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04-02-2008, 08:44 PM   #1
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Stop Down Metering with K20D and Tamron SP Lenses

I have 2 Tamron lens: a 90mm SP 2.5 macro and a 80-200SP Zoom.

I have one PK adaptall mount and one PKA adaptall mount.

If I use the PKA mount on either lens with the lens in A mode, then manual metering (M mode on the K20D) works just fine. That is, I stop down, press the green button next to shutter release and the shutter speed changes the correct amount. Photos look good.

If I use the PK mount, or use the PKA mount with the aperature not in A mode, I get wrong exposure readings.

Here's what I do:
1) set the f-stop to 2.5
2) press the Green button
3) note the shutter speed
4) stop down 1 stop
5)....continue to f22

Here's my results:

f-stop shutter
2.5 1/90
4 1/20
5.6 1/4
8 1/2
11 1
16 1
22 1.5

The correct exposure is f22 @1 sec.

The exposures aren't reciprocal for a even few stops. If they were I could at least extrapolate from them.

If I actually take a photo with the lens in manual mode using the with the correct exposure settings (taken via a light meter), then the image still doesnt get the correct exposure.

I can see the viewfinder getting darker as I stop down-so I know it's stopping down all they way. It happens on both lenses. It happens with either mount (either the PK or the PKA with the lens not in A mode).

Any ideas?

Pete

PS:I'm starting to see why the PKA adaptall mounts are selling for > $100!

04-02-2008, 09:10 PM   #2
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QuoteOriginally posted by petez Quote
Any ideas?
Be the first to try a DS screen in your camera..... That should fix the manual exposure errors (if it follows the k10 pattern.)
04-02-2008, 09:21 PM   #3
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Why would changing the screen fix it when the lens work just fine in A mode? I'm not seeing how it would change things.
04-02-2008, 10:25 PM   #4
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QuoteOriginally posted by jeffkrol Quote
Be the first to try a DS screen in your camera..... That should fix the manual exposure errors (if it follows the k10 pattern.)
I have one already
QuoteOriginally posted by petez Quote
Why would changing the screen fix it when the lens work just fine in A mode? I'm not seeing how it would change things.
In A + Av mode, the cam is just making a guess according to its Black Magic algorithms I don't pretend to understand. With my A 24-50mm in Av mode, it can be off as much as 2 stops.

In my layman's opinion, it meters more accurately with slower lenses since they're wide open (like the FA 80-320)...and as we all know, the more we stop down the more it overexposes on M42 lenses in particular. The light meter sees less light, so it leaves the shutter open longer to compensate.

I guess the short of the long is that the cam doesn't know what lens is hooked up, unless it communicates everything like the F series and newer. (Don't the lenses imprint their mark in the image EXIF? Could be wrong)... This is of course my opinion, and I (unfortunately) don't get paid to look into it further

Just enjoy your SPs...the 90 is one I'll never give up.

Edit: Lol...to answer the screen question: the screen is darker so the cam compensates for the extra light less...let me think about that last sentence and revise it tomorrow if necessary...I'm firing on 3 cylinders now.

04-03-2008, 06:35 AM   #5
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QuoteOriginally posted by petez Quote
Why would changing the screen fix it when the lens work just fine in A mode? I'm not seeing how it would change things.
The new super brite matte screen put into the k10 and probably k20 and k200 somehow has a hard time giving a normal meter reading in aperatures smaller than f4. Maybe due to scatter characteristics or some other such voodoo engineering. Nobody knows why the DS screens work (and Pentax is not telling) but w/ the K10 it did work........
There is a lot of posts, theories, conclusions out there. None are definitive. All I can suggest is try. The screen is half the price of a K/A Tamron mount..... but there is some evidence that the screen can, unfortunately, impact the "A" lens metering.......
Perils of the new (old) screen:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/1157-katz-eye-ll60...work-k10d.html
One of the many DS vs K10 screen threads:
Exposure Issue Using M lenses
Of course metering wide open and manually extrapolating down as necessary in M mode works just fine

