Highlight Tone Priority manages to preserve highlight detail that might otherwise be blown out due to overexposure. This might be a bright highlight on a portrait subject’s shiny forehead, or the brightest spot on a bride’s white wedding dress. With digital capture, overexposure is the worst because there’s no recovering detail in an overexposed white area. Once that detail is gone, it’s gone. HTP is designed to prevent that overexposure.
The problem is, you can accomplish much the same thing by doing it yourself. Simply by slightly underexposing, say a half stop or slightly more, you can be sure that the brightest highlights in an otherwise normally exposed scene are not going to be blown out. The downside, of course, is that everything else is a bit too dark. Simply adjusting for this in post processing, particularly if you’re shooting RAW image files, is a great way to have the best of both worlds—areas of shadow and midtone that are correctly exposed, while the highlights are kept in check.
Speaking of RAW, there’s a lot of differing opinion out there on whether Highlight Tone Priority affects the highlight detail in RAW image files or just for JPEGs. My understanding is that HTP does impact RAW image files; however, it’s really not much different than simply underexposing a RAW image yourself. You see, when you set your camera to enable Highlight Tone Priority you’ll notice something peculiar: Your top-of-camera LCD displays a D+ next to the ISO (referring to increased dynamic range, particularly in the highlights) and a new minimum ISO of 200. In effect, the minimum ISO is 200 but the camera underexposes as if it were at an ISO of 160 or even 125. And since ISO 100 is the lowest available, to underexpose it has to have a higher minimum ISO. That D+ 200 really isn’t 200; it’s an underexposed 200.
Here’s the thing: You can underexpose all on your own! If you want to maintain highlight detail, whether you’re shooting RAW or JPEG, simply be sure you underexpose the image file and sacrifice a little noise in the shadows in an effort to avoid blowing out all the detail in the highlights. Then adjust the curves in post processing and maintain total control over your exposure every time.
One last thing: Nikon shooters have a similar feature at their fingertips called Active D Lighting. The premise is the same even if the approach differs slightly. And, in the end, in each case you’ll retain more shadow and highlight detail by shooting RAW and making contrast adjustments in postprocessing.