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02-07-2008, 08:48 PM   #1
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M-Mode Green Button M42 mount Confusion...

I've been doing some experiments using my legacy M42 Takumars on my K10D, and am confused by my results to date. I am hoping someone here can either explain my results, or tell me what I am doing wrong!

First, the set-up. I'm using a small light box as a "constant" source; I measured the light box brightness using a Pentax 1/21 spotmeter and found it to be a pretty uniform 11-2/3 (11.67) EV. I mount one of the Takumars (35/3.5, 50/1.4, 85/1.8 or 105/2.8) on the K10D using a Pentax K Mount adapter. Using a tripod, and with the lens set to infinity, I position the lens
against the face of the light box. I set the camera to ISO 100, M-mode, center-weighted metering, EV steps set to 1/3, lens set to M (manual aperture). With the eyepiece covered, I step the lens through it's aperture range, pressing the green button 3-4 times for each aperture and record the indicated shutter speed.

Second, data reduction and analysis. For each Av,Tv pair I compute the equivalent Exposure Value (EV) using the standard formula {EV=log2(f*f/time/(ISO/100))}. I then graph EV versus Aperture for each lens.

Looking at the graphs is where things get confusing! My expectation, given a constant EV source, is that I'd get more or less horizontal lines. My results are anything but! Each lens produces a falling EV curve as the aperture is stopped down. The 50/1.4 and 85/1.8 lenses have an extra quirk in that the EV curve initially goes up, before following the downward trend after about f/2.8. The first attachment is a composite graph for the four lenses. Note that it appears from visual inspection that the 50/1.4 does not actually close down to f/16 (no change in the iris when the ring is moved from f/11 to f/16), so the upswing in its curve at f/16 is bogus.

To compound my confusion, I repeated the tests using three KA-mount lenses - a FA 50/1.4, a FA 100/3.5, and an A 50/1.7 - and obtained similar results, as shown in the second attached graph.

Next, I decided to see what I would get from the Takumars if I mounted them on my ZX-5n. Using the same set-up and procedure, I took data on the same four lenses. When graphed, these data were much more in line with my expectations as you can see by examining the third graph...

I was a little surprised that there was a definite spread in the detected EV across the lenses, but examination of the lenses suggested one possibility: the 50/1.4, which recorded the lowest EVs, seems to have a yellowish tint, while the other lenses seemed more neutral.

I made one final graph in an attempt to take out these possible 'transmission' effects. I plotted the difference in indicated EV for a particular lens/aperture between the K10D and the ZX-5n. This should leave only the difference in detected EV between the K10D and the ZX-5n metering systems. This graph is the last of the attachments.

As you can see, now all four lenses show a reasonably consistent behavior. Unfortunately the consistent behavior suggests that the K10D manual metering mode using the green button with legacy lenses, whether KA lenses set to manual apertures, or M42 lenses with manually set apertures, is not particularly accurate. Granted this is not a very real world test, but it is fairly well controlled, or at least I believe it to be.

So, where have I gone wrong? Or, if I've got it right, why would Pentax design the system this way? Inquiring minds want to know!!!

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02-07-2008, 10:01 PM   #2
Ole
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This has been discussed before. The general conclusion is that the K10D doesn't expose correctly over the range of aperture values when set manually. Some have speculated that the reason is whatever Pentax did to the focusing screen to make it brighter (the light meter measures off the focusing screen). Replacing the screen with one from an *istD should solve the problem.

Had the lens mount not been crippled we would not have had the problem. Your ZX-5N has a non-crippled lensmount with aperture simulator and therefore always measures the light at full opening no matter the aperture value set on the lens.
02-07-2008, 10:03 PM   #3
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Great work with the graphs and everything - thanks. As far as I know, this is a known problem with the K10D's focusing screen. It doesn't give very accurate exposure with a stopped-down lens. The possible solutions seem to be either replacing the focusing screen with one from one of the *ist cameras, or just learn and compensate for the error.
02-08-2008, 05:13 AM   #4
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Interesting that your graphs look backwards from mine. I wrote about it here.

As Ole says, I replaced my focus screen with an LL-60 and all is well.

02-08-2008, 06:31 AM   #5
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Thanks for the replies!