Last edited by jeffkrol; 04-03-2008 at 06:50 AM.
04-03-2008, 08:42 AM   #6
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QuoteOriginally posted by jeffkrol Quote
The new super brite matte screen put into the k10 and probably k20 and k200 somehow has a hard time giving a normal meter reading in aperatures smaller than f4. Maybe due to scatter characteristics or some other such voodoo engineering. Nobody knows why the DS screens work (and Pentax is not telling) but w/ the K10 it did work........
There is a lot of posts, theories, conclusions out there. None are definitive. All I can suggest is try. The screen is half the price of a K/A Tamron mount..... but there is some evidence that the screen can, unfortunately, impact the "A" lens metering.......
Agreed...I have the LI-60 and love it. The only thing I can think of (as I sort of touched on before) is the the cam isn't used to seeing weird apertures. What's the fastest Pentax brand lens...f/1.2? IMO and floating in my mind...I suspect the cam is programed in the averaging modes (Av, Tav) to have algorithms from something like f/1 to f/8 and the smaller the aperture, the more the calculation changes. Because, once again, the cam is used to metering wide open.

I don't get the big deal about the PKA mounts...I have 2 and the damn things work under their terms. If they do work, sometimes they won't go under f/4. So: they stay on the slow lenses, and the PKM stays on the 90, my most used lens.

Good thing I paid (literally) nothing for the PKAs, or I'd feel salty :ugh:
04-03-2008, 09:11 AM   #7
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Hang on folks. The camera is in manual mode. With ISO 200, F16 @ 1/180 is sunny 16. I should get a properly exposed image if it's sunny and I dont. I'm not talking about metering - I talking about exposure.

This just seems wrong.
04-03-2008, 09:16 AM   #8
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QuoteOriginally posted by petez Quote
Hang on folks. The camera is in manual mode. With ISO 200, F16 @ 1/180 is sunny 16. I should get a properly exposed image if it's sunny and I dont. I'm not talking about metering - I talking about exposure.

This just seems wrong.
I don't have that problem with my M 400/5.6, which I recently tested so that I would know how to expose properly. The sunny 16 gave me even exposures across the apertures I tested, and they were good exposures. Test shots on my Flickr site in the signature.

I would suggest, that somehow the adapters are hanging up.

04-03-2008, 09:41 AM   #9
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QuoteOriginally posted by petez Quote
I have 2 Tamron lens: a 90mm SP 2.5 macro and a 80-200SP Zoom.

I have one PK adaptall mount and one PKA adaptall mount.

If I use the PKA mount on either lens with the lens in A mode, then manual metering (M mode on the K20D) works just fine. That is, I stop down, press the green button next to shutter release and the shutter speed changes the correct amount. Photos look good.

If I use the PK mount, or use the PKA mount with the aperature not in A mode, I get wrong exposure readings.

Here's what I do:
1) set the f-stop to 2.5
2) press the Green button
3) note the shutter speed
4) stop down 1 stop
5)....continue to f22

Here's my results:

f-stop shutter
2.5 1/90
4 1/20
5.6 1/4
8 1/2
11 1
16 1
22 1.5

The correct exposure is f22 @1 sec.

The exposures aren't reciprocal for a even few stops. If they were I could at least extrapolate from them.

If I actually take a photo with the lens in manual mode using the with the correct exposure settings (taken via a light meter), then the image still doesnt get the correct exposure.

I can see the viewfinder getting darker as I stop down-so I know it's stopping down all they way. It happens on both lenses. It happens with either mount (either the PK or the PKA with the lens not in A mode).

Any ideas?

Pete

PS:I'm starting to see why the PKA adaptall mounts are selling for > $100!
If you consier the above in stops not exposure, I get the following


f-stop error
2.5 -1.2
4 0
5.6 +2
8 +2
11 +2
16 +1
22 +.5


This is very similar to the K10D, as I have reported many times the same issue.

