Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: Gary Ferguson on November 25, 2007, 08:29:43 am
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I'd be interested if anyone can help explain the method and practical advantages of Canon's new "Highlight Priority" setting, available on the 40D, 1D MkIII, and the forthcoming 1Ds MkIII. Bob Atkins gave a good analysis here,
http://www.bobatkins.com/photography/digit...D_review_6.html (http://www.bobatkins.com/photography/digital/canon_eos_40D_review_6.html)
but he concludes that there's still some unknowns in Canon's method. Does anyone know of further analysis, and has anyone any comments based on practical experience?
Thanks.
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I'd be interested if anyone can help explain the method and practical advantages of Canon's new "Highlight Priority" setting, available on the 40D, 1D MkIII, and the forthcoming 1Ds MkIII. Bob Atkins gave a good analysis here,
http://www.bobatkins.com/photography/digit...D_review_6.html (http://www.bobatkins.com/photography/digital/canon_eos_40D_review_6.html)
but he concludes that there's still some unknowns in Canon's method. Does anyone know of further analysis, and has anyone any comments based on practical experience?
Thanks.
[a href=\"index.php?act=findpost&pid=155741\"][{POST_SNAPBACK}][/a]
The only unknowns are how each converter deals with it.
The RAW data are just taken at double the ISO, with half the exposure. The conversions should bring the grey point up 1 stop, but roll in the extra highights. Allegedly, some converters, like ACR, bring up the grey point but don't roll in the highlights automatically.
There is definitely no intelligent assessment of highlights, if that is what you are hoping for; just a fixed, dumb exposure compensation at a higher ISO, which mandates noisier shadows.
EDIT - that should have been at a "lower" ISO, IOW, the higher ISO exposure is used with the gain and digitization of the lower ISO.
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John, are you saying it's the same as doubling the ISO then dialling in one stop's worth of underexposure?
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This has been discussed in this thread:
http://luminous-landscape.com/forum/index....50&#entry153950 (http://luminous-landscape.com/forum/index.php?showtopic=20984&st=0&p=153950&#entry153950)
A lengthy discussion on the same subject at DP Review site: http://forums.dpreview.com/forums/read.asp...essage=25080916 (http://forums.dpreview.com/forums/read.asp?forum=1019&message=25080916)
Here is my take: It is useful if you are shooting JPEG: it adds a different tone curve to open up the mid-tones and shadows. Not that you could not come up with the same (or better) curve yourself though. In RAW it does nothing, except underexposing one stop (by shooting as if at the next full-stop lower ISO).
I think Bot Atkins got it wrong by claiming that HTP impacts RAW too (he is admitting several times in the article he is not sure what is going on with HTP). His update (to the initial review) and examples prove nothing, IMHO. What he did is the following:
"... RAW images shot at ISO 200, "developed" with a -2 exposure compensation..."
He used the same method for both non- and HTP files, which appears to be logical, but this is what really happened in my opinion:
Since HTP exposures are already underexposed by one stop, developing them with the same -2 compensation resulted in non-HTP files being underexposed by 2 stops, and HTP files being underexposed by 3 stops. No wonder the 3 stops underexposure retains more detail in the highlights.
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That's clear, thanks.
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John, are you saying it's the same as doubling the ISO then dialling in one stop's worth of underexposure?
[a href=\"index.php?act=findpost&pid=155752\"][{POST_SNAPBACK}][/a]
I said it partly wrong, and I've edited the statement.
More precisely, if you have the camera set to ISO 200 and HTP, then the RAW image is exactly the same as if you had shot at ISO 100 with -1 EC.
The ironic thing, which shows how screwed up these big corporations get, is that it is not available for ISO 3200 on the 40D, yet ISO 3200 IS ISO 1600 under-exposed by one stop - they just double the numbers, tossing away a stop of highlight headroom.
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having read the linked discussions, I only have one question. with HTP turned on, does the down-stepping of the actual ISO only occur when the sensor senses that highlights would otherwise be clipped? or is the ISO stopped down regardless of what is being photographed?
ftr, I use HTP all the time and it works quite well although most of my images are dark and need exposure adjustments in Aperture after transfer from the camera. now I understand why.
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having read the linked discussions, I only have one question. with HTP turned on, does the down-stepping of the actual ISO only occur when the sensor senses that highlights would otherwise be clipped? or is the ISO stopped down regardless of what is being photographed?
ftr, I use HTP all the time and it works quite well although most of my images are dark and need exposure adjustments in Aperture after transfer from the camera. now I understand why.
[a href=\"index.php?act=findpost&pid=156837\"][{POST_SNAPBACK}][/a]
No, the sensor is not that smart, so it underexposes regardless of what is being photographed.
To the best of my knowledge, Aperture is not HTP-friendly, i.e., it does not recognize the HTP tag the way Canon DPP and Adobe CR do. I am not sure about other software.
