Proposed ISO-less Measurement

Started Jul 19, 2013 | Questions thread
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edhannon
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Proposed ISO-less Measurement
Jul 19, 2013

For my personal use the key practical question concerning ISO-less is, for a given camera model (that I own), is it better to use the in-camera ISO setting or to shoot at the lowest ISO and adjust in PP? This assumes of course that I shoot in RAW - which I always do.

I have followed some of the ISO-less threads. However, they tend to get long and tedious. And most answers are subjective.

I believe that I have come up with a way to decide the issue, at least for myself.

I have wrote some software that processes the raw files of some cameras (and TIFF files generated by dcraw)) and computes a 16-bit histogram.  One application of this is to compute the mean and standard deviation (RMS noise) of a gray patch.

Using that capability and data, I think I can answer the question for myself at least with the following procedure:

1.  Take two images of a gray card under constant light:

a. At ISO 1600 at suggested f-stop and exposure time suggested by internal meter.

b.  At lowest ISO at same f-stop and exposure time as the ISO 1600 exposure.

2.  Process the images with dcraw to produce four TIFF files:

a.  ISO 1600 without any exposure changes.

b.  ISO 100 without and exposure changes.

c. ISO 100 with exposure setting to bring mean ADU value up to one for ISO 1600

3.  Measure SNR for all three files.

If the SNR for ISO 100 with increased exposure setting has a SNR as good or better than the ISO 1600 files, then I should keep the ISO at base for all RAW shots.

I have a Pentax K10 and a Canon 40D.  I understand that the Pentax is ISO-less (or near ISO-less) and the Canon is not ISO-less.  I would do test for both as a control on my software.  Expect to see a difference in results between the two cameras.

Do the members who have more image measurement experience than I do think that this will work?

Again my goal is not to show which camera is "better".  Rather it is to make up my mind if I should shoot with ISO fixed at base value when I shoot raw.

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Canon EOS 40D Pentax K10D
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