I've noticed that several people have searched this blog for the string "highlight tone priority," and since inquiring minds want to know, I figured I would write a post about it. Highlight Tone Priority is a setting option on Canon's DSLRs that is often misunderstood. Some people swear by it, and others avoid it like the plague. It can be a useful tool in some circumstances, but just like the choice of "native" vs. "non-native" ISO settings, it is important to understand what it does, how it works, and just what the trade-offs are before deciding whether or not to do use it.
The purpose of Highlight Tone Priority is to increase the effective dynamic range of the camera, specifically by giving you extra protection in the highlights, while keeping the overall exposure the same. In other words, if two identical pictures are taken at the same exposure, one with HTP on and one with it off, the picture taken with HTP on will have fewer blown out highlights. Below is an example.
Both of the above images were shot at ISO 1600, at f2.8, with a shutter speed of 1/50th, and with the same amount of ND. The top one was shot with HTP off and the bottom one was shot with HTP on. The difference is immediately apparent. While the darker areas of the image remain very similar (the vertical blinds, the house and tree at the bottom right of the frame, etc), the bottom image clearly has much more highlight detail. Whereas in the top image the whole sky and much of the building are clearly blown out, in the bottom image the building and sky both retain significant detail.
Sounds great, right? After all, blown out highlights is one of those ugly qualities that screams out "video", and its far superior dynamic range is one of the last things keeping film alive at all. So, why wouldn't anyone want to use such a feature? Well, to answer that question you need to know how Highlight Tone Priority works, and as you might have guessed, there's no such thing as a free lunch.