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Old Tuesday 21st July 2009, 06:29   #1
cab1024
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Highlight Tone Priority and RAW files

Has anyone done any testing to confirm whether or not highlight tone priority (on a 40D) actually does anything when shooting RAW files?

Given that RAW files are supposed to capture everything that registers on the sensor I'm inclined to think it doesn't make any difference except to the preview thumbnail generated for the back of the camera. But I may be wrong, and I figure you folks would be best to point that out

This thread covers the topic but not necessarily for RAW files:
http://birdforum.net/showthread.php?...+tone+priority

If it does do something it might be worthwhile using, especially for white birds in flight against a whitish sky.

What do you think?
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Old Tuesday 21st July 2009, 08:06   #2
Roy C
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Quote:
Originally Posted by cab1024 View Post
Has anyone done any testing to confirm whether or not highlight tone priority (on a 40D) actually does anything when shooting RAW files?

Given that RAW files are supposed to capture everything that registers on the sensor I'm inclined to think it doesn't make any difference except to the preview thumbnail generated for the back of the camera. But I may be wrong, and I figure you folks would be best to point that out

This thread covers the topic but not necessarily for RAW files:
http://birdforum.net/showthread.php?...+tone+priority

If it does do something it might be worthwhile using, especially for white birds in flight against a whitish sky.

What do you think?
I have shot landscapes in highlight tone priority in raw on a 40D and it does work on the raw file. I have found it useful when a shot has white houses in sunlight but never bother these days. Not sure there is any use for it in bird photography though but that's just my opinion.
As far as I can see when in this mode it underexposes by a stop and then pushes the non highlights back up to where they should be.
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Old Tuesday 21st July 2009, 09:17   #3
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This post on DPReview shows the difference in the raw capture with HTP on and HTP off. The histograms you see are from the actual raw data captured, before the camera got its mits on it and faked the "JPEG" histogram you see in the camera.

http://forums.dpreview.com/forums/re...ssage=24943011

Using HTP when shooting raw is, in my opinion, not a smart thing to do, especially if it is your aim to "expose to the right". You will be underexposing the raw capture by 1 stop and the camera's internal processing will conceal that fact from you when it generates the internal preview image from which the camera's own histogram is created.

There may conceivably be a tiny benefit to HTP with raw, if you use DPP to process your raw files, but otherwise no benefit whatsoever. AFAIK DPP is the only software that actually understands what to do with files captured with HTP on. Other software, like Lightroom, just applies pretty dumb processing to "mend" the underexposed file. Since I only shoot raw I never use HTP, ever. As I use Lightroom for raw processing I have even less reason to use HTP.

If I shot to JPEG I might use HTP, but only in conditions with troublesome dynamic range and important details in the highlights and shadows. In flat lighting (overcast day), or low contrast scenes - e.g. a mid toned bird against blue sky or foliage - then turn it off.

To prove this for yourself shoot a scene in raw with identical exposure settings under identical lighting, once with HTP on, once with HTP off. Then use the free software available here - http://www.cryptobola.com/PhotoBola/Rawnalyze.htm - to look at the true raw histograms.

Here is my own example of the raw histograms, one without HTP and one with HTP. All exposure settings were supposedly identical - 200 ISO, f/5.6, 1/200. Exposure was set to maximise my ETTR data capture, without clipping. The first raw histogram shows that I achieved my goal. The camera was on a tripod and the images were shot 5 seconds apart - just long enough to switch HTP on and fire again. If you look at the data reported by Rawnalyze you will see that for the shot with HTP enabled the ISO is shown as 200/100, meaning that the camera "pretended" to be shooting at 200 ISO but was "secretly" only shooting at 100 ISO, to effect a 1 stop underexposure.

I've also attached the two files as converted by DPP with no edits, Neutral picture style, 0 sharpening, just as shot. You can see that the brightness of the sky has been toned down, while the rest of the scene seems barely changed. HTP is doing its job here, but we do have a slightly underexposed file - i.e. not an optimum ETTR capture.
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Old Tuesday 21st July 2009, 10:17   #4
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If I open the same two files in Lightroom and turn on highlight and shadow clipping warnings we see that the two files look as near as dammit identical. That is because all that Lightroom does is to blindly double the exposure values of each and every pixel to compensate for the 1 stop underexposure that HTP caused. The files essentially look the same, but one is underexposed and will have greater shadow noise and less smooth tonal graduation. There is no intelligence applied to the "correction", unlike with DPP. HTP has brought no benefit here whatsoever, just a reduction in IQ. You could pretty much accomplish the same thing yourself by deliberatly choosing to underexpose a little, in order to protect the highlights. Of course, if you make your own choice then you don't have to stick with 1 stop underexposure. You can pick 1/3, 2/3, 1, 1 1/3 or whatever you want.

The reason the files appear clipped, even though neither was overexposed, is simply due to Lightroom's default camera profile (Adobe Standard) and default tone curve. The images both look blown but the raw data isn't. You can easily pull back the clipping with the highlight recovery slider.

By the way, if you use the zone system of metering, focusing your interest at the highlight end of things then you really, really, really do not need HTP, or any kind of exposure protection. Your own careful metering will secure perfect highlight exposure with no trickery or IQ loss. To accomplish this, simply spot meter off the brightest part of the scene for which you want to retain detail (ignore specular highlights) and set the exposure for that highlight area to be at +3 stops. e.g. meter off the brightest part of the sky at +3 and let everything else end up wherever it may. That was exactly my approach when shooting the third image below, which has had no edits except WB. The histogram is a gnat's whisker from the right hand edge - a perfect exposure for the scene, maximising the detail captured for the bird while not making the sky look a pure blanket of white. The resulting image (still no edits) is attachment 4.
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Last edited by tdodd : Tuesday 21st July 2009 at 10:40.
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Old Wednesday 22nd July 2009, 22:02   #5
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Tim,

That was an awesome answer, and definitely gives me a lot of food for thought. A lot has changed in how I determine my exposure since I started shooting RAW about a month ago.

First off, while shooting jpegs, I tended to overexpose by 2/3-stop since the camera's evaluative metering mode seems to expose for the lightest part of the scene, and my subjects were often in darker areas relative to the sky. I don't do that anymore since I now realize there is a lot more detail hiding in the shadows, and none in blown highlights.

Now that I'm shooting RAW I can get even more detail, and have control over the noise, in the shadows. I started another thread awhile back about exposure modes, but that was prior to going RAW, and now I'd probably recant all of my conclusions from that thread. Now, I've switched back to evaluative mode and as long as my subject is not blown out or in deep, black shadow, then I figure the exposure is optimal. And if in shadow or against the sky, I'll overexpose slightly with EC.

I've only just started reading about ETTR so I'm not sure how that plays into my new way of doing things. I too use Lightroom, so by your last post it sounds like I should avoid HTP entirely, especially using my personal version of the zone system. But, my method is evolving...

Thanks for all the insight, Tim.
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Old Thursday 23rd July 2009, 13:10   #6
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As Tim says, HTP only makes any sense if you convert in DPP (and even then, I don't seen any real value in it).

Set the camera to HTP and convert in any other software, and all you've got is a file underexposed by about a stop - hardly a great place to start, as a rule.
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