I mentioned about this feature on a forum recently and it was suggested that should feature as a blogpost - so here you go Calum!!
Firstly, I did a little bit of web searching as you do to check it out - it is remarkable how little information there actually is around. Which suggests that either people don't know what it is for, don't use it much, or that anyone has been that much interested in it.
Well I am no techno geek when it comes to some of these finer points - I use what is available to me, and try to find out it's limitations.
Using the Canon 1Dmk111 as an example, Highlight Tone Priority (HTP) can be found in the Custom Function setting, in the second group under Image/Flash exp./Disp and scroll through to C.Fn 11. You have the option to enable or disable.
I leave mine as disabled unless I am working in certain lighting and subject conditions.
So what does the manual advise us?
By using this setting, we will get improved highlight detail. The dynamic range is expanded from the standard 18% grey to the bright highlights. The gradation between the greys (or grays if you are US I guess) and the highlights become smoother.
There is a caution that with setting 1 (i.e. enabled) that noise in the shadows may be slightly more than usual.
The main visual clue is that the ISO setting on the top plate goes from upper case to lower case on the zeros. So for example ISO 400 with the HTP setting will appear as ISO 4oo. The minimum ISO becomes ISO200, as the camera is effectively carrying out a split exposure processing between the areas above and below 18% grey.
I understand that the selection of this feature will affect frame rate or buffer capacity, although I cannot confirm that from my own experiences.
Here is a bit of info I cribbed from a Google search which is attributed to the fine work by Arthur Morris on his BIRDS AS ART blog
"Canon has not publicly disclosed all the details of the method used to achieve Highlight Tone Priority. I can confirm that Canon’s method involves lowering the gain on the image sensor and modifying the image processing algorithm to simulate the ISO speed set by the user, but the details of the algorithm are confidential. Since HTP does involve lower gain than standard camera settings, it affects both RAW data and JPEG images. Although it would be possible for users to come close to simulating the effects of HTP for RAW images by shooting at a lower ISO speed and altering the tone curve of the image during post-processing, this method requires advance planning and furthermore offers no advantages in terms of noise reduction compared to setting HTP on the camera. Finally, as stated in the camera instruction manuals, “noise in the shadow areas may be slightly more than usual” when HTP is activated. Therefore, it’s basically up to users to decide if the benefits of HTP outweigh the potential drawbacks.” - all as written by Chuck Westfall of Canon USA
So does it work in RAW and jpeg. It clearly works in jpeg, and I understand to get the best of the results you should process your RAW images via Canon's own Digital Photo Professional software (DPP). This would make sense as the the software is designed specifically for Canon files.
Does it work with Adobe products to get the full benefit? Maybe somebody can answer that one. I do understand that it works correctly with Breezebrowser Pro - but you all know that Dave and I are big fans of that particular software.
Here are a couple of image that with and with HTP on. I used manual exposure to ensure that the settings remained the same. Certainly on the back of the camera, the black 'blinkies' were showing on the version that didn't use HTP. You will note that the shadows are clipped on both versions from the little blue triangle.
The main thing to note though is the shift leftwards of the highlights on the histogram with the HTP on version., and that the red clipping warning is extinguished on that version.
So when and where would I use it?
Bright light conditions, with very light subjects - so Egrets for starters, plus Kingfishers in the sunshine, when the sun is beating down on their white neck patch Calum :-).
When wouldn't I use it? - the rest of the time - as I said before I set it to default OFF.
So try for yourself and see what you find.