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08-18-2010, 01:22 PM   #1
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k20d exposure compensation with legacy glass - a possible solution!

Please test this and let me know how it works for you. Here are some preliminary findings, that may help those having trouble metering with M or K lenses. I have not tried screwmount.

The general consensus is that the camera underexposes with faster apertures. I have found that the camera exposes "properly" at f8, and error increases as you deviate form this fstop.

NB: My definition of proper is to have at least some of the scene exposed in the final quadrant of the histogram. Consider me a conservative ETTR-er.

Try this: set the compensation for 0. Set the lens to f8, and CENTER WEIGHT for middle grey. I have had a terrible time spot metering with M lenses at small apertures.

Now, every full f stop you move towards the maximum aperture (in my case, 1.8), move the EV compensation +0.3 and vica versa.

So (I am using the K55mm 1/1.8 as a reference):

f8 - 0
f5.6 - +0.3
f4 - +0.5
f2.8 - +0.7
f1.8 - +1.0 (this one is approximate).

The histograms were very similar. Note that this formula works in 1/3 or 1/2 steps, but 1/3 is more accurate (works for 4 out of 5 stops, 1/2 only works for 3/5 stops). I prefer 1/2 for speed, myself.

As we move past f8, metering seems to be very inaccurate, and it best done compensated from f8.

Any criticisms of my findings would be greatly appreciated. I have tested this method with both an Takumar Bayonet 80-200 f4.5 and SMC K 55 1.8.


Last edited by paperbag846; 08-19-2010 at 04:08 PM. Reason: I have adjusted these values as the the original data was skewed to the left.
08-18-2010, 03:01 PM   #2
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I've found that it all depends on the lens. Some A lenses overexpose at every aperture setting except wide open, as far as I've tested.

I can check those two specific lenses later on

Adam
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08-18-2010, 03:03 PM   #3
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There was alot of discussion on the M lens meter innaccuracy after the k10d came out. The focus screen seems to be the culprit, same as k20d. I put a *ist D screen in my k10d and it was pretty much cured.Lowell Goudge did alot of work on it if you can find the old threads. He made some charts on it too.
08-18-2010, 03:11 PM   #4
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Thanks for the feedback.

I tested out my takumar bayonet 80-200 4.5, a much slower lens, and my formula worked for it as well!

But I should comment, that between f8 and the minimum aperture, things work pretty much as I stated above. However, every lens behaved differently at apertures above f8 ish (i only tested full stops).

This formula has helped me a lot in getting the exposure right on the first try. My success rate has been exponentially better.

I'm sure focal length plays a major role in this equation, but I have gotten very reliable results from 28mm up to 200mm, between fmin and f8. The slowest apertures work rather well in this method, but sometimes still come out a bit dark.

This was my only real criticism of the k20d (NB: not the supposed "underexposure" nonsense, but difficulty metering with legacy glass). I'm very happy.

Any corrections or contradictory evidence would be much appreciated!

08-18-2010, 03:49 PM   #5
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QuoteOriginally posted by Adam Quote
Some A lenses overexpose at every aperture setting except wide open, as far as I've tested.
This is what I get as well, with a Cosina-manufactured A 28mm 2.8. I tried it with a K100DS, K20D and K-x. all behaved the same way; perfect exposure wide open, overexposed when stopped down (about one full stop at f8). The .3 Ev per stop might work, but in the opposite direction to the OP's findings of course.
08-18-2010, 04:12 PM   #6
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QuoteOriginally posted by borno Quote
There was alot of discussion on the M lens meter innaccuracy after the k10d came out. The focus screen seems to be the culprit, same as k20d. I put a *ist D screen in my k10d and it was pretty much cured.Lowell Goudge did alot of work on it if you can find the old threads. He made some charts on it too.
To avoid any confusion it is the *ist DS screen that corrects this exposure problem when used on a K10
08-19-2010, 10:26 AM   #7
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QuoteOriginally posted by pschlute Quote
To avoid any confusion it is the *ist DS screen that corrects this exposure problem when used on a K10
Ping Lowell about that. Remember to ask him too whether metering stayed true with lenses in the A position. IIRC, the trade-of was that accuracy with non-A glass meant loss of accuracy with A lenses.

At one point I did a lot of testing with various lenses and exchanged quite a bit of information with other users. My conclusion was that there is no magic bullet. It is easy to blame the focus screen, but the true fault lies with metering logic that reads off the screen and applies correction factors based on lens maximum aperture. Pentax was good enough to correct the issue with M-mode stop-down metering on the K-x and K-7, but for those of us with the older cameras, we are left with the meter-chimp-adjust pattern.

