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07-10-2010, 07:27 AM   #1
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non-linearity of K20 sensor

In attempting to calibrate my Sekonic 558 meter and K20d, I've noticed something strange: in bright sunlight at iso 100, the camera never goes below f11.3@125. Even at the height of summer at noon, it never gets up to f16. in Bright sunlight, however, the Sekonic indicates f16, and in fact tests with slide film are exposed correctly. This would seem to indicate that the native sensitivity of the k20 is actually about iso 64.

The problem is that at lower light levels, and with studio strobes, the k20 seems to gain sensitivity, and the exposure of it and the Sekonic start to converge. In other words, it starts to behave like a 100 iso sensor.

Has anyone found this? I'd think it unusual to have to open up a stop only in bright sunlight to match my meter.

thanks

07-10-2010, 07:58 AM   #2
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Maybe you set the program line in a way that limits the aperture values available? MTF priority, perhaps?
07-10-2010, 09:45 AM   #3
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QuoteOriginally posted by bdery Quote
Maybe you set the program line in a way that limits the aperture values available? MTF priority, perhaps?
It would be the first thing I'd check.
Also, the Sekonic has almost 2 more stops of metering range on both sides.
The Pentax is 0-21 EV, the Sekonic is -2 to 22.9, and it would surprise me not at all to find the Pentax specification to be somewhat optimistic.
07-10-2010, 10:21 AM   #4
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Hi,
I don't use program. When I was attempting to establish some kind of calibration between my Sekonic and the k20, I used manual, took a variety of exposures, and noted which looked best.

I would have thought that if the k20 looked best at f11.3 in bright sunlight, then some sort of linear relationship would also apply in other situations. In other words, if it acts like iso 64 in the sun, it should also act like it in the shade.

07-10-2010, 01:54 PM   #5
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QuoteOriginally posted by colesf Quote
This would seem to indicate that the native sensitivity of the k20 is actually about iso 64.
No, it doesn't really say anything about the native ISO at all. It could be anything based on that. All this says is that the ISO settings across the board are higher than you think they should be (eg, ISO 100 on the camera is really 64; 200 is really 128, 400 is really 2456, etc). But native could be anything.

I don't know your meter and how reliable it is, but folks that test cameras for adherence to ISO standards generally find all Pentax DSLR's with a couple of percent of accurate at all settings (see for instance the dxomark site - at least, I think that's one of them). So if you meter disagrees, I'd tend to assume it's the one not adhering to ISO standards. Most likely, it was calibrated to work well with a particular film, not to actually adhere to ISO standards. Perhaps it was also calibrated to match the reciprocity failure of that film, and that's why you see a differences at different light levels.
07-11-2010, 05:24 AM   #6
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QuoteQuote:
it would surprise me not at all to find the Pentax specification to be somewhat optimistic
If Pentax are ISO certified (and I believe they are) then all their qualification and specification testings and uncertainties and tolerances etc are audited by the ISO group regularly.

That does not garantee quality, of course (I work in R&D and we often test competitors' devices and find them not compliant) but I think it's fait to assume that the specs are not "optimistic".
07-11-2010, 06:09 AM   #7
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Just wondering if any of the following were set: Auto ISO is set, sensitivity is set to expanded in the Custom menu, Expand Dynamic Range function is turned on, Auto EV Compensation is turned on. Any of these could affect exposure.

Also I'm not sure how you're using you Sekonic meter... Are you taking incident or reflected light meter readings and comparing it to the camera's meter (reflected light)? If you are comparing reflected light readings, the angle of coverage of the light meter is important.
07-11-2010, 09:41 AM   #8
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QuoteOriginally posted by creampuff Quote
Just wondering if any of the following were set: Auto ISO is set, sensitivity is set to expanded in the Custom menu, Expand Dynamic Range function is turned on, Auto EV Compensation is turned on. Any of these could affect exposure.

