Question to HighlightTonePriority (Canon)

Submitted by bolitho on Tue, 07/01/2025 - 09:50
Forums: 
General discussion

Canon cameras have the "Highlight Tone Priority" setting which is stored in the Makernotes. (Other brands have similar but differently called settings.)
If I am not mistaken this setting finally makes Canon's DPP or Adobe's ACR and LR(C) apply a specific tonecurve to save highlights.
I just reread the chapters to histograms and exposure correction in the FRV help. (Thank you for the very comprehensive information!)
Do I understand correctly FRV ignores this setting?
I won't raise a feature request if not - just trying to understand what FRV displays.
 

LibRaw

Tue, 07/01/2025 - 10:33

With HTP set to 'On' the exposure remains the same, but (on my Canon cameras) ISO (conversion gain) is lowered by 2x. This adds 1 stop to the headroom in highlights.

When a camera is set to ISO 400, the maximum value in raw is about 14,000. HTP lowers the conversion gain by the factor of 2, and the previous reading of 14,000 becomes 7,000 - that's an extra one stop.

You can take 3 shots under the same light with the same shutter speed and aperture setting, one with ISO set to 400 and HTP set to 'Off', next with ISO set to 400 and HTP set to 'On', and the last one with ISO set to 200 and HTP set to 'Off', and compare them, looking at raw histogram. You can also use any file exchange server (Dropbox, Google Drive, WeTransfer are some of the free ones) to upload the raw files and send us the links to downloads.

bolitho

Tue, 07/01/2025 - 10:58

Thank you for the swift response.
I'll carry out the suggested investigation (and probably come back to you).
 

bolitho

Wed, 07/02/2025 - 09:12

Just want to share my findings:
Did as suggested:
Did "... take 3 shots under the same light with the same shutter speed and aperture setting, one with ISO set to 400 and HTP set to 'Off', next with ISO set to 400 and HTP set to 'On', and the last one with ISO set to 200 and HTP set to 'Off', and compare them, looking at raw histogram."
FRV "ignores" the HTP setting - it just displays the files as they are, i.e. the picture taken with HTP "on" is darker (and the histogram shifted to the left) than the one taken with HTP "off". (Same picture under the same shooting conditions.)
This is as expected (and basically why I asked for confirmation/clarification). 
Adobe (I used LRC) compensates for the HTP setting - after import files are equally "bright" whether taken with HTP "on" or "off". (i.e. HTP "on" shifts the histogram to the right.)
Also a tone curve is applied (depending on the HTP setting: newer camera models have two settings: "D+" and "D+2").
Actually I tested with both settings: The overall shift seems to be the same but D2+ corrects highlights more aggressively.
I made a further test:
I copied a file with HTP "on". Using exiftool I set HTP to "off" in the copied file. (Thus I had identical files but HTP different.)  
As expected: FRV displays identical bright images with identical histograms; LRC brightens up the image with HTP set to "on".
This leaves me with the question how to use FRV for culling in the future (as HTPed images are displayed too dark). The Adobe "hidden exposure correction" is applied but HTP ignored.
I don't want to raise a feature request because this would open a can of worms (brands with different HTP implementations...).
Probably there already is a simple/pragmatic approach (e.g. just an exposure shift without applying a tone curve)?
Any recommendation? 
Thank you for the already great product! 
 

LibRaw

Mon, 07/14/2025 - 21:16

Please, "You can ... use any file exchange server (Dropbox, Google Drive, WeTransfer are some of the free ones) to upload the raw files and send us the links to downloads."
Without having sample files we can't be sure what's happening.

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