Highlight tone priority

photon

With all the hubub about the new cameras coming, my good ol' 20D is looking really long in the tooth! In my research, I came across the term 'Highlight tone priority'.

At first I thought, great! This is what I always wanted: a mode that will meter such that the highlights are never blown out... In other words the highlights will always be at right side of the histogram, but never clipped. When I try to get the exposure right, this is what I'm doing somewhat manually, so having the camera doing it for me seems like a good idea.

However, apparently in the Canon cameras it seems like a non-linear gain applied to the hardware-level signal from the chip, so it seems to introduce some noise in the shadows as a trade-off for about one stop more dynamic range in the highlights. (Good discussion here at Bob Atkins)

My question is: is there any technical reason why a camera could not be programmed not to clip the highlights? And is the 'Highlight priority mode' a worthwhile tradeoff for noisy shadows?

Thanks for the discussion...

Tee Why

I think highlight tone priority HTP as they call it is for when the shot is so dynamic that the highlights may be overexposed as the dynamic range of the lighting is wider than what the camera can capture.

From the tests I've seen on dpreview, I'm not sure it's that effective.
Here take a look.
http://www.dpreview.com/reviews/canoneos40d/page21.asp

ziggy53

photon wrote:

...

My question is: is there any technical reason why a camera could not be programmed not to clip the highlights? And is the 'Highlight priority mode' a worthwhile tradeoff for noisy shadows?

Thanks for the discussion...

The short answer is that you would not want a camera exposure system that would always "not clip the highlights". The problem is, how does a camera know what is a highlight?

If you shoot towards a subject and the scene includes a spectral highlight, for instance a shiny glint off of a mirror or shiny metal, how do you describe to the camera that the spectral highlight is not to be measured as an important highlight and detail in the scene?

Our mind can do it but a camera cannot.

The exposure system of a modern camera tries to expose for middle gray unless a special program/mode is invoked. Using that system the exposure calculation can be designed so that small parts of the scene can be "ignored" if they are ridiculously out of range of the rest of the image.

Designing an exposure system to only measure for highlights would require considerably more assumptions about the scene, which could wind up in error. (BTW, there are camera systems which attempt to do just that but you don't hear too much about them because they don't work that well and they need to be invoked as needed.)

The correct method to expose for a digital camera is to use the default exposure, as determined by the camera, and adjust the exposure compensation by either experience or by reviewing a trial exposure and the resulting histogram and "blinkies".

In the case of the histogram and blinkies, once you know what the histogram means for "your camera", you can adjust the exposure accordingly and put the highlights, middle-tones and shadows where needed. As long as the "blinkies" don't impact an important area of the scene, they may be acceptable and blown purposely.

In the case of experience you are using your brain in partnership with the camera and exposure system to interpolate the exposure to what it should be, based on your previous results and understanding of the camera and the scene.

There will always be scenes which include tonality beyond the ability of the camera to capture in a single exposure. HDR technique may be required in those instances.

In my humble opinion the Highlight Tone Priority mode is an improvement when it is needed. It is not a panacea and it is not intended to be such.

The human eye and brain are more conscious of loss of detail in the highlights than additional noise in the shadows.

dgeddie

Raw
Something to consider... If I'm correct all these bells and whistles - great for marketing - only apply to JPG output and have no impact if you shoot raw.

I'm new to DSLR, coming from a bridge, and thought how wonderful these new additions would be. However, once I could shoot multiple RAWs with the better buffer, I don't even look at the JPG output any more.

So these have no value to me 8 weeks after buying the camera

ziggy53

dgeddie wrote:

Something to consider... If I'm correct all these bells and whistles - great for marketing - only apply to JPG output and have no impact if you shoot raw.

I'm new to DSLR, coming from a bridge, and thought how wonderful these new additions would be. However, once I could shoot multiple RAWs with the better buffer, I don't even look at the JPG output any more.

So these have no value to me 8 weeks after buying the camera

Dgeddie, welcome to the Digital Grin.

Highlight Tone Priority (HTP) does affect RAW files as well as JPGs.

When you need it, it adds around 1 extra stop of dynamic range.

evoryware

Agree with Ziggy. I use it now more than I used to, but you lose ISO 100 with it enabled.

photon

Thanks Ziggy!
Thanks for the reply Ziggy... I always look forward to your thoughtful and thorough posts!

ziggy53 wrote:

The short answer is that you would not want a camera exposure system that would always "not clip the highlights". The problem is, how does a camera know what is a highlight?

Of course, you're right when you say that only a human can decide what is blown out on purpose, and what is by accident, and specular highlights are certainly in this category. Probably another example is when you're exposing for a dark, shadowed building and want to see what is in the shadows, it may be acceptable to blow out the sky. However, it may be possible that the photographer could leverage the metering mode to indicate the correct highlights to save.

On the Canon's (and probably Nikon and other makes as well), the different metering modes (evaluative, center weighted, partial, and spot) indicate the 'region of interest' of the scene to correctly meter on. [I use this a lot when I'm using Aperture mode to get the 'right part' of the scene correctly exposed, and then adjust afterwards]. Anyway, the camera would use the current metering mode to determine what highlights wouldn't be clipped. So as the photographer could choose what area of the scene I didn't want clipped, and the camera would expose so that area isn't clipped.