As otehrs have eluded to, thisseems to be related to the super bright viewing screen.

This exposure problem holds true even with brand new lenses when taken out of the A mode.

It does not exist on my *istD, but that has a different viewing screen.

What I suspect is that pentax has known about this and uses the maximum apature data from the lens to correct for the problem,

If you change the view finder screen, I have heard reports from some people that the after market screens or DS screens resolve this problem, but for every solution there is a down side. Fixing the exposure in manual will screw up every lens in A mode. Therefore you cant win everything back. The only difference between changing the screen for manual lenses and then having a problem with A lenses, and the problem you have presently is that for any A lens, the result will be a constant error with that lens, up to potentially +/- 1-2 stops, as opposed to an error that is highly variable over the entire apature range for your Non A lenses.
04-03-2008, 11:30 AM   #10
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QuoteOriginally posted by Lowell Goudge Quote
This exposure problem holds true even with brand new lenses when taken out of the A mode.

What I suspect is that pentax has known about this and uses the maximum apature data from the lens to correct for the problem,

If you change the view finder screen, I have heard reports from some people that the after market screens or DS screens resolve this problem, but for every solution there is a down side. Fixing the exposure in manual will screw up every lens in A mode. Therefore you cant win everything back. The only difference between changing the screen for manual lenses and then having a problem with A lenses, and the problem you have presently is that for any A lens, the result will be a constant error with that lens, up to potentially +/- 1-2 stops, as opposed to an error that is highly variable over the entire apature range for your Non A lenses.


As I said before, I prefer to use my Tam with the M Adaptall. Having said that, I also prefer to use my other lenses' aperture rings. Even with a PKA mount, I still tend to use the ring.

Green button in M mode is just the same as Av mode, except that Av constantly adjusts while M locks it in until you meter again.

(I was mad at first about the overexposure issue. I've worked around it, and can predict it. Let's put the math aside and work through it. You want to have some fun (aka hell)? I'll let you borrow my 300D in Av with an M42 lens Overexposure by up to 5 stops )
04-03-2008, 02:35 PM   #11
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I just got off the phone w/ Pentax. The gentleman was kind enough to test a K20D and a manual lens. He could not reproduce the problem. I explained my frustration and the frustration of others who experience the problem and he is willing to help find a solution. The only catch is it must be with a Pentax lens. So, if you are seeing manual exposure or manual metering errors with a Pentax lens, please reply w/ the lens model and the problem and I'll get him to test it.

Pete
04-03-2008, 03:02 PM   #12
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QuoteOriginally posted by ryan s Quote


As I said before, I prefer to use my Tam with the M Adaptall. Having said that, I also prefer to use my other lenses' aperture rings. Even with a PKA mount, I still tend to use the ring.

Green button in M mode is just the same as Av mode, except that Av constantly adjusts while M locks it in until you meter again.

(I was mad at first about the overexposure issue. I've worked around it, and can predict it. Let's put the math aside and work through it. You want to have some fun (aka hell)? I'll let you borrow my 300D in Av with an M42 lens Overexposure by up to 5 stops )
seems like your the perfect candidate for the DS screen.. Instead of your cyphering all the time, most will behave normally and fairly consistent... in most cases. At worst your error will be way less then you are now seeing. Save your brain for $50
Pretty graph of bad exposures:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/20316-m-mode-green...confusion.html

Wanted to post the graphs of some manual lenses AFTER the screen swap but seemed to have lost them.......
https://www.pentaxforums.com/forums/pentax-dslr-discussion/6724-k10d-ll-60-fo...-m-lenses.html
Good photo comparison....
DigitalFotoNetz.de :: Thema anzeigen - Wrap-around Thread für div. Probleme div. Kameras...
OT: to Blende8: Pentax SLR Talk Forum: Digital Photography Review

Last edited by jeffkrol; 04-03-2008 at 03:30 PM.
04-03-2008, 05:17 PM   #13
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See post #6

