What is "highlight tone priority" good for anyway?

[Marsu42]

I don't know if this is implemented in more expensive bodies than my 60d - but htp doubles the iso and applies a non-linear curve so that highlights get more resolution while the shadows are compressed. In good light, I used to use it all the time at the beginning ("why not?"), but then discovered that the darks get so little resolution that raising them in postprocessing introduces noise or banding.

Nowadays, I'm using it on shots that have very bright highlights that I don't want to blow - like an animal's fur with the sun behind it or white flowers when shooting a macro of an insect on them. I've read some about htp elsewhere, but I have to admit the only people with a clue seem to be around here... thus the questions:

* Why does htp raise the iso value - does htp increase dynamic range (it doesn't exchange the sensor behind your back...)? Or is it just a convenience that saves you heavy underexposing and thus you are able to see the result on the camera screen and not only later after postprocessing?

* Do you use htp at all? If yes, what for?

I'm wondering because htp is the only camera gimmick that is applied to raw, too - so it might be something to it after all...

[Bosman]

I don't know if this is implemented in more expensive bodies than my 60d - but htp doubles the iso and applies a non-linear curve so that highlights get more resolution while the shadows are compressed. In good light, I used to use it all the time at the beginning ("why not?"), but then discovered that the darks get so little resolution that raising them in postprocessing introduces noise or banding.

Nowadays, I'm using it on shots that have very bright highlights that I don't want to blow - like an animal's fur with the sun behind it or white flowers when shooting a macro of an insect on them. I've read some about htp elsewhere, but I have to admit the only people with a clue seem to be around here... thus the questions:

* Why does htp raise the iso value - does htp increase dynamic range (it doesn't exchange the sensor behind your back...)? Or is it just a convenience that saves you heavy underexposing and thus you are able to see the result on the camera screen and not only later after postprocessing?

* Do you use htp at all? If yes, what for?

I'm wondering because htp is the only camera gimmick that is applied to raw, too - so it might be something to it after all...

I recommend doing the research in Google. If you need more look it up. To sum it up, you lose info in shadow details and lose the use of iso's lower than 200. You gain more info in highlight detail with the setting on. The info is embedded in raw and jpg. This is something you can do in post if needed. Essentially Chuck Westfall from Canon says it is a Curve applied in camera rather than in photoshop. I have tried using it but don't like adding limitations to my camera and i do my best to get proper exposures when i shoot. All that said it does work and makes me consider the idea of using it if my output on a super sunny day could use some help.
http://www.imaging-resource.com/PRODS/E1DMK3/E1DMK3EXPOSURE_HTPMODE.HTM
http://digitalprotalk.blogspot.com/2007/10/highlight-tone-priority-image-salvation.html

[Marsu42]

I recommend doing the research in Google. If you need more look it up.

I already knew and wrote what htp does, I've taken tenth of thousands of pictures with it. And I'm perfectly able to type something into google, but as you may or may not have discovered that not everything an internet search returns is very reliable. So I guessed it'd do no harm asking here if anyone has separated the usual user theories from the real information, no need for anybody to reply...

That being said, thanks for the links. But my question still is: If it's just an in-camera tone curve, why does it raise the iso value? Is it really 100% something that can be done in postprocessing?

[Mt Spokane Photography]

I recommend doing the research in Google. If you need more look it up.

I already knew and wrote what htp does, I've taken tenth of thousands of pictures with it. And I'm perfectly able to type something into google, but as you may or may not have discovered that not everything an internet search returns is very reliable. So I guessed it'd do no harm asking here if anyone has separated the usual user theories from the real information, no need for anybody to reply...

That being said, thanks for the links. But my question still is: If it's just an in-camera tone curve, why does it raise the iso value? Is it really 100% something that can be done in postprocessing?

If you use jpeg and send your images electronically straight to be published, or printed, it is useful.  While I normally use raw and post process, many users have valid reasons to use jpeg and it will certainly improve shadows in images where I use jpeg for straight to the printer applications.
 
The setting  is usually not used by raw processors except for DPP, so it has no effect on raw images for most photographers.  Its just a trigger telling DPP to apply a exposure curve to the raw image as it processes it, no harm done, you can change the curve any way you like.

