According to DPReview...
They then go on to say this...Highlight Tone Priority
Highlight Tone Priority was introduced into Canon's DSLR line in 2007, and has since become a standard feature. HTP is designed to recover roughly one stop of extra tonal information in highlight areas, which is especially useful in scenes containing a range of tones - the classic example being a bridal couple, in a dark suit, and bright white wedding dress respectively. This sort of scene is one where normally, you might have to trade highlight for shadow detail, or vice-versa.
Highlight Tone Priority works by reducing the amplification of the sensor by one stop, so that it takes one stop more light before it clips to white (and, as a result, you cannot select ISO 100 because this amplification mode is needed to provide HTP ISO 200). A different tone curve is then applied so that images are rendered at the correct brightness.
This one-stop reduction in amplification results in a raw file that is effectively push-processed by the new tone curve and would be underexposed if the normal tone curve were applied. There's a downside, of course - that additional highlight dynamic range comes at the expense of slightly greater shadow noise.
Do any of you use this? If so, all the time, or in what situations?Most of the time, the effect of HTP is likely to be fairly subtle in our experience, but it can make a huge difference to some scenes, like the one we've shown here. These four images are the product of two photographs taken in Raw + JPEG simultaneous capture, at the same ISO sensitivity and exposure values. With HTP turned off, the brightest areas of this scene are obviously clipped, but when HTP is turned on, these areas are restored to their true color, and a lot of detail is revealed which is lost to clipping in the original shot.
The same goes for raw files as well. These two raw files were converted in ACR with -2EV digital exposure compensation, and the 'brightness' value increased to achieve equivalent luminance in the sunlit brickwork to the left of the image (which is the nearest thing to a midtone in this shot). You can see that the raw file recorded with HTP turned on contains a lot more detail in the highlight areas, and a much truer color tone. The raw file created with HTP turned off looks washed out by comparison, and the color of the bright areas is inaccurate.
In simple terms, what this tells us is that where you'd normally expect around 1EV of extra dynamic range in the highlights in a raw file compared to a JPEG, turning HTP on will get you around 2EV of extra range (i.e. 1EV more than a simultaneously captured JPEG).