Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Martin.au Forum Pro • Posts: 13,247
Re: Don't listen to me, try it for yourself - it's easy

Mjankor wrote: Can you please explain how they can be arbitrary, with respect to the ISO standard?

Hey Mjankor, SOS depends on in-camera processing. No two cameras process an image the same way, in fact there are very large variations, even from mode to mode within the same camera. But don't take my word for it, check it out yourself. Set your camera to a Neutral mode (say Portrait), meter off a uniform wall and take a shot. What value does the wall show in sRGB?

Now change the camera mode to Landscape, meter off the same wall and take a second shot.
What value is the wall now in sRGB? Is it 118? is it the same?

Try both scenarios with your other camera. How consistent are the four figures?

Can you also please tell us what DPR are doing, when they do their ISO tests to the 12232 standard?

"If you want to compare ISOs between cameras from our reviews, look at the ISO test section (on the Noise and Noise Reduction page). This is where controlled test results are reported, essentially using the SOS method of ISO 12232:2006. The rest of the testing is based on knowing this SOS ISO calibration, as all test scenes have specific grey patches that are white-balanced and exposed as closely to L=50 as possible (i.e. within 1/6 stop)." - Andy Westlake

I am not an expert on DPR testing procedures, so I am not the right person to ask. But the first question that comes to mind is 'Do cameras set up as above capture a consistent tonal range, or merely present output images of the same average brightness/contrast' for a given exposure?

And, finally, can you please tell us how every camera I've looked at is consistent in this test? Why do all the cameras agree, if it's an arbitrary standard (oxymoron, I know)?

I assume this is an experiment you've done. Please tell us your results and methodology.

I am starting to get the impression that you are not genuinely interested in my answers but that your questions have an agenda behing them, and that you are never going to be satisfied with any answer I give unless itìs what you want to hear. My opinions have been made clear. Perhaps now you can ask other knowledgrable contributors like bobn2 what they think. Or report back with your findings for the exercise above and answer your own question.



I was very interested in your answers, because they would demonstrate one of two things.

The first possibility, is that I could use your posts to improve my knowledge. That would have been a good option.

The second possibility, is that you're quite happy to blather about something that you don't understand, such as the ISO standard, while assuming that your understanding of a different subject, such as sensor behaviour, makes you an expert.

Unfortunately as your answers generally demonstrate the latter, it places everything you say into doubt.

There's no agenda behind my questions. Challenging people is a very good way to learn because you can learn not only what they have to say, but also verify the quality of their knowledge. This thread is a great example. My main point of interest is the claim, from you, that comparing cameras with similar settings is wrong, and that they should be compared based on their DXO ISOs as their manufacturers ISOs were completely arbitrary.

So, I challenged you on your claim about arbitrary ISOs and have learnt that you basically have no idea what the standard is, or how DPR test them, and so forth.

Ergo, I now know that possibility number 2 is likely, and that I should look elsewhere for quality information. Everything you say now has to be vetted against reliable sources. By researching this subject and reading the work of the reliable sources I learn more. I guess you could say that's my ulterior motive. You posts are a catalyst to me learning more.


No. Bad test, bad. Start with the standard and work from that, rather than guessing.

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