[Marsu42]

The setting  is usually not used by raw processors except for DPP, so it has no effect on raw images for most photographers.  Its just a trigger telling DPP to apply a exposure curve to the raw image as it processes it, no harm done, you can change the curve any way you like.

This is interesting and unexpected (and still doesn't explain the raised iso) because Adobe raw in Lightroom does the same thing. I really expected that htp alters the actual raw data, too. It might be futile to ask but are you 100% sure about this? Any sources?

[markIVantony]

Bosman might have been replying to your title, which is a bit different than the ISO question you ask.
Regarding ISO, raising the minimum ISO might be to provide a buffer range for the firmware to allow -up to- a 150 ISO reduction in the entire image, and then to have room to shift the non-highlights back up to the original values before writing the image values.  Else, you could lose the black values (they would become less than zero).  However, I can only imagine there are several other equally effective algorithms to give highlight tones priority.

[Jake Townsen]

HTP used with raw obviously helps retain a lot more detail in the highlights. I could complelty blow out a shot where a wedding dress has no information and recover 90% of the detail in post. Without HTP a lot of that detail is lost.  From what I understand the reason iso 200 is used is because iso 100 is a computerized iso, kind of like iso 50. Whereas iso 200 is more native and able to record more information layers. Thus allowing more leeway when recovering highlights.  Actually when I've used iso 50 I've noticed how much less information is recorded in the raw. Check the camera manual as i recall there is something written about this, probably explains it better than I can.

[keithfullermusic]

Maybe I'm totally wrong, but I thought it exposes for the highlights at ISO 100 and exposes everything else at 200.

Everything I've heard about it is that you really shouldn't use it unless you're shooting jpeg because you can do it in post, and probably a better job, but you have to make sure to expose for the highlights.

[Marsu42]

Maybe I'm totally wrong, but I thought it exposes for the highlights at ISO 100 and exposes everything else at 200.

First time I've read that, but it would explain the iso increase... sources?

HTP used with raw obviously helps retain a lot more detail in the highlights. I could complelty blow out a shot where a wedding dress has no information and recover 90% of the detail in post. Without HTP a lot of that detail is lost.

The 60d manual? Explaining about inner iso workings? Not really :-) ... I'll get this information from magic lantern, in the latest builds you can separately tune the analog iso and digic digital gain (only full ev steps are native, the other ones are a hack I'm only using when shooting m with auto iso). But you're basically saying that there is a "real" data difference between htp and vanilla shots, and not just a reversible tone curve?

[Mt Spokane Photography]

The setting  is usually not used by raw processors except for DPP, so it has no effect on raw images for most photographers.  Its just a trigger telling DPP to apply a exposure curve to the raw image as it processes it, no harm done, you can change the curve any way you like.

This is interesting and unexpected (and still doesn't explain the raised iso) because Adobe raw in Lightroom does the same thing. I really expected that htp alters the actual raw data, too. It might be futile to ask but are you 100% sure about this? Any sources?

I turn it off for Raw, and forgot about the ISO change.  There seems to be a lot of opinion as to what curves image editors apply or don't apply, and no authoritative agreement.  We know what happens in jpeg.
 
In raw, unless you use DPP, I'd turn it off, and even if you use DPP, its a matter of personal opinion as to its benig better or not.
 
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I think you have brought up a good point, and its likely that each raw processor handles the situation differently as far as if or how they apply the tone curves.
 
I might try it with lightroom 4 RC1 and see what differences it makes, but I do not know if i'd be able to see any tone curves applied.

[markIVantony]

Maybe I'm totally wrong, but I thought it exposes for the highlights at ISO 100 and exposes everything else at 200.

HTP sets the -minimum- ISO to 200, meaning you can expose at 200 or higher.  It doesn't force exposure at 100/200 ISO.

