OT? Is Canon's HTP (highlight tone priority) just digital or really changes RAW values?

Submitted by l_d_allan on

Forums: 

My understanding is that Canon's HTP (highlight tone priority) really does change RAW values, and I've also read that it is just digital, so leave it off.

IIRC, the DPReviews of Canon DSLR's over the years have indicated that HTP does what it is supposed to do ... change the sensor's RAW response curve to increase RAW dynamic range at the cost of some noise (ISO 100 vs ISO 200).

I wrote to Canon tech support, and their reply was that HTP changed the sensor response curve for RAW, not just JPEG.

My experience from watching "blinkies" is that HTP increases the dynamic range, but that may be an artifact of JPEG processing. I don't recall if I've tested that since MagicLantern (ML) implemented RAW histograms and RAW blinkies.

My question came specifically in a OP to the ML forum asking about Dual-ISO and HTP:
http://www.magiclantern.fm/forum/index.php?topic=8442.0

I consider the ML developers to be true Canon gurus, yet I find myself in disagreement with them on HTP. However, it is possible I misunderstood a cryptic reply.

BTW: my understanding of ALO (auto lighting optimizer) is that it is just for JPEG, and doesn't make a difference for RAW. Or not?

Dear Allan,

Submitted by Iliah Borg on

Dear Allan,

I think HTP is just another word for underexposure. There is no way to change sensor response curve in general purpose cameras. To check, you can shoot with HTP on and off and compare histograms in RawDigger.

--
Best regards,
Iliah Borg

HTP

Submitted by Jonathan (not verified) on

If what you say is true, when you set exposure for HTP wouldn't you see a change in the shutter speed, aperture,ISO or some combination of both? I have experimented by photographing a light bulb with HTP on/off. I saw no changes in the in the exposure values. Often I will shoot -1/3ev to -1/2ev on bright sunny days which appears to give similar results vs. HTP. On my 6d I can't set HTP and manipulate -ev. The meter freezes at "0". According to canon the mid tone values on the Dynamic Range scale are shifted to help preserve highlights. Don't quite understand what they mean..isn't that what happens when you shoot -ev?

Dear Allan,

Submitted by Iliah Borg on

Dear Allan,

Additionally, here is a quote from Chuck Westfull TechTips:

 

Recent Canon EOS cameras have both Highlight Tone Priority and Auto Lighting Optimizer settings. What is the difference between them and when would I use each? It seems to me that they both address high dynamic range situations.
 
Auto Lighting Optimizer (ALO) analyzes contrast in captured images and modifies both shadows and highlights via tone curve adjustments to minimize loss of detail in contrasty lighting conditions. Current EOS models including the EOS 50D and 5D Mark II provide four settings for ALO: Off (Disable), Low, Standard and Strong. ALO can be used at any ISO speed setting. Sample images showing the effect of ALO can be seen here:
 
http://web.canon.jp/imaging/eosd/eos50d/01.html#03
 
Highlight Tone Priority (HTP) is available with all current EOS models excluding the Rebel XS/1000D. HTP has no effect on the actual dynamic range of the image sensor. It's just an alternative method of image processing that preserves more highlight detail than Canon's standard processing, without significantly altering midtones or shadows. The effect of HTP is enhanced by Canon's 14-bit A/D converter, which provides finer tonal gradations than the previous 12-bit system. HTP is a Custom Function with a simple on/off setting, and the available range of ISO speed settings is slightly limited when it is on. Take a look at the following Web page for some sample images that show the capabilities of Highlight Tone Priority:
 
http://www.imaging-resource.com/PRODS/E1DMK3/E1DMK3EXPOSURE_HTPMODE.HTM
 
ALO does not affect RAW image data. It is at its best for in-camera JPEGs shot in extremely contrasty lighting conditions. Examples would include backlit portraits and urban landscapes on sunny days, where the tops of buildings are brightly illuminated by the sun but subject matter at street level is in heavy shadow. HTP affects RAW data as well as in-camera JPEGs. It is very useful in high-key shooting conditions such as wedding photography and certain kinds of sunsets. ALO can be combined with HTP with cameras that have both features.

