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05-21-2009, 09:33 AM   #1
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K10D, SMC-M lenses, stop-down metering

I've read quite a bit now on the web about the exposure problems associated with using pre-A series lenses on the K10D (the stop-down metering is inaccurate and the error varies with aperture). I've run into the problem myself, with overexposed images at small apertures and underexposed ones at large apertures. A number of people seem to have had success switching over to an LL-60 focussing screen, and that's a route I may have to go down, although I'm nervous about the work involved in swapping the screens over. In the meantime, though, I was hoping I could kludge around it by metering at a certain known 'good' aperture and then making the necessary adjustments for the aperture / shutter speed I want. Have others found this to be a successful (albeit fiddly) approach? To that end, I'd need to be sure that a given aperture does have a correct exposure, for which I imagine I need to use the histogram. Any tips for doing this 'calibration' would be most welcome - I'm something of a newbie in this area.

On the subject of histograms, am I not right in thinking that the histogram should not vary at all according to aperture (as long as the corresponding shutter speed is set, of course)? Solid evidence of this metering problem is provided by histograms which are different for each aperture, the apex of the curve moving over to the right as the aperture reduces (i.e., f-stop increases).

05-21-2009, 09:45 AM   #2
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I spent a lot of time understanding exposure errors with K mount lenses as I have 8 of them.

I do 2 tests for each lens I own, on each body (*istD and K10D at present)

test #1

for each aperture click on the lens I meter with the green button, and take a shot. my "test subject" is a block wall or paved road, uniformly lit by sunlight.

I then check the greyscale histogram value in a photo editor (I use PSPX2) on a selection of about 10% of the total area in the middle of the shot. PSPX2 lets you view the histogram for either the selection or full frame.

I plot greyscale vs apature (apature on a log scale. that's how I produced the graph for this post:

https://www.pentaxforums.com/forums/241716-post69.html

Test #2

is to meter at something around the middle of the metering range (F5.6.-F8) and as you change aperture, change the shutter speed correspondingly, i.e. each 1/2 stop click should be replicated with a 1/2 stop change in shutter speed. make the same type of plot. THis will tell you if your aperture is absolutely accurate.

Usually there are errors in both ends, or a gradual drift, i.e. a tendancy to over/under expose progressively as you stop down, but this error is usually less than 1/2 stop over the entire range.

Note between about 25 grey scale and 225 greyscale each F stop is between 40 or 50 greyscale value (depending on low or high contrast respectively)

The bottom line is to know your lenses.

As for changing the focusing screen, I swapped between K10D and *istD in about 10 minutes. It's pretty easy.

right now, I have a split image screen in my K10D
05-21-2009, 10:05 AM   #3
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Lowell, thanks for that - you were one of the people I hoped would reply! I'll give the testing a go - I imagine Photoshop will have something comparable to the PSPX2 feature you use.

If you are not using the LL-60 screen (or the one from the *ist), am I to take it that you do the kind of overriding of the metering that we're talking about? or does your split image screen sort the problem?
05-21-2009, 10:12 AM   #4
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QuoteOriginally posted by J2R Quote
Lowell, thanks for that - you were one of the people I hoped would reply! I'll give the testing a go - I imagine Photoshop will have something comparable to the PSPX2 feature you use.

If you are not using the LL-60 screen (or the one from the *ist), am I to take it that you do the kind of overriding of the metering that we're talking about? or does your split image screen sort the problem?
No split image makes spot metering especially, worse. What I do is use the histogram, and select a suitible grey surface. (tree trunk or road is good, although I have again doug out my 18% grey card also) I then do a test shot on that surface, check the histogram, and manually adjust either shutter or aperture so that the peak of the histogram (from the grey surface) is in the middle. Once set, unless you change lighting conditions, you just leave it alone. I did the same approach (except without histogram obviously) when I shot film i.e. meter off a grey surface and go and shoot. Many people who shoot manual do a similar thing.

With some combos, if I ever get a K20, that allows exposure compensation, I will be able to just dial in an offset. When I use my SMC 300F4 anmd SMC-F 1.7x AFTC the effective aperture is F6.8 (Ithink) and always tends to over expose by 1 1.2 stops. right now, I simply set aperture, meter and then dial up the shutter speed, but it would be nice to have it always entered as compensation

05-21-2009, 11:14 AM   #5
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Lowell, you are evidently a patient man - most commendable!

