I had read about Canon's Highlight Tone Priority (HTP) function but had never bothered to check it out on my Mk III until now. I have had questions about it as well, as in how was it applied to a RAW file (since I shoot all RAW)? I had read some opinions based on 3rd party testing that the camera collects the image data of one stop lower ISO then sets a "flag" in the RAW file that a RAW converter reads -- for a jpeg the camera boosts the image data digitally with a curve that applies less boosting to the highlights.

But, that was based on users' tests, not technical information from Canon, and, what's more, I had no idea as to whether HTP would benefit me, and no idea what my RAW converter (Lightroom) did with it compared to Canon's processing.

So, camera on tripod using manual settings (after metering), I took a series of shots of a section of my kitchen ceiling with some shadows and a florescent light for the highlights:

Shot 1: ISO 200 with HTP on
Shot 2: ISO 200 with HTP off
Shot 3: ISO 100, HTP off
Shots 4,5: ISO 100, decreasing shutter speed by 1 stop each
Shot 6: Canon's in-camera jpeg, ISO 200, HTP on

First off, pretty much any decent exposure (for shadows and midtones) produced some blown highlights. My concern was if HTP did a good job of preserving highlights compared to what I might do, and how did Lightroom's treatment compare to Canon's in-camera treatment?

As I imported my images into Lightroom, the first thing I noticed is that the shot with HTP On (the first image in the series) went through some kind of adjustment as it was rendering the Lightroom preview. That may or may not have been as a result of Lightroom reading some kind of "HTP Flag". I don't have the inside scoop on that, though, so I can't state definitively.

When I compared the HTP version and the non-HTP version, there was definitely less blown highlights with more detail, although I wasn't all that impressed with the HTP version (the camera's jpeg did better). By using Lightroom's Recover Slider, though, it brought the Lightroom version much closer to the jpeg version.

So: Image 1 is the whole scene shot with HTP, image 2 is the a closeup of the HTP version's detail, image 3 is a closeup of the shot of ISO 200 with HTP off, image 4 is a closeup of the jpeg version, and image 5 is a closeup of the RAW with HTP and Lightroom's highlight Recovery slider adjusted.

Now for some messing around! First, there was no way of matching the last image, with the adjusted HTP, using the shot taken at ISO 200 with HTP off. Just to show you, image 6 shows it after moving the Revovery slider. However, when you take the shot by lowering the ISO to 100 (no HTP),underexposing by 1 stop, and then boost the Exposure in Lightroom by one stop and slide the Revovery slider over, all of a suddent you get results that match the best (adjusted) HTP image. Image 7 shows the ISO 100 shot unadjusted, #8 shows the same image with the two adjustments.

Lastly, I threw image 9 and 10 as an exercise: the shot was taken with a 1 stop slower shutter speed than the first ISO 100 shot, underexposing by two stops (#9) then I boosted the exposure in Lightroom by two stops and again maxed out the Recovery slider, producing nice results with no noticeable noise in the shadows, thanks to RAW and a clean ISO 100!

My conclusion? It seems like HTP does no harm at low ISO settings and does something useful to some degree. On the other hand, you have to do some tweaking to get the best results.

On the downside, a few things. First, there's no "undoing" HTP: it's applied while importing the file and the adjustments, whatever they are, are invisible in your software. Second, those of us who are used to working with exposure might not have a lot of faith in letting the camera fine-tune and tweak our exposure settings. It might save some work, and might in fact save highlights that we didn't catch with our settings, but we may not want to surrender that control to the camera. Third, the HTP process in all lilelihood sacrifices range in the shadows. Lastly, I'd be careful of using this at higher ISO settings. Maybe it'll work just fine, but I'm leary of letting the camera do too much stuff when higher ISO settings tend toward more noise. That would take more testing...