change the focus screen! (I'm still amazed Pentax never advertized the fact that for the past 20 years, every reflex camera they produced were having removable focus screens!)

Well, to make a long story short, just buy a LL-60 focus screen (yep, that's right, the one for the *ist line, and NOT the LL-80 that suffer from the same non-linearity), and everything will be allright...

01-25-2009, 04:51 PM   #5
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QuoteOriginally posted by blende8 Quote
It has to do with a non-linear behaviour of the screen.
QuoteOriginally posted by Canada_Rockies Quote
Wieland pointed out the cause of the problem . . .
QuoteOriginally posted by dlacouture Quote
just buy a LL-60 focus screen (yep, that's right, the one for the *ist line
Thanks much for satisfying my curiosity.

Jer
01-25-2009, 05:47 PM   #6
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QuoteOriginally posted by dlacouture Quote
Well, there is both a simple explanation tothis choice by Pentax and (fortunately!) an easy fix...

Pentax simply tried to create an A-lens-friendly focus screen : the original focus screen does increase the perceived brightness of lenses between f/1.2 and f/4.5 (approx.), thus inducing an underexposure with these apertures (but then, everybody moons over the Highlight-friendliness of the Pentax camera, so...).
What they didn't a) check or b) cared about, was that below f/4.5, this screen will induce an overexposure, making the whole lot quite useless with old lenses...

Anyway, thank God Pentax is so user-friendly and provides us with such customizable cameras, because the solution is simply to change the focus screen! (I'm still amazed Pentax never advertized the fact that for the past 20 years, every reflex camera they produced were having removable focus screens!)

Well, to make a long story short, just buy a LL-60 focus screen (yep, that's right, the one for the *ist line, and NOT the LL-80 that suffer from the same non-linearity), and everything will be allright...
Ummmm...where does one get a LL-60 screen?

Steve

(BTW...various aftermarket screens suffer from the same issue, even those without the "brightening" treatment...)
01-26-2009, 12:44 AM   #7
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SteveBrot : well, I bought one new at Digit-photo.fr for 28€ (they still have plenty, it seems)...

But I agree they are not that common around the web...
01-26-2009, 12:53 PM   #8
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May have to wait a bit

QuoteOriginally posted by stevebrot Quote
Ummmm...where does one get a LL-60 screen?

Steve

(BTW...various aftermarket screens suffer from the same issue, even those without the "brightening" treatment...)
Pentax | LL-60 AF Divided Matte Focusing Screen | 38572 | B&H

01-26-2009, 01:07 PM   #9
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I have reported on this at length, and have substituted the screen from my *istD into the K10D for a test.

the link below shows the results.
https://www.pentaxforums.com/forums/241716-post69.html
Note however, while chaging the screen solves the problem for manual lenses, you now introduce a problem with all A lenses (and newer).

each lens will suffer an exposure error, as a function of maximum apature, so you will have to re-check each lens, and keep a mental note of the adjustment you need to make.

I went as far as to ask pentax if they would re-calibrate my metering with the *istD screen. They considered this a modification and said NO.
01-26-2009, 01:54 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
...Note however, while chaging the screen solves the problem for manual lenses, you now introduce a problem with all A lenses (and newer).

each lens will suffer an exposure error, as a function of maximum apature, so you will have to re-check each lens, and keep a mental note of the adjustment you need to make.

I went as far as to ask pentax if they would re-calibrate my metering with the *istD screen. They considered this a modification and said NO.

Ohhhhh...

I was afraid of something like that.

Steve
01-26-2009, 02:01 PM   #11
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Thanks Jeff, I found the B&H listing just last night.


Steve
01-26-2009, 07:41 PM   #12
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QuoteOriginally posted by Lowell Goudge Quote
I have reported on this at length, and have substituted the screen from my *istD into the K10D for a test.

the link below shows the results.
https://www.pentaxforums.com/forums/241716-post69.html
Note however, while chaging the screen solves the problem for manual lenses, you now introduce a problem with all A lenses (and newer).

each lens will suffer an exposure error, as a function of maximum apature, so you will have to re-check each lens, and keep a mental note of the adjustment you need to make.

I went as far as to ask pentax if they would re-calibrate my metering with the *istD screen. They considered this a modification and said NO.
Wheatfield never bought the argument huh.... Anyways I'm still a bit confused about your findings. You found exposure errors on A lenses when they were in "A" mode (not using the DOF or green buttons)...??
If apertures are linear (meter reads from full open and extrapolates) then any errors are not screen caused except if equal throughout the aperture range from the "base" metering. There is no reason to do "stop down" metering on an A or better lens..
Pass through A teleconverters shouldn't make much of a difference either since, again, it starts from "base reading" and extrapolates up. One assumption is the metering algorithm doesn't have a "cheat function" built in, which is a possibility or the meter starts getting "dodgy" w/ starting apertures of f8 or so (f4 lens doubled)....

