Highlight Tone Priority
Here's how it works. Your exposure is determined by three factors: shutter speed, aperture, and ISO. Once you (manually or using the automatic features camera) determine the proper exposure, your camera lowers the ISO setting by one stop, effectively underexposing your image. This is why when HTP is on your minimum ISO is 200. This of course will also underexpose the darker portions of your photograph, so the camera brightens the darker portions so that they will match the exposure you would have achieved without HTP turned on. In effect, the camera is doing what I do in bright scenes when I want to make sure that I have highlight detail. I underexpose the image and then correct for the shadows with software.
|Anhinga wing detail with HTP on|
Unless you're examining the image at or near 1:1, you may not notice this. However, the bottom line is that HTP only does what you can do more effectively with exposure compensation and adjusting levels in Adobe Lightroom. And since HTP does not eliminate the need for either, I don't see it's benefit of HTP. I can evaluate my scene and underexpose by any value I choose from -1/3 stop to -2 stops, and HTP always chooses -1 stop. And most image editing software programs give you much more control to fix the shadows as well. I could only recommend HTP to those who need to avoid exposure compensation or image editing with software.