Canon Highlight Tone Priority – usefull for raw shooting?

Highlight Tone Priority, HTP, is a feature from Canon with the goal to preserve highlights. From the Canon EOS 6D manual:

”Highlight details are improved. The dynamic range is expanded from the standard 18% gray to bright highlights. The gradation between the grays and highlights becomes smoother.”

With HTP enabled, the lowest ISO setting becomes ISO 200 (D+). This function underexposes the image by 1 EV to help prevent highlight clipping. The camera then brightens the JPEG image before saving it to the memory card. When shooting in raw, ISO 200 (D+) appears to be the same as ISO 100 but the camera meters and exposes the scene as if ISO 200 were used.

Not for raw files?

In terms of raw, there appears to be no difference between shooting at ISO 100 or ISO 200 (D+), ISO 200 or ISO 400 (D+), and so on.

On a Canon EOS 6D, ISO 50, ISO 100, and ISO 200 (D+) show no difference in dynamic range. With the same shutter speed and aperture, the same amount of highlight clipping occurs.

Based on this, it seems that Highlight Tone Priority is primarily intended for JPEG users.

Below are two raw files that have been analyzed using Hraw. Both were shot with the same shutter speed and aperture. In the second image, Highlight Tone Priority was enabled.

As you can see, there is no difference in clipped highlights more than some moving objects like white birds.

The same raw files opened in RawDigger look like this:

Take a look at the number of overexposed pixels — they’re almost identical.

There is one thing that stands out, though: the maximum value of 16,383 in the second green channel in the ISO 200 (D+) image. This seems to be caused by a single irregular pixel, which is visible in the darktable screenshot below.

But there is a difference between the files in my raw converter

This is likely due to the Makernote tag HighlightTonePriority. When it is set to On, some raw converters, such as Canon Digital Photo Professional and Lightroom, apply a different tone curve. In the examples below, I changed the tag from On to Off, and Canon Digital Photo Professional now treats the ISO 200 (D+) file as if it were shot at ISO 100.

Lightroom

For Lightroom the preview images use the embedded ones from the raw files.

When comparing the highlights reconstructed by Lightroom there is no difference between ISO 100 and ISO 200 (D+).

darktable

RawTherapee

What about D+ and D+₂?

Highlight Tone Priority, D+, has been available since 2007. With models like the R6 and R7, we now also find something called Enhanced, D+₂.

As already written, RawTherapee tries to imitate the tone curve of the embedded JPEG in the raw file. Therefore, it is possible to see the differences between the two tone curves in RawTherapee.

The difference is minor, but the D+₂ tone curve is less aggressive in the highlights.