Also I'm not sure how you're using you Sekonic meter... Are you taking incident or reflected light meter readings and comparing it to the camera's meter (reflected light)? If you are comparing reflected light readings, the angle of coverage of the light meter is important.
He should be metering a gray card with the camera and then holding his incident meter at the gray card if he is hoping for any sort of accuracy.
I missed this in the original post, he hasn't outlined his "test" methodology, and may well be botching his procedure.

07-11-2010, 09:47 AM   #9
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Yes,
I've also used a gray card, and my camera is not set to any of the optional things like expanded DR.
My Sekonic is very accurate for iso 100 slide film.

So I'm wondering, for those with the k20, what reading do you get for a typical clear sunny summer mid-day? (iso 100, 1/125).
07-11-2010, 10:53 AM   #10
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I don't have the K20D, but I don't get anything close to "Sunny 16" except for average or light colored subjects in full direct sun with no shadows. Apparently the ISO exposure standards don't actually support Sunny 16 all that well except in those cases. But see, for instance, DxOMark - Pentax K20D for the K20D actual measured sensitivities according to the standards. *Just* shy of nominal; nothing remotely close to the almost full stop difference you are suggesting.

Last edited by Marc Sabatella; 07-11-2010 at 11:15 AM.
07-11-2010, 10:57 AM   #11
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I use a 'Sunny 11 Rule'. This has held true for my ist D, K10, and K20.
With film and my old cameras I use a Sekonic L398 meter. Exposures on NEG rather than Slide film follow the Sunny 16 Rule closely. Not exact but close enough.
Playing around one day I used the meter with the ist D and saw what you've seen.
Why ? Who knows.....I just adapted to the Sunny 11 and adjust from there.
07-11-2010, 02:01 PM   #12
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QuoteOriginally posted by bdery Quote
If Pentax are ISO certified (and I believe they are) then all their qualification and specification testings and uncertainties and tolerances etc are audited by the ISO group regularly.

That does not garantee quality, of course (I work in R&D and we often test competitors' devices and find them not compliant) but I think it's fait to assume that the specs are not "optimistic".
Prior to retirement I was ISO certfied as a Lead Auditor. Our audit consisted of determining if the facilitiy was following their own Quality Manual and set procedures. We did not presume to know or even care about design specifications.
07-11-2010, 02:14 PM   #13
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QuoteOriginally posted by Ahab Quote
Prior to retirement I was ISO certfied as a Lead Auditor. Our audit consisted of determining if the facilitiy was following their own Quality Manual and set procedures. We did not presume to know or even care about design specifications.
Interesting. the ISO team visiting us from time to time is very torough and often asks to see calculations and procedurs.
07-11-2010, 02:18 PM   #14
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QuoteOriginally posted by thazooo Quote
I use a 'Sunny 11 Rule'. This has held true for my ist D, K10, and K20.
With film and my old cameras I use a Sekonic L398 meter. Exposures on NEG rather than Slide film follow the Sunny 16 Rule closely. Not exact but close enough.
Playing around one day I used the meter with the ist D and saw what you've seen.
Why ? Who knows.....I just adapted to the Sunny 11 and adjust from there.
This makes me wonder if, perhaps, 'sunny 11' is a more appropriate rule of thumb for aps-c than 'sunny 16'.

Thoughts, anyone?

Disclaimer: just got back home from 3 hours at a local winery, so I may not be thinking all that clearly at the moment.
07-11-2010, 05:24 PM   #15
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QuoteOriginally posted by Ahab Quote
Prior to retirement I was ISO certfied as a Lead Auditor. Our audit consisted of determining if the facilitiy was following their own Quality Manual and set procedures. We did not presume to know or even care about design specifications.
Well, sure - but that's an entirely different ISO standard. They maintain *thousands* of different standards. I'm guessing you were dealing some of the business practice standards, like the ISO 9000 standard. Actually verifying the exposure standards (eg, ISO 12232) would be a highly technical affair. Not that I'd expect ISO would actually be certifying camera compliance with these standards - but independent reviewers do.

BTW, yes, I think "Sunny 11" seems pretty appropriate for many situations on the digital cameras I've used. I doubt it's APS-C specific. It's probably true of any digital camera accurately following the ISO standards.
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