[Mt Spokane Photography]

Marsu42, In checking my copy of Victoria Brampton's book for Lightroom, here is what she says. (Victoria is one of the most knowledgable Adobe users and frequently helps photographers out on the Adobe forums as well as publishing books about lightroom.)
  "Canon’s Highlight Tone Priority automatically underexposes the raw data by one stop to ensure you retain the highlights, leaves a tag in the file noting that this setting was applied, and applies its own special processing to the JPEG preview that you see on the back of the camera. Lightroom understands that tag and increases the exposure by one stop behind the scenes to compensate, but if you accidentally underexpose the image with HTP turned on too, you can end up with a very noisy file. When shooting raw for use in Lightroom, there’s no advantage to using that setting instead of changing the exposure compensation yourself, so you may wish to turn off HTP and set your exposure to retain the highlights manually.Canon’s Auto Lighting Optimizer and Nikon’s Active D-Lighting don’t affect the raw data itself, but Lightroom has no idea that you’ve used those settings, and even if it did, the processing applied by the camera is variable. When ALO or ADL are turned on, that special processing is applied to the JPEG preview that you see on the back of the camera, as well as to the resulting histogram. Seeing that false brighter preview could cause you to unknowingly underexpose the file, and then be disappointed to find it’s very underexposed when you view the raw photo in Lightroom, so it’s a good idea to turn those settings off unless you’re shooting JPEG or only using the manufacturer’s own software."
 
 
 
So, its pretty much worthless or even detrimental to most users of raw, and should be turned off unless you need it for straight to jpeg shots. You can always under expose bright scenes as you see the need if you are using raw.[/size]

[Policar]

I don't know if this is implemented in more expensive bodies than my 60d - but htp doubles the iso and applies a non-linear curve so that highlights get more resolution while the shadows are compressed. In good light, I used to use it all the time at the beginning ("why not?"), but then discovered that the darks get so little resolution that raising them in postprocessing introduces noise or banding.

It doesn't double the ISO, it halves it (hence 200 ISO being the lowest available with this feature--the camera is really shooting at 100ISO in this case) and then applies a curve that doubles the exposure of everything but the highlights, which roll off smoothly.  I love it.  If you bracket or shoot raw and expose to the right instead of exposing for the shot it's kind of pointless, but if you shoot jpeg and videos or ever use auto exposure outside it's amazing (if you shoot raw you can just expose at -1EV, granted).  If it weren't for Canon's noisy circuitry it wouldn't mess up the shadows, but unfortunately it does just a bit.  But most landscapes worth shooting have a very low scene dynamic range so for that just expose to the right and shoot raw since you might end up shooting at "50ISO" or something after pulling the image digitally and you can get nice contrast and tonality that way.

[Marsu42]

It doesn't double the ISO, it halves it (hence 200 ISO being the lowest available with this feature--the camera is really shooting at 100ISO in this case) and then applies a curve that doubles the exposure of everything but the highlights, which roll off smoothly.

Thanks Policar & Mt Spokane - that explains it and verifies the theory that it's indeed just a tone curve.

And if I understand it correctly, I can indeed simply reproduce this by shooting at iso 100 with 1ev underexposure and then apply a tone curve in postprocessing - which might be the better option for still shots because the shadow noise is treated better in postprocessing (see LR4 intelligent shadow recovery for this).

[Aglet]

So, its pretty much worthless or even detrimental to most users of raw, and should be turned off unless you need it for straight to jpeg shots. You can always under expose bright scenes as you see the need if you are using raw.[/size][/color]

Agreed.

I inadvertently left HTP ON in my 7D when shooting some extremely high DR landscape scenes only to find out MUCH later in post that the camera had significantly underexposed the shot compared to what I expected.  When trying to recover the shadow areas later in post using LR3 and only a setting of 17 on Fill Light slider all the shadows turned into corduroy with very obvious 8 pixel wide vertical noise stripes.  A complete loss of otherwise excellent images and the first blow that sent me into a love-hate relationship with the 7D.

OTOH, I can keep the shadow areas darker than I like and other people still liked the shot.
But I don't want to have to accept these sorts of compromises if I can avoid them so things I remember from that lesson:

- never use 7D for high DR landscape use
- never use HTP on any of my cameras because it's just as easy to get the effect I want in post as long as I expose to not clip the hilites
- ETTR is a good method to use on the 7D as it seems to meter even farther below midtone than some other cameras if the raw histogram info on DPP can be trusted. This'll give you a bit more DR and working room in post.
- visit my techblog to see about pushing shadows on various camera bodies