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04-26-2007, 06:37 PM   #1
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K10D Exposure w/Green Button in M mode

As a follow-up to this thread where we discussed using M or K lenses on a K10D and the variable results using the 'Green Button', I performed a quick little test.

I don't own a gray card (although I probably should) so I substituted the cardboard back of a spiral-bound notebook. (Note: This means that the numbers I'm posting are not true 'gray card' numbers! This is not so much an exercise in evaluating the meter's accuracy but rather its consistency.) These are also shot handheld, center-weight average, manually focused with my eye to the viewfinder under dismal overcast skies. It was about 55° F.

I shot the same scene at ISO 100 with the Super-Multi-Coated Takumar 35/2 and the Super-Multi-Coated Takumar 50/1.4. I shot the binder one last time with the SMC Pentax (i.e. the K version) 200/4 at ISO 200 (the last three at ISO 800 due to slow shutters). With all three lenses I fired a shot at every single aperture click. (11 shots with the 35/2, 13 shots with the 50/1.4, and 11 shots with the 200/4.)

The pictures were opened in Photoshop CS2 using default ACR values (not 'Auto'), a linear curve, and no noise reduction, cropped identically to 800x800 center pixels. What I'm reporting here is the median pixel (brightness) value of the resultant crop.


The graph:


A sample of the scene, this one at f/8 with the S-M-C Takumar 50/1.4:

(On the right is a print of this picture.)

My conclusion is that the K10 experiences a gradual increase in calculated exposure as you stop down. I found it fairly linear and predictable until around f/16. To my tastes, I think the 'correct' number for my test was about 180 - meaning the number that fairly reproduced the scene. (As jeffkrol points out this number doesn't mean much.)


Last edited by carpents; 04-26-2007 at 08:21 PM. Reason: Pbase is stupid.
04-26-2007, 07:08 PM   #2
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Where's your graph? And why do you think 180 is correct? White wall, grey card, doesn't matter. Meter should peak from 90 (old D ), 110 (DS ect) and possibly 126 w/ the K series. Where do you get 180 from... way too hot w/ 0 exp compensation.

Fotogenetic - D100 and D70 Custom Tone Curves
Here's a funny one: In the middle of the article he states"Note that with the proper exposure (EV 0) the data is all in the middle as it should be." but if you look at the actual chart, you'll see that the meani s 110. Not thew middle (126ish).
Digital Tones and Exposure Zones

Last edited by jeffkrol; 04-26-2007 at 07:16 PM.
04-26-2007, 07:40 PM   #3
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QuoteOriginally posted by jeffkrol Quote
Where's your graph?
It doesn't show up?

QuoteOriginally posted by jeffkrol Quote
And why do you think 180 is correct?
I meant as far as which pictures came out fairly representing the scene. It of course isn't going to be the same number as a 'real' gray card would give because the spiral notebook is brighter/darker than 18% (or whatever) gray.
04-26-2007, 08:01 PM   #4
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QuoteOriginally posted by carpents Quote
It doesn't show up?


I meant as far as which pictures came out fairly representing the scene. It of course isn't going to be the same number as a 'real' gray card would give because the spiral notebook is brighter/darker than 18% (or whatever) gray.
It shows up now, but not before. Anyways that's the fun thing about light meters. Grey, white, they should all look the same as to the histogram. BTW Ugly graph...
the data that is.......
Odd the graph disappearred again... now I'm baffled.
opps, back again, and the notebook.
Now it's gone again.....
Driving me crazy, I'm reposting your image if you don't mind


SCREEN capture, so you don't think I'm too crazy:



Last edited by jeffkrol; 04-26-2007 at 08:17 PM.
04-26-2007, 08:24 PM   #5
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Because I hate pbase so much I updated the links and created a gallery.


It only contains crops from the 50mm:
Images relevant to this conversation.

So, Jeff, you don't like the slope on the graph?
04-26-2007, 08:35 PM   #6
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Nope, don't want to do that much thinking in the field......
That's about the ugliest exposure curve I ever saw from a camera. If I were you I'd buy a DS screen from Pentax pronto.... Actually that I can live with. 2 screens one for AF/A lenses, one for K/M / screwmount/preset lenses.
04-26-2007, 08:37 PM   #7
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How satisfying! A quantitative representation of exactly what I see qualitatively with my K10D and M/K lenses with the green button. I've "superceded" most of these old lenses with DA zooms and primes, so I don't use them much, but there are a couple (a K28/2 and K300/4) I used occasionally with my DS2.

A number of anecdotal accounts of this sort of behavior with the the K10D have appeared on various forums since the camera was introduced, but some apparently don't experience such overexposure with their examples so who knows the fraction of K10Ds overexpose with the old lenses? As far as I know, Pentax has never commented on this issue, but I may be behind the curve.

Thanks for going to the trouble of running the experiment and posting the results.

Jer
04-26-2007, 08:42 PM   #8
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Actually, there is the "don't ask, don't tell school" ... Many that say they "don't have problems" just mean they manually compensate for the gross errors. Many K10 users have no "manual lenses"... ect.
If this "engineering marvel" was OK'd by Urano, it's good he got canned.... I'm baffled by what they were thinking. And even more baffled Pentax didn't get slammed hard from reviewers on this issue...

04-26-2007, 08:50 PM   #9
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QuoteOriginally posted by jeffkrol Quote
Actually, there is the "don't ask, don't tell school" ... Many that say they "don't have problems" just mean they manually compensate for the gross errors. Many K10 users have no "manual lenses"... ect.
If this "engineering marvel" was OK'd by Urano, it's good he got canned.... I'm baffled by what they were thinking. And even more baffled Pentax didn't get slammed hard from reviewers on this issue...
Ha ha, I'm on the 'compensate' side. Some have reported wild and un-usable exposure at f/8 and smaller, which I don't see.

