Highlight Tone Priority explained
Applies to: Canon EOS digital Rebel DSLR cameras, general concept applies to entire line of Canon DSLRs
There is a lot of confusion regarding the highlight tone priority and how it works. The internet is full of misleading and completely weird information on the subject while Canon does not actually reveal anything much in the camera user manuals. So here are the common questions answered very clearly as facts. I have verified these myself after consulting Canon's firmware operations and carrying out controlled tests.
Belief #1: No in-camera settings affect RAW file
That is absolutely wrong. There are settings that drastically affect the RAW file and produce notable changes in the file size and appearance. To make the best of your camera, you need to keep in mind what affects RAW files.
The biggest ones (custom functions) are:
1) Long exposure noise reduction - recommended setting: Auto
2) High ISO noise reduction - recommended setting: standard (high will slow down your camera)
3) Highlight tone priority - recommended setting: subject for another whole article
Belief #2: HTP (highlight tone priority) expands dynamic range
This is again quite wrong. It only APPEARS to extend dynamic range and include highlights. But if that were true, wouldn't Canon keep HTP on by default? Extra range doesn't hurt, right?
Fact is, HTP switches the ISO down one stop (i.e. if you select 200, sensor ISO will be set to 100). This causes an underexposure, thus including highlights. On this underexposed image, the processor applies a tone-curve adjustment and raises the shadows up. So in other words - since actual ISO was lower than selected, some darkest areas will be absolute blacks and tone curve adjustment cannot save them. This means you lost shadow detail to save highlight detail - yes, it is a bargain.
Belief #3: HTP produces better files
Depends on the situation. If you have a wholly bright situation, HTP will produce a bigger RAW file and have more details than otherwise. Otherwise if the shadows contain most details and form the dominant part of the image, switch off HTP to reduce shadow noise and increase contrast in shadow regions.
Program the menu settings to help you change HTP in two button presses.
Belief #4: HTP "exposes to the right"
No, HTP does not affect your metering or exposure at all. The mid tone is EXACTLY similarly exposed with HTP on or off. It doesn't matter there. HTP only decides where you have more detail - in highlights or shadows.
To make the most out of your camera:
Learn to frequently change the HTP setting, especially while doing travel photography. On a cloudy day, HTP should be on. On a clear sky day, turning off HTP will sometimes increase the size of your RAW files by even 2MB ...and render midtones beautifully. That's what Canon is known for - vivid colors.
Also, for HDR photography, it is highly recommended to keep HTP off. Why let the camera mess with the sensor data when you are already doing multiple exposures? This will keep noise under control in high ISO HDR images like handheld HDR of a church interior (as using tripods is not always allowed)
Finally, if you just don't like the HTP - my suggestion is, turn it off. Permanently.
And shoot photos in such a way that nothing is overexposed too much. Then brighten up shadows in post processing. That's all HTP actually does anyway.
Feel free to leave any questions or comments regarding this topic.
Of course, proper exposure is necessary and is to be taken care of regardless of whether HTP is on or off.
Photo credits: Pratik Panda.