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01-08-2011, 11:32 AM - 1 Like   #1
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K-5 with A/M lenses

I tried something I have done previously with the k-x (see here). The enclosed pictures have been taken from the same tripod position under the light from a single 40W tungsten spot (*) with AWB, Pentax-A 50mm 1:1.7, mode=M, ISO=200, t=0.8s, f=8 the only difference being the aperture ring in A (#1) and 8 (#2). The images have been converted in camera to 0.3M for posting. [Firmware 1.02]

Observation: Comparing the images, there is a yellowish tint to #2.

Conclusion: The K-5 (too) treats A- and M-lenses differently. With the latter AWB would seem to be off.

I'm not saying this is a big deal as such, however, this begs the question of the rationale of the different treatment as well as what the difference might be exactly.

(*) There is a window in the room, but as it was dark outside this shouldn't matter.

[edit: for some reason these ended sideways when uploaded, they illustrate the point just the same :-)]


Last edited by jolepp; 03-03-2017 at 02:41 AM.
01-08-2011, 12:24 PM   #2
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An interesting experiment. I take it you found this to be a consistent and repeatable behavior (on my K100D, I sometimes find the AWB has changed between consecutive shots when I haven't moved). Have you only noticed this in tungsten light, or does this occur in other light as well?
01-08-2011, 12:40 PM   #3
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I originally noticed this with the k-x when comparing the M50 and A50 with the same tungsten lit scene. I haven't done any extensive testing, but this would seem to be repeatable; anyway the best way to test repeatability would be other people trying out a similar thing.

I'd speculate that this is a feature of a shared FW code base. If so, the more interesting question is what else (and why) might also be different between older and newer lenses as the older ones are quite popular, and in fact an often cited reason to choose Pentax; a point with which I quite agree :-)
01-09-2011, 04:58 AM   #4
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When I opened the above as .DNGs in ufraw the "camera WB" values were:

#1 : 3173 K
#2 : 3749 K

That would pretty much explain the difference in color tint. Using the value from #1 to #2 did not quite make these identical, but #2 needed to be adjusted -1/6 EV to make them about as bright, the color casts were still slightly different.

01-09-2011, 01:26 PM   #5
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I found the same thing using M42 S-M-C 50/1.4. I also used to have K-x and my recollection is that the color cast wasn't nearly so pronounced on that body. It's corrected easily enough, but the K-5 needs more compensation - another thing to fiddle with when using the lens.. Just out of interest, anyone have a theory why WB is affected by what lens you have on?

Last edited by syoon1970; 01-09-2011 at 01:34 PM.
01-09-2011, 02:08 PM   #6
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Could it not be the coatings on the lenses are a bit different given they were likely made at different times? I notice this too on my *ist with my 50's but I've always chalked it up to a slight probably difference in the multicoatings. My older 50 does have a tiny bit of a gold cast if you look very closely while my later 50 does not and they are both Takumars. One is all manual, one is not.
01-09-2011, 02:42 PM   #7
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I have a feeling that it has to do with the metering. A lenses can use Matrix Metering while M lenses are restricted to Center Weight Average.
01-10-2011, 08:54 AM   #8
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A clarification: the shots above are from the very same Pentax A 50mm 1:1.7; I simply took pic#1, switched the camera off and moved the ring from the 'A' position to '8', turned camera back on, and took pic#2. I'm assuming that the body treats an A lens as an M lens when the aperture ring is not in the A position.

Also, the metering switch stayed in the CW position; the only change between #1 and #2 was moving the aperture ring.

01-10-2011, 10:28 AM   #9
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This is all very interesting. I may have to try this on my K100D to see what it does.

Perhaps the firmware does some sort of correction for the white balance based upon the minimum aperture of the lens (which would not be too shocking as it affects the amount of light coming in), and so without that information it uses some other default value. In other words it includes the minimum aperture as a figure in it's calculation of the AWB, unless the lens is not on A. That would be my best guess as to why this is happening (could also explain the slight difference is exposure, as that same correction may occur there as well)
01-10-2011, 10:30 AM   #10
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Jolepp, I assume the mode dial is in "M" for each shot? What happens if you shoot the "A" lens in Av priority? (ie "A" lens in Av and "M" lens in M) Does that change anything? What about other apertures? f/2.8? f/16? Also what about screwmount lenses?

NaCl(curious phenomenon)H2O
01-10-2011, 11:00 AM   #11
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Mode dial stayed at M too from #1 to #2. I did not try stop down metering with the aperture ring at '8', could do that and some other apertures under similar conditions. I suppose I might get the body to treat the A50 as if was a typical M42, by isolating all the contacts with something suitable ...
01-10-2011, 11:24 AM   #12
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i haven't done a controlled test of it, but i do notice a difference in wb handling on my M lenses, sometimes it's to the benefit of the shot others not, however as i shoot in RAW exclusively i tend to fine tune WB in lightroom by picking a white area to set the custom WB, occasionally I revert to the as shot though as I like the way it looked. I think it may well be in the lens coatings, on older Taks it may be the radioactive coating has yellowed a little as well
01-10-2011, 10:24 PM   #13
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Well, I did one very quick test on my K100D with my A28/2.8 with the aperture ring in A and at the numerical stop (I used f2.8), both in M mode, and there was no WB difference between the shots (overcast natural light).

Jolepp, have you noticed this (or can you test this) in lighting other than Tungsten. I was doing some more thinking, and I remembered that Pentax did change how it dealt with Tungsten white balance on the K-7. With it there was added a custom setting for subtle or strong correction, and so too on the K-5 (Custom setting 12). The default is subtle--is that still what you have it set at? If the AWB differential is restricted to only tungsten lighting, then I would suspect that the firmware is simply a little buggy and these two newer guidelines only got applied when a A lens is attached; whereas, it is still using a (older) less-correcting WB guide for the M lenses.
01-11-2011, 02:24 AM   #14
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I've found the K-5 AWB to be less accurate than the K-x's and it has nothing to do with the lens being used.
01-14-2011, 10:07 AM - 1 Like   #15
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QuoteOriginally posted by macTak Quote
...
Jolepp, have you noticed this (or can you test this) in lighting other than Tungsten. I was doing some more thinking, and I remembered that Pentax did change how it dealt with Tungsten white balance on the K-7. With it there was added a custom setting for subtle or strong correction, and so too on the K-5 (Custom setting 12). The default is subtle--is that still what you have it set at? If the AWB differential is restricted to only tungsten lighting, then I would suspect that the firmware is simply a little buggy and these two newer guidelines only got applied when a A lens is attached; whereas, it is still using a (older) less-correcting WB guide for the M lenses.
This sounds a likely explanation to me; the tungsten light correction might be in the same bin (share a sw flag?) with matrix metering, using a pttl flash (and other minor differences as to how the bodies deal with A / M / M42 (isolated base) lenses).

As for testing for other color temperatures, that would ideally need a light source where that could be controlled. I tried some more snaps with the household tungsten spot though; it seems that using other apertures (2.8/16) at least doesn't make a difference with the A50 in 'A' position vs. aperture selected with the ring. Also, I tried some thin paper between the lens and body to isolate the contacs for M42 lens emulation. It would seem that at least fully open M42s and Ms are treated the same (the body would seem to like hold the aperture lever at fully open ...).
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