I read the link that RH posted about manually focusing and I would imagine that has to play a part. It sounds like manufacturers are more concerned with making their new lenses more usable...sounds like a good game plan. It also seems like these problems stem from the focus screen.

Perhaps we can get Pentax to make a 'manual focus' screen that has a flatter curve and works all the way down to f/1.2. But would the darkness trade-off be worth it?

I just dial in exposure compensation...
04-26-2007, 09:04 PM   #10
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Actually if you look at Jonas's response and the one to blende8 on the german forum, the "loss of brightness" seems not so great. Field compensation is OK, but to state "useable w/ 1000's of old lenses", though true, is really bordering on good HONEST (is there really such a thing business practice. And as much as I like the "retro" feel of things, this would border on using a hand held meter and straight M mode, completely ignoring the built in meter. No, I really can't accept this (or to be honest VPN). On the bright side, it still is a good camera, for many, but I'll wait. My D still works fine. As to RH, that link he posted (still pretty well ignore his slant on things) was an interesting thing regarding possible manual focus errors. (which also can apparently be solved w/ a DS screen). If you go to Steves forums, you can see the problems this has caused ennacac
http://www.stevesforums.com/forums/view_topic.php?id=569114&forum_id=80
All over a dumb focus screen. good thing it's easily rectified w/ $50............
But the problems are real, not just figments of newbies imagination (unfortunately they are usually more impatient). Being a "old timer" from the D exposure wars has made me a bit outspoken..

Last edited by jeffkrol; 04-26-2007 at 09:12 PM.
04-26-2007, 09:19 PM   #11
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Thanks, carpents, for taking the time to document the behavior. Now perhaps I should put my K 24/2.8 to use again!
04-26-2007, 09:58 PM   #12
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I just got my K10D this week and I tested my Tokina 28-70mm f/2.8-4.3 lens this evening. In Av mode with aperture ring set to A, I'm at least 2 stops over exposed.
Do you guys leave the setting to permit aperture ring left enabled all the time for the times you use a lens without the auto setting or to jump from manual mode to say Av mode?? Does it hurt anything to leave it enabled?

I had read the thread referred to here but after rereading it I see the reply there about the lens staying wide open in Av mode.

I probably shouldn't have said anything, the more I read that thread the more confused I'm becoming, which is a common occurrence:-). I need to play with the camera more.

Last edited by Eaglerapids; 04-26-2007 at 10:23 PM.
04-26-2007, 10:37 PM   #13
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QuoteOriginally posted by Eaglerapids Quote
I just got my K10D this week and I tested my Tokina 28-70mm f/2.8-4.3 lens this evening. In Av mode with aperture ring set to A, I'm at least 2 stops over exposed.
Do you guys leave the setting to permit aperture ring left enabled all the time for the times you use a lens without the auto setting or to jump from manual mode to say Av mode?? Does it hurt anything to leave it enabled?

I had read the thread referred to here but after rereading it I see the reply there about the lens staying wide open in Av mode.

I probably shouldn't have said anything, the more I read that thread the more confused I'm becoming, which is a common occurrence:-). I need to play with the camera more.
Staying wide open in AV (or any of the auto modes) is fine for an "A" lens. The camera will meter wide open, but also will mechanically be able to "close the aperature" to the desired setting. The discussion here involves the case where the mechanical linkage is not available or not really fully functionnally coupled to the electronics (M lenses, K lenses, A lenses OFF the A position, and screw mounts).
If your lens is behaving, and your bodies behaving, and you don't have exposure compensation dialed in, or your not using the wrong flash (I may have missed one or 2 scenarios), then the camera should not overexpose by 2 stops....
It is generally pointless to use any setting other than A with a KA lens. Or M mode w/ a KA lens for that matter (actually I guess that one's not too bad). AV and TV and when needed Exposure compensation is just fine.

Last edited by jeffkrol; 04-26-2007 at 10:56 PM.
04-27-2007, 12:02 AM   #14
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I did such a test, too, and posted it on the German Pentax forum and also told Pentax about it.
Please do so, too, tell Pentax!
The more they know and hear about it, and see "scientific" tests, the more we can hope that they will do something about it.
04-27-2007, 03:20 AM   #15
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Thank you for that Sean. I understand what you mean with the difference between a grey card and your cardboard: as the whole scene is bigger than the cardboard it doesn't give a true "grey card value" in return. In my series the grey card filled the viewfinder. OTOH, to my eyes is doesn't the rest of your scene differ a lot from the cardboard.

I have put the numbers I posted in the preceeding thread
https://www.pentaxforums.com/forums/pentax-dslr-discussion/6262-k10d-m-k-lenses.html
into a diagram, and as a tribute to my personality I also added the same for a lens with the Canon 5D:

Note: The white EV0 refernce line is placed at RGB 97. It can be discussed if it should be a bit higher, perhaps at 110. There are different preferences and theories over time on this one. I just reused an old diagram and you can feel free to move around the reference lines according to your own take on this.

I wonder how the differences between mine and Seans curves can be explained? Has Pentax done anything about this during the production (I know my camera is a late one compared to yours)?

I may have to drop this over at King Phil's place as well and see what comes out from that.

My interpretion is that the K10D is nearly compatible with 25 millions of lenses if you tweak it a little. (OTOH it's obvious that the Canon is more compatible, and with more lenses as well.)
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