Highlight tone priority mode

[Alis]

I was wondering if anyone can explain what this mode is and if they use it. I assume it is a Canon term but don't know it's equivalent (if there is any) for Nikon or other brands.

[Colin Southern]

I was wondering if anyone can explain what this mode is and if they use it. I assume it is a Canon term but don't know it's equivalent (if there any) for Nikon or other brands.

Hi Ali,

It is a Canon term. It's a "juggle" to preserve more detail in highlights, but at the expense of potentially a little more noise in the shadows. It's achieved by manipulating the RAW data before it's written to the card - unfortunately, Canon needed to give themselves some wiggle room to be able to achieve this, and so the flip-side is there's a 200 ISO minimum when this is enabled.

I've used it - can't say the results were anything spectacular. Biggest downside for me is that I really need LOWER ISO modes - even 100 is a curse when shooting landscape, so even though it would be nice to have on, usually the ISO 200 minimum creates more problems for me than it solves.

For day-to-day shooting though (probably the kind that you do) it's a good thing as the noise difference between 100 and 200 ISO will be virtually undetectable, but you get the benefit of higher shutter speeds and more highlight tone detail.

[_GUI_]

It's achieved by manipulating the RAW data before it's written to the card.

Colin, let me correct this a bit. HTP is much simpler: it consists in shooting at an ISO one stop lower than set by the user (that is why ISO100 is not available when using this mode). That gives 1 extra f-stop of headroom in the highlights at the cost of more noise in the deep shadows (it's known that if keeping aperture/shutter we reduce ISO, we get more final noise).

If that was not clear enough: when you set HTP, shoot at 1/200 f/5.6 ISO200, you obtain _exactly_ the same RAW data as shooting at 1/200 f/5.6 ISO100, i.e. internally the shot is amplified at ISO100 gain.

The only point of using HTP is if you are shooting JPEG or you shoot RAW but you will use DPP for development, because in that case the RW development software will take into account that HTP was set and will process the RAW data accordingly, i.e. will lift more the shadows without blowing the extra highlights. But regarding the hardware and the obtained RAW data, HTP doesn't provide anything that could not be achieved without HTP.

An article about HTP demonstrates this: CANON'S HTP. USELESS IN RAW.

Regards.

[Colin Southern]

HTP is much simpler: it consists in shooting at an ISO one stop lower than set by the user (that is why ISO100 is not available when using this mode). That gives 1 extra f-stop of headroom in the highlights at the cost of more noise in the deep shadows (it's known that if keeping aperture/shutter we reduce ISO, we get more final noise).

Interesting ... so if I'm understanding what your saying correctly, in essence, you'd also get the same result at any ISO by dialing in 1 stop of -ve EC?

[Colin Southern]

Interestingly, Rob Galbraith had some interesting comments on HTP

http://www.robgalbraith.com/bins/con...id=7-8739-8970

"A Highlight Tone Priority image is processed differently in the camera, regardless of whether the camera is set to CR2 or JPEG. Specifically, the amount of gain applied during the analog-to-digital conversion step is less. For instance, if the camera is set to ISO 200, the amount of gain applied is similar to or the same as ISO 100, which means more of the highlight detail captured by the sensor is preserved during this early in-camera processing step. This is the main reason the camera can't be set lower than ISO 200 when Highlight Tone Priority is enabled; it requires the extra highlight headroom it gets by applying a level of analog signal boost to the sensor data that is commensurate with a lower ISO setting.

After that, it's all secret sauce: Canon isn't publicly describing what is done to the image once it's in digital form, but it obviously involves a modified tone curve that's meant to give Highlight Tone Priority ON photos the same overall tonal look as Highlight Tone Priority OFF ones, but with more detail and smoother gradation in the highlights."

In essence, agreeing with your comments about - essentially - under-exposing by 1 stop, but still feels that there's additional modification of the highlight tone curve in both the RAW and JPEG data.

Any additional thoughts?

[_GUI_]

In essence, agreeing with your comments about - essentially - under-exposing by 1 stop, but still feels that there's additional modification of the highlight tone curve in both the RAW and JPEG data.

Any additional thoughts?

Of course, the software part is the 'secret' of the HTP. My additional thought is that since I don't shoot JPEG, nor use DPP (the only two ways of enjoying that processing), HTP means nothing to me.

Regards.

[Dave Humphries]

Hi both,

in essence, you'd also get the same result at any ISO by dialing in 1 stop of -ve EC?

Having read the article, I would say the answer is a qualified yes (if I have understood correctly):
i.e. "Yes" if you shoot RAW and do not use Canon DPP to convert from RAW, (e.g. because you use ACR)

This is because only DPP correctly interprets the meta data that knew HTP was on when the shot was taken and it then applies the reverse process in RAW conversion to correct the otherwise under-exposed RAW file.

In the real world, it does seem to be something of benefit to jpg shooters only*, since if you're into RAW shooting, you'll probably apply more common sense to the conversion manually and possibly obtain a better than one stop improvement.

For the camera's jpg conversion, the normal S shape conversion (?) to get the sensor's 12 bits into jpg's 8 bits must (I assume) be biassed down a stop in favour of protecting the highlights with a corresponding increase in gain at the bottom end to get the shadow detail back in (along with some more noise).

At least, I think that's it ...

* Since jpg shooters are probably a large part of Canon's customer base, it probably explains why the feature exists - along with Auto, P, scene modes, etc. All the stuff we never use these days!

[Colin Southern]

Of course, the software part is the 'secret' of the HTP. My additional thought is that since I don't shoot JPEG, nor use DPP (the only two ways of enjoying that processing), HTP means nothing to me.

Regards.

I must admit that I'm pretty much in the same boat.

[_GUI_]

* Since jpg shooters are probably a large part of Canon's customer base, it probably explains why the feature exists - along with Auto, P, scene modes, etc. All the stuff we never use these days!

In fact I think JPEG shooters are the biggest piece of the cake. Digital photography forums gather advanced users (like us) who want the best from our cameras, so we are not a good sample to find out how the average camera owner is.

Thousands of low end DSLR users just want to take their camera on holidays and weekends to shoot JPEG to share with their friends and family, and they will not go to the forums to discuss about RAW or try to understand the inner functions of their camera, like not everyone owning a car enters car&driver forums to discuss advanced details about their vehicles.

That is why IMO JPEG-oriented features are a priority for camera vendors and will remain so.

Regards.

[Colin Southern]

This is because only DPP correctly interprets the meta data that knew HTP was on when the shot was taken and it then applies the reverse process in RAW conversion to correct the otherwise under-exposed RAW file.

To be honest Dave, I haven't done any testing personally - and don't intend to. It was a pretty hot topic for a while though, and the general consensus from those who were getting all excited about it was that Canon were under-amplifying the gain from the A/D converter (as _GUI_ points out), but were also manipulating the RAW data around the highlight tone curve.

Personally, I find that I often have to under-expose images by up to a couple of stops to get great highlight detail across all channels.

[Dave Humphries]

Personally, I find that I often have to under-expose images by up to a couple of stops to get great highlight detail across all channels.

Weeelll, it gives all those ND filters you have a use I suppose

[Colin Southern]

Weeelll, it gives all those ND filters you have a use I suppose

Nah - they don't help. GND filters on the other hand

[Alis]

Thanks everyone for the great discussion.

I was playing with the settings a while back, then noticed afterward that I can not go below ISO 200. Then had to go back and disable those functions one by one to find out which one did it (reverse engineering?

But my guess was it had something to do with RAW and it should not be something you can achive in the comuter completely, as (I may be wrong) I do not remember having this feature on my Rebel XTi.