40D: When to use highlight tone priority

Started Sep 15, 2007 | Discussions thread
GaborSch Veteran Member • Posts: 7,203
Let's test the effect of HTP

You have the camera, you can make a proper test.

If there are really dark parts of the scenery, which would be underexposed with ISO100 at the exposure selected for ISO200, then if you compensate for that in raw processing, the result will be more noisy than the same shot with ISO200. You can test this easily.

The images I analyzed (the same scenery once with ISO200 HTP on and once with the same exposure but HTP off) show, that a "compression" occurs even at the low (dark) end, i.e. more pixels are in the very low range.

Here are the raw histograms:

Canon had no reason to make a cheap trick, for on that level, i.e. in DIGIC, it is possible to combine two ISO values in a single raw file - and this is, what they are saying. All sensor sites are read with ISO200, and the values are transformed non-linearly.

I have the suspicion, that the true advantage of the 14 bit depth lies in this "HTP compressed" data. I would need more test images to analyze. Do you want to contribute? (I am developing a raw analyzer, see http://www.cryptobola.com/PhotoBola/Rawnalyze.htm ).

Btw, I see a problem in the non-linearity of the data. I don't think the raw processors are prepared for this (not even DPP!).

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