Enough gibberish: here's an example of what I mean. Imagine a beautiful silver and chrome sports car in front of a lovely lake and some snow capped mountains. If I'm in partial metering mode, I could use this new 'don't clip my highlights' mode and point at the snow on the mountains. The exposure would not clip the mountains, but may still clip some of the shiny chrome speculars and probably some of the sky. Or I could use 'whole scene' evaluative metering with the don't-clip mode and it would ensure I don't clip anything. Seems to me it would be a nice feature to have, as it would allow the photographer to indicate 'what is a highlight' by pointing in the direction of the non-highlights

Let me know if I'm still talking gibberish, or if it makes any sense whatsoever. In any case, the feature as I originally thought doesn't exist, so this whole conversation is pretty academic!

Curtis

ziggy53

photon wrote:

... Imagine a beautiful silver and chrome sports car in front of a lovely lake and some snow capped mountains. If I'm in partial metering mode, I could use this new 'don't clip my highlights' mode and point at the snow on the mountains. The exposure would not clip the mountains, but may still clip some of the shiny chrome speculars and probably some of the sky. Or I could use 'whole scene' evaluative metering with the don't-clip mode and it would ensure I don't clip anything. Seems to me it would be a nice feature to have, as it would allow the photographer to indicate 'what is a highlight' by pointing in the direction of the non-highlights

Let me know if I'm still talking gibberish, or if it makes any sense whatsoever. In any case, the feature as I originally thought doesn't exist, so this whole conversation is pretty academic!

Curtis

If you use Spot Metering mode and point to the whitest part of snow that you want to preserve, and then adjust the EC by +2 to +3, you might very well get the control you are looking for.

The camera will attempt to read the metered spot as a medium gray and the EC adjustment tells to exposure to add extra exposure, effectively the same as saying to the camera, "This spot needs to be an EV (exposure value) 2 or 3 stops brighter than your meter reading."

The "nice" thing about this method is that "you", the camera operator, get to decide what value the highlight is above medium gray. Sometimes you may want the highlight to be more conservative than other times. You get to be in control, rather than trusting someone else's determination of what the highlight should be. (Generally a "highlight" can be anything from 2 to 4 stops over the middle gray. Remember, pure white is not "necessarily" a highlight.)

Again, you should determine in advance what the histogram of the camera is telling you and you should watch the "blinkies" to make sure that any clipped highlights are in non-critical areas of the scene.

photon

What about 'expose to the right'?

ziggy53 wrote:

The "nice" thing about this method is that "you", the camera operator, get to decide what value the highlight is above medium gray.

It's a good point... But isn't there also the dogma that in digital you should 'expose to the right' to use all the high-value bits so you have the best signal to noise ratio, and then you can bring it back down to reasonable levels in post?

Great discussion!

Curtis

ziggy53

photon wrote:

It's a good point... But isn't there also the dogma that in digital you should 'expose to the right' to use all the high-value bits so you have the best signal to noise ratio, and then you can bring it back down to reasonable levels in post?

Great discussion!

Curtis

"Expose to the right" only relates to scenes with a total dynamic range less than that of the camera.

If you have a camera of say, 9 stops of dynamic range, and you have a scene of only 6 stops of dynamic range, shooting-to-the-right is a good idea in that you can still preserve all the tonality and you reduce noise in the lower tones at the same time.

If you had a scene which contained a dynamic range of 9 stops, your strategy should be to try to expose the imager without clipping the highlights or squashing the shadows. You will have filled the histogram to-the-right and to-the-left.

If you had a scene which had a dynamic range which exceeds the range of the imager you either decide to expose for lower tones or for higher tones, but you won't be able to expose for the whole scene in a single capture. HDR technique might alternately be required or desired.

NeilL

ziggy53 wrote:

If you use Spot Metering mode and point to the whitest part of snow that you want to preserve, and then adjust the EC by +2 to +3, you might very well get the control you are looking for.

The camera will attempt to read the metered spot as a medium gray and the EC adjustment tells to exposure to add extra exposure, effectively the same as saying to the camera, "This spot needs to be an EV (exposure value) 2 or 3 stops brighter than your meter reading."

The "nice" thing about this method is that "you", the camera operator, get to decide what value the highlight is above medium gray. Sometimes you may want the highlight to be more conservative than other times. You get to be in control, rather than trusting someone else's determination of what the highlight should be. (Generally a "highlight" can be anything from 2 to 4 stops over the middle gray. Remember, pure white is not "necessarily" a highlight.)

Again, you should determine in advance what the histogram of the camera is telling you and you should watch the "blinkies" to make sure that any clipped highlights are in non-critical areas of the scene.

Nice!

Neil

NeilL

ziggy53 wrote:

"Expose to the right" only relates to scenes with a total dynamic range less than that of the camera.

If you have a camera of say, 9 stops of dynamic range, and you have a scene of only 6 stops of dynamic range, shooting-to-the-right is a good idea in that you can still preserve all the tonality and you reduce noise in the lower tones at the same time.

If you had a scene which contained a dynamic range of 9 stops, your strategy should be to try to expose the imager without clipping the highlights or squashing the shadows. You will have filled the histogram to-the-right and to-the-left.

If you had a scene which had a dynamic range which exceeds the range of the imager you either decide to expose for lower tones or for higher tones, but you won't be able to expose for the whole scene in a single capture. HDR technique might alternately be required or desired.

Also nice!

Neil