As for keeping HTP on all the time, I am not sure it is a good strategy. It makes sense if you are shooting scenes with a lot of important highlights at risk of clipping, but for general work the downside is that you are simply underexposing majority of your shots, risking more noise, for instance.
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The ironic thing, which shows how screwed up these big corporations get, is that it is not available for ISO 3200 on the 40D, yet ISO 3200 IS ISO 1600 under-exposed by one stop - they just double the numbers, tossing away a stop of highlight headroom.
[a href=\"index.php?act=findpost&pid=155981\"][{POST_SNAPBACK}][/a]
I just read mypost again, and thought that perhaps I could be clearer. Basically, ISO 3200 on the 40D, which does not offer HTP for that ISO, is actually the same thing as HTP at ISO 3200 would be (same noise levels), but with the extra highlights thrown away (when the RAW numbers are doubled and clipped at 4095).
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You all might get some real answers over at prophotohome, where Chuch Westfall seems to answer questions on a regular basis.
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You all might get some real answers over at prophotohome, where Chuch Westfall seems to answer questions on a regular basis.
[a href=\"index.php?act=findpost&pid=157592\"][{POST_SNAPBACK}][/a]
Chuck appears on other forums I've been on, and very tight-lipped about anything going on under the hood - you'll get much more useful answers by reverse-engineering the camera's output.
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Here's some info:
http://www.prophotohome.com/forum/canon-1-...light-prio.html (http://www.prophotohome.com/forum/canon-1-series-digital-slr-eos-5d/75125-mr-westfall-question-about-highlight-prio.html)
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Here's some info:
http://www.prophotohome.com/forum/canon-1-...light-prio.html (http://www.prophotohome.com/forum/canon-1-series-digital-slr-eos-5d/75125-mr-westfall-question-about-highlight-prio.html)
[a href=\"index.php?act=findpost&pid=157966\"][{POST_SNAPBACK}][/a]
Not much help as far I could see (without a subscription) - am I missing something, or could someone who has seen it précis the answer?
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Not much help as far I could see (without a subscription) - am I missing something, or could someone who has seen it précis the answer?
[a href=\"index.php?act=findpost&pid=158401\"][{POST_SNAPBACK}][/a]
This is the response from Chuck Westfall:
"We haven't made any claims about increasing dynamic range with our current image sensors. Instead, we've said that dynamic range remains about the same or similar to previous generations despite higher resolution and smaller photosites. (Check 1Ds Mark III vs. 1Ds Mark II for example.)
Highlight Tone Priority mode 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 our standard processing, without significantly altering midtones or shadows. The effect of HTP is enhanced by our 14-bit A/D converter, which provides finer tonal gradations than the previous 12-bit system."
__________________
Chuck Westfall
Director/Media & Customer Relationship
Camera Division/Canon U.S.A., Inc.
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This is the response from Chuck Westfall:
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Thanks very much for that.
I've only done a few (very) quick comparisons with my 1Ds3 since I'd suspected that I could get much the same results with appropriate exposure and subsequent attention to raw processing.
It does make a difference with in camera jpegs (as expected) but I don't think I'll be using it in normal day to day raw shooting
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The HTP is something which I will try and probably use a lot. From tests I've seen, it works well to preserve highlights AND also it seems to render much more shadow detail. The less time I spend in post production futzing around with these files trying to get highlights back, the better, IMO.
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... The less time I spend in post production futzing around with these files trying to get highlights back, the better, IMO.
I'd hardly call having a tone curve (or two) that offer levels of HTP much 'futzing around'... Still I suppose it depends on the type of work you do and your workflow. It's just that in a series of images there may be as many I don't want it applied to, as to ones I -do- want it applied to. The tone curve approach gives a flexibility that I think would more suit my own workflow.
But who knows, I've barely had it for a week so far :-) :-)
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If you are shooting raw would HTP be any better than applying a good curve to an ISO 100 shot?
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Might not the two approaches be somewhat complementary? HTP does affect the raw data (I believe?) and curves is post-shoot. And there's the old "choose your exposure" thing too ...
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If you are shooting raw would HTP be any better than applying a good curve to an ISO 100 shot?
[a href=\"index.php?act=findpost&pid=158440\"][{POST_SNAPBACK}][/a]
I think the trick is finding a curve which does as good a job as the HTP does. It seems to be easier said than done. If this was just a curve issue, then Canon could have simply updated DPP and been done with it, I presume.
In any case, I have had pretty good luck with the Photoshop Highlight and Shadow tools...
-r
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Quote from Chuck Westfall:
Re: Highlight Prio.
Dynamic range of the 40D remains the same as the 30D despite higher resolution and smaller pixels.