What strikes me as strange about this whole business is that the error was not picked up by Pentax before going into full production. It's not a subtle, hard to reproduce problem - I was surprised how gross the error was (at least with the lenses I've tried), even after reading about it here. Am I right in thinking that it does not apply at all to A series lenses? Unfortunately I only have 1 of those (a 28mm), whereas I have 35mm, 50mm, 135mm and 300mm prime lenses and an 80-200mm zoom all in the M series (along with a couple of Ricohs).
05-21-2009, 11:27 AM   #6
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QuoteOriginally posted by J2R Quote
Am I right in thinking that it does not apply at all to A series lenses?
You are correct.

As for Pentax letting this slip by...The legacy glass exposure issue was known at the time the user's manual was printed. There is a small disclaimer for AV and green-button exposure.

Steve
05-21-2009, 11:45 AM   #7
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Stevebrot : in fact, I think it does have an impact on A-lenses metering, but as it's constant over the whole aperture range (as metering is done wide-open), it's not so blabant as on M-lenses...

But fast A-lenses surely underexpose quite a bit.... My FA 50/1.4 could easily do with a +1 ev comp, without blowing the highlights...
05-21-2009, 01:40 PM   #8
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I find this interesting since the owner's manaul for the ZX-5n says fast lenses will over expose by +1 stop. What's really going on here?

05-21-2009, 01:57 PM   #9
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QuoteOriginally posted by dlacouture Quote
Stevebrot : in fact, I think it does have an impact on A-lenses metering, but as it's constant over the whole aperture range (as metering is done wide-open), it's not so blabant as on M-lenses...

But fast A-lenses surely underexpose quite a bit.... My FA 50/1.4 could easily do with a +1 ev comp, without blowing the highlights...
I only have the A 50/1.7 and the FA 35/2 and both expose accurately and consistently at all apertures. Ditto for my Tamrons when the PK/A adapter is used. The only exposure issue I have noticed with my A-contact lenses is 1/2 stop difference between exposure at infinity vs. close focus with the A 50/1.7. Go figure...

Out of curiosity, what do you think is happening with your FA 50/1.4? Excess light scatter biasing the meter?

Steve

(Almost forgot about the Sigma macro...it is fine too...)

Last edited by stevebrot; 05-21-2009 at 02:07 PM.
05-21-2009, 01:57 PM   #10
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QuoteOriginally posted by J2R Quote
Lowell, you are evidently a patient man - most commendable!

What strikes me as strange about this whole business is that the error was not picked up by Pentax before going into full production. It's not a subtle, hard to reproduce problem - I was surprised how gross the error was (at least with the lenses I've tried), even after reading about it here. Am I right in thinking that it does not apply at all to A series lenses? Unfortunately I only have 1 of those (a 28mm), whereas I have 35mm, 50mm, 135mm and 300mm prime lenses and an 80-200mm zoom all in the M series (along with a couple of Ricohs).
I think pentax elected to "live with the issue" so to speak.

I advised them of the problem in writing, but had not fully investigated it. What is interesting is that an A lens does the same think, when it is taken out of A.

I think what they did, was realize the error existed, and in the quest for a brighter viewing screen, account for it in exposure compensation based on maximum apature. What is interesiting is that my sigma TCs are simply feed through for apature data, and as a result putting a TC on my 70-200 introduces a constant exposure error because the real apature has changed byt the body does not know it. Note the SMC-F 1.7x AF TC modifies the apature reported to the body of an A lens so metering is not impacted.

I also asked if they would re-calibrate my metering based on having an *istD screen installed, the answer was no to that also. that was called a modified camera.

for me personally, I would have preferred being given the choise, just like with focus adjustment etc.

Edit note

one add on point, if we had on the input menu maximum apature as well as focal length, pentax could program the green button to follow the error curve and fix the whole issue

Last edited by Lowell Goudge; 05-21-2009 at 02:57 PM.
05-22-2009, 03:26 AM   #11
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stevebrot: Interesting that the problem is alluded to in the manual. I keep forgetting that I don't actually have a K10D but a Samsung GX-10, and I've not seen any mention of this in the GX-10 manual.
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