My general experience with a multitude of legacy non-A contact glass might be summarized as follows:
  • Exposure issues and the pattern of inaccuracy vary by lens
  • Apertures greater than f/4 tend to underexpose
  • The underexposure may be as much as 2.5 stops at f/2 and wider for some lenses (e.g. Jupiter-9 in my collection), but usually is no more than 1-1.5 stops
  • Exposure is generally pretty accurate at f/4 and narrower under bright conditions
  • Underexposure is a hazard at f/11 and narrower under dim conditions due to lack of meter linearity/sensitivity below about EI 4@100 at the sensor (remember, stop down metering means less light to the sensor)


Steve

Last edited by stevebrot; 08-19-2010 at 10:31 AM.
08-19-2010, 11:42 AM   #8
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Are you spot metering exposure in this middle gray in your frame? Perhaps the underexposure in larger apertures is happening because of vignetting: the light lost from the corners creates some haze in the rest of the frame; then your middle gray appears brighter pushing the camera to underexpose. You can check this by putting your middle gray object in some corner of your frame. Perhaps it isn't even haze. Perhaps the center of the frame really becomes brighter because of the border vignetting. I suppose that depends on the lens you are using. In this case your exposure is correct, you don't have underexposure.

08-19-2010, 03:01 PM   #9
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QuoteOriginally posted by climit Quote
Are you spot metering exposure in this middle gray in your frame? Perhaps the underexposure in larger apertures is happening because of vignetting: the light lost from the corners creates some haze in the rest of the frame; then your middle gray appears brighter pushing the camera to underexpose. You can check this by putting your middle gray object in some corner of your frame. Perhaps it isn't even haze. Perhaps the center of the frame really becomes brighter because of the border vignetting. I suppose that depends on the lens you are using. In this case your exposure is correct, you don't have underexposure.
I have done some further testing and realize that I was incorrect in my OP. I will fix it.

Last edited by paperbag846; 08-19-2010 at 03:18 PM.
08-19-2010, 03:03 PM   #10
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Well, it was all beaten to death before... K10 and K20 suffered from a really bad focusing screen, changing it for a *istDS really did lessen under/over exposure somewhat (but still left some), and Pentax fixed the focus screen on both Kx and K7 (funny thing is that K10/K20 screens are the only digital screens labeled '??-80', whereas every other focus screens are '??-60').

Now, I've not seen any detrimental effect on my A lenses by using the 'xx-60' focus screen in both my K10 and K20... On the contrary, they metered quite better than with the factory focus screen...
08-19-2010, 04:15 PM   #11
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QuoteOriginally posted by dlacouture Quote
Well, it was all beaten to death before... K10 and K20 suffered from a really bad focusing screen, changing it for a *istDS really did lessen under/over exposure somewhat (but still left some), and Pentax fixed the focus screen on both Kx and K7 (funny thing is that K10/K20 screens are the only digital screens labeled '??-80', whereas every other focus screens are '??-60').

Now, I've not seen any detrimental effect on my A lenses by using the 'xx-60' focus screen in both my K10 and K20... On the contrary, they metered quite better than with the factory focus screen...
The LL-60 was the one I got too, workes fine in my k10d.
08-19-2010, 05:11 PM   #12
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QuoteOriginally posted by dlacouture Quote
Well, it was all beaten to death before... K10 and K20 suffered from a really bad focusing screen, changing it for a *istDS really did lessen under/over exposure somewhat (but still left some), and Pentax fixed the focus screen on both Kx and K7 (funny thing is that K10/K20 screens are the only digital screens labeled '??-80', whereas every other focus screens are '??-60').

Now, I've not seen any detrimental effect on my A lenses by using the 'xx-60' focus screen in both my K10 and K20... On the contrary, they metered quite better than with the factory focus screen...
Now if we could only let the Katz Eye people in on the perfect screen secret, we would have a winner. My Katz Eye meters the same as the factory ??-80 screen.

BTW...If it is the screen design, why is it that both the K-x and the K-7 still suffer from underexposure when used in Av mode with M42 lenses but function properly in M mode with the same lenses? This is in the manual for both cameras.


Steve
08-19-2010, 06:12 PM   #13
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QuoteOriginally posted by dlacouture Quote
Well, it was all beaten to death before... K10 and K20 suffered from a really bad focusing screen, changing it for a *istDS really did lessen under/over exposure somewhat (but still left some), and Pentax fixed the focus screen on both Kx and K7 (funny thing is that K10/K20 screens are the only digital screens labeled '??-80', whereas every other focus screens are '??-60').

Now, I've not seen any detrimental effect on my A lenses by using the 'xx-60' focus screen in both my K10 and K20... On the contrary, they metered quite better than with the factory focus screen...
Thanks for the suggestion. So this new focusing screen will improve metering with my newer lens as well? I can find the ML-60 on the pentax webstore.

Last edited by paperbag846; 08-19-2010 at 06:36 PM.
08-19-2010, 09:32 PM   #14
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QuoteOriginally posted by borno Quote
There was alot of discussion on the M lens meter innaccuracy after the k10d came out. The focus screen seems to be the culprit, same as k20d. I put a *ist D screen in my k10d and it was pretty much cured.Lowell Goudge did alot of work on it if you can find the old threads. He made some charts on it too.
Lowell convinced me to try it out, and you are not going to pry my LL-60 *ist screen out of my camera any time soon. All of a sudden both my M lenses have no metering troubles at all, and it had zero effect on the A lenses.

PS: the grainier screen is quite a bit better for manual focus as well.
08-19-2010, 09:51 PM   #15
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Would the ML-60 achieve the same effect? I can't seem to find the LL-60 anywhere...

PentaxWebstore Focusing Screen ML-60 Divided Matte
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