The -60 series of screens are supposed to make the issues better, the cam comes with -80 series...
04-03-2008, 05:26 PM   #14
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QuoteOriginally posted by ryan s Quote
See post #6

The -60 series of screens are supposed to make the issues better, the cam comes with -80 series...
Never forgetting that the LL 60 screens will upset the exposure when the lens is in A mode. I just use manual exposure to get around this.
04-03-2008, 05:53 PM   #15
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QuoteOriginally posted by petez Quote
I just got off the phone w/ Pentax. The gentleman was kind enough to test a K20D and a manual lens. He could not reproduce the problem. I explained my frustration and the frustration of others who experience the problem and he is willing to help find a solution. The only catch is it must be with a Pentax lens. So, if you are seeing manual exposure or manual metering errors with a Pentax lens, please reply w/ the lens model and the problem and I'll get him to test it.

Pete
I went through this with my K10D, and sent pentax a spread sheet with all the lenses I had tested up to that time.

As I have bought more I have kept the spread sheet updated.

In the end, I have seen nothing on the K10D to resolve the issue.

I even offered them the technical concept, simply allow us to enter minimum and maximum apature and the camera then controls the stopping down of the lens as it does with KA mounts and newer. The only catch is for manual lenses pentax has to recalibrate the apature actuator. This is because KA mounts and newer have the apature area linear with movement of the lever, where as K mounts have the apature diameter move linearly with the lever. As a result, unless they make this change in calibration, the exposure will be off by the same amount.

They could of course, simply avoid all the software issues and give us a viewing screen that works, but at the rate they shipped K10Ds (I think about 10-15000 per month) they have to retrofit a LOT of cameras.

Lots of luck with them and your K20. I have learned to live with the issue, but wish I didn't have to.

Lenses tested to date on K10D

Pentax K 50mm F1.4
Pentax K 105mm F2.8
Pentax K 135mm F2,5
Pentax K 300mm F4 with and without 2x Takmur TC
Pentax M 100mm F4 Macro
Tamron XR LD 28-75 F2.8 Di (note tested using the apature ring)
Vivitar Series 1 70-210 F3.5
Vivitar 400mm F5.6

All lenses exhibit the same behavior

the general curve is -1 to -1.5 stops under expose at F1.4 moving to correct exposure at about F4 to +1.5 to +2 stops from F8-F16 then coming back slightly to +1 to +1.5 stops by F22/32

All these same lenses are within +/- 1/2 stop over the entire range of f-stops on my *istD.

It is curious to note that my Tamron 28-75 when using the manual apature ring as opposed to the camera controling the apature exhibited the same problem, but is perfect when using the apature controlled by the camera.

Still to test.

Tamron adaptall 2 25mm f2.5
Sigma APO 70-210mm F2.8 EX
Sigma APO 10-20mm EX DC
Pentax SMC - FA 28-105 f4-F5.6
Pentax SMC - FA 28-80mm F3.5-4.5

For all those interested, the most reliable way to perform the test is to take a series of photos, one at each manual F-Stop detent, using the green button to set exposure in manual mode.
Your subject should be a block wall, paved or concrete roadway or path.

For each shotuse a photo-editor's histogram to calculate the grey scale value of the central 10% of the image area.

The camera should be set to 1/2 stop increments, because this matches the mechanical detents of the lens.

ISO should be set to one value. preferrably below 800. Contrast shouldbe set to neutral the camera should not be in bright mode.

With these settings you should have the grey scale (for no exposure error) at about 110, and each stop is a change ingrey scale of 40-45 over the greyscale range of about 25 and 230. You can actually test this out by setting your camera to the middle F stop, set the exposure, and then mechanically changing apature both ways without changing the shutter speed. If you plot grey scale vs f-stop (Fstop on a log scale) you will get an s shaped curve streight in the middle, showing the fariation of grey scale with F stops.

Last edited by Lowell Goudge; 04-03-2008 at 06:20 PM.
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