Highlight Tone Priority

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Many of Canon's newer DSLRs contain a feature called "highlight tone priority" (HTP). Canon claims this feature will help to prevent you from overexposing the highlights of your images. If you enable this feature, you'll see a D+ appear on the panel of your camera. I'm always concerned about retaining detail in the highlights of my images, so I decided to find out how this feature works and test it out to see if there's any improvement to the image quality of my photographs.

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.
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Anhinga wing detail with HTP on
But how well does it work? I've shot numerous photos of scenes with bright highlights (sunlight reflected off of metal surfaces, etc), and can defintitely say that it helps preserve detail in the highlights. But when I examined the shadowy portions of these images, I've usually been disappointed. I took the above photograph with HTP turned on. I then imported it into Adobe Lightroom and examined the Anhinga's wings. Not only do the Anhinga's wings appear noisier than I would expect, but the noise appears patterned in a way that decreases the attractiveness of the image. Canon's in-camera software just doesn't seem to be able to increase the exposure of the shadows as well as can be done with computer software.

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.