 

 

--
Best regards,
Iliah Borg

I've studied HTP (D+) on my

Submitted by lexa on

I've studied HTP (D+) on my Canon 5D Mark II about 5 years ago.

This is just 1-stop underexposure and +1EV middle tone correction in RAW converter (or in-camera JPEG) later.

A later article on Canon's

Submitted by JPL on

A later article on Canon's site (sorry, but I didn't save the link) indicated that the changes made by HTP were applied to RAW development in DPP only, not in other RAW converters.

We can't really comment as no

Submitted by Iliah Borg on

We can't really comment as no link is provided. In raw, HTP amounts to what is commonly called "underexposing by one stop". In conversion, HTP needs to be compensated for, not applied.

--
Best regards,
Iliah Borg

Dear Sir:

Submitted by Iliah Borg on

Dear Sir:

You wrote "only", while the page you linked to states "usually".

HTP directly affects raw data, at the time the shot is taken. That is why further on the page it is said "Please note that when HTP is active, the highest and lowest ISO settings of your camera may not be available".  To elaborate, what happens is ISO setting is dropped down 1 stop while shutter speed and aperture setting are preserved (this amounts to decreasing the raw levels by one stop - commonly (mis-)labelled as 1-stop underexposure), and that is why at base ISO setting HTP is not available. Dig it with RawDigger ;)

--
Best regards,
Iliah Borg

OP still confused. Does ACR handle HTP ok?

Submitted by l_d_allan on

I'm still unclear on whether it makes a different to the "effective RAW values" if HTP is used or not when the option of RAW is set in the camera.

My impression is that if Canon's DPP is used, then it "usually" makes a difference. Or not?

If so, does Adobe's ACR or LR able to increase effective dynamic range if HTP is ON with the camera set to RAW?

Or does it not really matter if ACR / LR are used, so just leave it OFF?

Dear Allan,

Submitted by Iliah Borg on

Dear Allan,

HTP is just highlights protection. It does change the raw values, because the internal gain is changed (folks call it dropping ISO down 1 stop). You know of exposure compensation, which allows  to adjust shutter speed and aperture to achieve an exposure different from metered. This one is "-1 EV ISO compensation". Check with RawDigger, and you will see.

I would not  bring dynamic range into a general HTP discussion, as there are too many variables involved, like camera model and particular ISO setting.

With any converter you can do a simple experiment shooting with HTP on and off and comparing the results.

If a converter does not support HTP it is easy enough to design a convex curve in the converter to counteract HTP, lifting the midpoint 1 stop, compressing the highlights, and expanding shadows.

 

 

--
Best regards,
Iliah Borg

HTP seems to help reduce "blinkies", inc with ML RAW blinkies

Submitted by l_d_allan on

Thanks for the info.

What I have noticed is that "blinkies" are less likely to happen when I have HTP enabled and RAW mode in the camera.

If I have a "sunny 16 rule" situation outdoors in full sun (not cloudy and during summer), I'll notice "blinkies" showing up more with HTP disabled and ISO 200. If I enable HTP and use ISO 200, then "blinkies" become visible at a higher exposure value.

For example: HTP OFF, blinkies at ISO 200, f16, 1/160th second.
HTP ON, blinkies at 1/100th second.

With the above example, my understanding is that HTP has increased "highlight protection" by 2/3rd stop. Correct?

I've also seen something very similar with Magic Lantern's RAW histogram/blinkies enabled. However, with ML, the blinkies don't show up until even greater exposure, such as 1/50th second.

The above seems to indicate that HTP does actually change how the sensor is recording values. It's not something that could be accomplished the same with post processing. Or not?

Also, thanks for pointing out that I had a misconception about "highlight protection" and "dynamic range". If I understand your comment correctly, I now think that DR doesn't improve, because the improved "highlight protection" comes at the cost of increased noise in the shadows. Is that more or less correct?

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