Ole, on the ZX-5n, I only took data with the M42 Takumars, so the metering was done stopped down. As you say, the ZX-5n does not permit stopped down metering with KA lenses...

ankit, I too suspect the focusing screen, based on other posts. Where can I buy a focusing screen from another Pentax? Haven't been able to locate same at B&H or Adorama...

carpents, I think our graphs actually say the same thing... mine are in 'perceived' scene brightness, which drops as the aperture gets smaller, which means the camera thinks there is less light than there really is, so the image taken would be over exposed. And thanks for identifying the part number of a suitable screen (LL-60); maybe now I can find one!

By the way, since I also don't like manual focusing with the K10D's screen, would I be better off going with a Katz Eye replacement, versus the LL-60?

Curt
02-08-2008, 07:12 AM   #6
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QuoteOriginally posted by MNCurt-K10D Quote

By the way, since I also don't like manual focusing with the K10D's screen, would I be better off going with a Katz Eye replacement, versus the LL-60?
That was my immediate reaction. Anyone know if the same issue obtains with the Katz Eye?

woof
02-08-2008, 08:07 AM   #7
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This and the linked thread are interesting. Although I wasn't nearly as precise in my own testing, I noticed the same results with K,M,M42 lenses. I posted some questions about this over-exposure issue (on another forum) and got mildly slammed for saying there was a metering issue, so I have since shut up. Basically metering at f8 and adjusting as needed for each lens's varying characteristics.

So If you replace the screens, how does that effect F, FA and DA lenses? I'd assume there's no issue as they meter wide open. Is that correct?
02-08-2008, 08:18 AM   #8
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QuoteOriginally posted by Peter Zack Quote
This and the linked thread are interesting. Although I wasn't nearly as precise in my own testing, I noticed the same results with K,M,M42 lenses. I posted some questions about this over-exposure issue (on another forum) and got mildly slammed for saying there was a metering issue, so I have since shut up. Basically metering at f8 and adjusting as needed for each lens's varying characteristics.

So If you replace the screens, how does that effect F, FA and DA lenses? I'd assume there's no issue as they meter wide open. Is that correct?
There was some debate about that. Will have to find the work by jensR I believe.
,,,,,,,,,,,,,,,,,,,,,,,
TO WOOF: Katz eye are still problematic for mf. Only the DS screens help

02-08-2008, 08:31 AM   #9
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QuoteOriginally posted by Peter Zack Quote
This and the linked thread are interesting. Although I wasn't nearly as precise in my own testing, I noticed the same results with K,M,M42 lenses. I posted some questions about this over-exposure issue (on another forum) and got mildly slammed for saying there was a metering issue, so I have since shut up. Basically metering at f8 and adjusting as needed for each lens's varying characteristics.

So If you replace the screens, how does that effect F, FA and DA lenses? I'd assume there's no issue as they meter wide open. Is that correct?
I'd be happy to run tests with my FA lenses. I have not noticed metering problems with any of my lenses since changing to the LL-60 focus screen.
02-08-2008, 08:34 AM   #10
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That would be great Sean before investing in yet another part...
02-08-2008, 05:37 PM   #11
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QuoteOriginally posted by carpents Quote
I'd be happy to run tests with my FA lenses. I have not noticed metering problems with any of my lenses since changing to the LL-60 focus screen.
As Peter says, that would be great. I have a few FAs and would love to know the results here.

woof
02-08-2008, 07:50 PM   #12
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Lance had problems....

but as far as I know, nobody else has had any issues (there's a good chance I missed a few).
Maybe most swap when using AF lenses.....
https://www.pentaxforums.com/forums/pentax-dslr-discussion/1157-katz-eye-ll60...work-k10d.html
To add to the confusion it may be firmware dependent......
https://www.pentaxforums.com/forums/pentax-dslr-discussion/9624-focusing-screens.html
Anyways short of the REALLY bad metering w/ m lenses and the orig screen, there isn't much choice.....
02-08-2008, 10:02 PM   #13
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I've had interesting results with manual and "A" lenses (and the original focus screen). I keep meaning to do some proper testing and make some graphs, if nothing else to help me get proper exposures What I can say is that I seem to get overexposed when shooting wide open, and a tad underxposed when stopping down. I won't say anything else and hopefully I'll find time soon to run the tests and add them to this thread. I also have a split-focus screen that I'll run a second set of tests with after I've done them with the original focus screen.
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