How does Canon's Highlight Tone Priority work? Is there any reason not to use it?

September 22, 2010 1:32 PM

I recently bought a Canon 5D Mk II DSLR and am curious about one of its features: "Highlight Tone Priority"

My understanding is that is somehow uses more than one ISO setting for different parts of the image. I have two questions:

1) Does anyone have information on how it works, technically? Does it change the operation of different parts of the CMOS sensor dynamically?

2) Are there any disadvantages to using it? In particular, would it be better to shoot in an ordinary mode and then post-process the images to address any issues with highlights. I am curious about whether using HTP reduces the quality of the original files in any way.

posted by sindark to Technology (2 answers total)

The camera is always using one ISO setting for the entire sensor. The general consensus for HTP is that is works by under-exposing the image by one stop by using a lower ISO setting than the nominal setting (hence why ISO 200 is the minimum - HTP 200 is actually ISO 100 on the sensor) and scaling up the image one stop while using a different tone curve for the brighter part of the resulting exposure. Tests done with darker scenes show no difference between an ISO 100 image and an HTP 200 image; tests done with brighter scenes show improved tonal variation in those bright areas. This is why there seems to be a lot of dissent on the interwubs - people doing studio-style tests of low-dynamic-range scenes aren't hitting the effected part of the tone curve, but people doing real-life tests in more varied lightning conditions are.

This gives you higher image noise in the darker parts of the image as you're doubling a less-precisely-measured value (remember that brighter areas will always be less noisy than darker areas because they have less shot noise). It also means you have one stop of "safety" on the bright end that you would have lost to clipping before, and since the tone curve is different, you'll get improved tonal detail in those brighter areas of the image.

General recommendation, then, would be to use it in situations where you are much more concerned with keeping detail in subtly graded bright areas of the image - wedding dresses, snowfields, white buildings, etc, and to definitely avoid it where noise in dark areas of the image would be undesirable, or when there's nothing of interest in the rightmost ~1/4th of the histogram.
posted by 0xFCAF at 2:32 PM on September 22, 2010 [4 favorites]

Biggest drawback: with highlight tone priority enabled, you may lose access to the very highest ISO setting, check the owner's manual. On the 40D, if I am not mistaken, I cannot expand my ISO to include "H" (ISO 3200) if I have highlight tone priority enabled.

Basically HTP weights the exposure to make it darker, prioritizing the fact that you do not want to over-expose highlights in your image. I have a 40D, and leave HTP off, since I shoot RAW files and check my histogram regularly while shooting (just personal preference).

Copied from "The Digital Picture" - "First seen on the 1D Mark III and now found on the 40D is HTP (Highlight Tone Priority) which is designed to prevent blown (RGB=255,255,255) highlights. An ISO value of at least 200 is required for this setting. The feature works. It should primarily be used when the scene to be captured has some very bright highlights in it along with dark colors. A black tux beside a white wedding dress is the classic illustration. But, I don't recommend using it unless necessary as I prefer the contrast/tone curve of a standard shot better. Shadow noise will be slightly higher in an HTP image."

And from the original 1diii review - "The 1D III holds highlights much better if the new Highlight Tone Priority is enabled (C.Fn II 3). To use HTP (Highlight Tone Priority), an ISO setting of at least 200 must be selected (which is displayed as 2oo when HTP is enabled). Here is the HTP feature description directly from the manual: "The dynamic range is expanded from the standard 18% gray to bright highlights. The gradation between the grays and highlights becomes smoother." The feature works. It becomes much harder to blow the whites (solid 255,255,255 RGB values). However, it reduces the brightness of the highlights slightly. If you can hold the highlights with HTP set off, you will get crisper images. Highlight Tone Priority will appeal to some photographers and circumstances - especially at weddings where highly reflective white wedding gowns show up next to black tuxes with great regularity."
posted by kenbennedy at 1:27 PM on September 23, 2010