Anyways older tests. I could probably find another 1/2 dozen of these graphs....:
Thanks to carpents......
https://www.pentaxforums.com/forums/pentax-dslr-discussion/6286-k10d-exposure...on-m-mode.html


http://forums.dpreview.com/forums/read.asp?forum=1036&message=29780755
This was for a Chinse made split screen but note the text:
These measures follow quite exactly what I found at the time with the original Lx-80 family screen, but with a slight +1/3ev exposure compared to it.

And this is what I experienced on the field, with both the original screen AND this split screen.

My brother (having a K10, the original screen, a new LL-60 screen, the FA 50 f/1.4 and a nice collection of manual lenses) repeated these tests with the original screen and the LL-60.
Our conclusions:

- original screen (and by extension all Lx-80 screens) and the split screen behave exactly the same way, with a slight, constant overexposure (around +1/3ev) for the split screen.
- LL60 really handles exposure in a better way (I'll post results later today) :

+ quite constant exposure between f/3.5 and f/11 (around +1/3ev from 3.5 to 11)
+ slight underexposure above f/3.5 and below f/11

Last edited by jeffkrol; 01-26-2009 at 08:11 PM. Reason: Another chart for effect...
01-26-2009, 10:22 PM   #13
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So stop down meter and bracket the exposure 1/3 ev ? This is what I do and it seems to work well.
01-27-2009, 07:48 AM   #14
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QuoteOriginally posted by 71Matador250 Quote
So stop down meter and bracket the exposure 1/3 ev ? This is what I do and it seems to work well.
you need a lot of brackets to make up a 11/2-2 EV error.....
One thing: as far as I know nobody did a test w/ a k20. I'm not expecting any difference since the meter/screen is the same or very similar.
Interest in this subject is waning as it seems (and likely caused by the hassles,pricing, and shortage of good M lenses) K20 users are picking up new lenses and not bothering as much w/ the old, except possibly the seasoned users who are already used to this. I was planning on the k10 but the manual lens thing (a a few other nits) kept me from getting it.


http://forums.dpreview.com/forums/read.asp?forum=1036&message=28916185

Last edited by jeffkrol; 01-27-2009 at 07:58 AM.
01-27-2009, 08:07 AM   #15
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QuoteOriginally posted by jeffkrol Quote
Wheatfield never bought the argument huh.... Anyways I'm still a bit confused about your findings. You found exposure errors on A lenses when they were in "A" mode (not using the DOF or green buttons)...??
no on A lenses when not in A they behaved just like manual lenses
QuoteQuote:
If apertures are linear (meter reads from full open and extrapolates) then any errors are not screen caused except if equal throughout the aperture range from the "base" metering. There is no reason to do "stop down" metering on an A or better lens..
I agree and the camera does not stop doen the lens on A lenses in the A position, it just meters and sets the shutter but it knows the maximum apature, which it does not know on a manual apature lens, or when you take an A lens out of A mode
QuoteQuote:
Pass through A teleconverters shouldn't make much of a difference either since, again, it starts from "base reading" and extrapolates up. One assumption is the metering algorithm doesn't have a "cheat function" built in, which is a possibility or the meter starts getting "dodgy" w/ starting apertures of f8 or so (f4 lens doubled)....
this is the problem, the camera K10D for sure and K20D probable (i only say this as I don't own one to prove it) I checked metering with my tamron 28-75 F2.8 and found that ion the A mode (camera meters only not stops down) that there was about 1/2 to 34/ stop error (leading from correct to over exposed as I stopped down from F2.8 to F32. I assume that is an apature error in the lens as my Sigma 70-200 F2.8 does not exhibit this behavior more or less eliminating the body as the cause. BUT if I take the tamron out of A mode, and use it as a manual apature lens (I am not sure why anyone would do this, but what the hell) it behaved just like an origonal K mount manual apature lens, errors indluced. This led me to believe that pentax has a compensation factor, for the exposure, when it knows the actual maximum apature.
I then out of curriosity changed the focusing screen to that of the *istD, and found magically that the K10D metering (using my K50 F1.4) changed and matched perfectly the metering of the *istD. This led me to conclude that the cause of the K10D metering issue is the focusing screen, which behaves differently as a function of apature. And what I suspect is that it has to do with the reflective nature of the screen, and the scatter of light off each point on the screen, back to the metering. As you stop a lens down, the light hitting the focusing screen becomes more parallel, reducing scatter.


Additionally, when I use my 70-200 F2.8 all on its own, it meters correctly, but adding a 1.4x TC it over exposes and adding the 2x tc it over exposes more. adding the TC is really the same as stopping down because it changes the angles that light hits the focusing screen.

What is interesting is that the change in exposure compensation when adding the TC is exactly the same as the difference (error) in exposure between F2.8, F4 and F5.5 on manual apature lenses. Since the Sigma TCs only pass lens information streight through, it reports F2.8 for maximum apature even with the TC installed again leading to the possibility that the K10/K20 are calibrated with a fudge factor to compensate for the viewing screen.

Last edited by Lowell Goudge; 01-27-2009 at 08:14 AM.
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