What you're seeing with HTP is a modified image processing algorithm combined with the effects of 14-bit A/D conversion of the RAW image data. Reviewers who discuss the benefits of HTP are effectively pointing out how image processing modifications combined with a higher bit-depth in the RAW image data can improve the rendition of highlight details compared to earlier cameras without those features.
And once again, the 14-bit A/D conversion enhances the effect of HTP because it provides 4 times as many gradations to choose from as the RAW image data from cameras with 12-bit A/D conversion.
__________________
Chuck Westfall
Director/Media & Customer Relationship
Camera Division/Canon U.S.A., Inc.
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Quote from Chuck Westfall:
Re: Highlight Prio.
And once again, the 14-bit A/D conversion enhances the effect of HTP because it provides 4 times as many gradations to choose from as the RAW image data from cameras with 12-bit A/D conversion.[a href=\"index.php?act=findpost&pid=158949\"][{POST_SNAPBACK}][/a]
If the 14 bits can't improve DR, it really can't improve anything else, because noise, which is the barrier to DR, is also the barrier to being able to benefit from smooth "gradients".
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If the 14 bits can't improve DR, it really can't improve anything else, because noise, which is the barrier to DR, is also the barrier to being able to benefit from smooth "gradients".
[a href=\"index.php?act=findpost&pid=158953\"][{POST_SNAPBACK}][/a]
It seems that everyone who has reviewed these cameras has stated that the files look better in color, shadow detail, highlight retention, etc.
So, uh, I am going to take Chuck's word over yours :-)
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It seems that everyone who has reviewed these cameras has stated that the files look better in color, shadow detail, highlight retention, etc.
So, uh, I am going to take Chuck's word over yours :-)
[a href=\"index.php?act=findpost&pid=158961\"][{POST_SNAPBACK}][/a]
I think the idea here is that the in-camera processed jpegs look better, not necessarily Raw images that have been underexposed by one stop at a lower ISO and processed accordingly in ACR. Sometimes just one click on 'auto' in ACR can correct an image, whether underexposed or fully exposed to the right.
On the occasions that I've resorted to shooting in jpeg mode, I've found that at least some of the shots have had irrecoverable highlight detail loss, despite my attempts at being careful about exposure.
I think this HTP feature is strictly for jpeg shooters. Of course it works in RAW mode, but I can't think why you would use it in RAW mode.
BTW, an image which is fully exposed to the right in jpeg mode will suffer from irrecoverable loss of highlight detail, without the HTP feature.
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It seems that everyone who has reviewed these cameras has stated that the files look better in color, shadow detail, highlight retention, etc.
So, uh, I am going to take Chuck's word over yours :-)
[a href=\"index.php?act=findpost&pid=158961\"][{POST_SNAPBACK}][/a]
Chuck is a business person. What he is going to say is what he thinks is good for the company (and his own standing there), and within the limits of his own knowledge and understanding.
Canon's official statements have proven to be incorrect in the past; remember the white papers that said how the "big pixel" cameras (5D and 1Dmk2) have better high-ISO performance because they collected more photons with their bigger photosites? The fact is, these cameras have a lower quantum efficiency than the Canon 20D, and even less than some P&S cameras. The high-ISO pixel performance is almost exactly the same on all three cameras. The only reason the 1Dmk2 gives better high-ISO images than the 20D is because the coarser pixel spacing requires less from the optics for a sharp image, and the anti-aliasing filter is weaker, meaning the sharpness of "image" is greater, relative to the sharpness of noise. Nothing electrical, or related to photons at all. The bigger pixels captured more photons total, but that requires a greater exposure (ISO 50).
Never trust a company, or its representatives, completely. Always verify what they say.
As far as 14-bit Canons are concerned, it is impossible to get a 12-bit RAW out of the same camera, so you are comparing 14-bit from one, newer camera, along with the possibility that it is forcing the converter to work with more shadow precision (something it could have always done without the extra two bits), to another, older camera.
Those of us who have taken 14-bit RAW data and quantized the data to 12-bit, have found no loss in image detail in any tonal range.
Do you have any idea how much pixel values vary due to noise alone? The difference between values at 14-bit, and what they are rounded to when quantized to 12 bits, is only a very tiny fraction of the amount the pixel values are varying due to noise, except in the very deepest shadows of the lowest ISOs. In fact, for the midtone and highlight areas, the rounding is virtually infinitessimal compared to the shot noise.
All the apparent benefits of 14-bit RAW processing are available by just promoting 12-bit RAWs to 14 bits by padding them with '10' (binary), just like you get better results by converting an 8-bit JPEG to 16-bit in photoshop before extensively editing it.
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As far as 14-bit Canons are concerned, it is impossible to get a 12-bit RAW out of the same camera, so you are comparing 14-bit from one, newer camera, along with the possibility that it is forcing the converter to work with more shadow precision.......
[a href=\"index.php?act=findpost&pid=159011\"][{POST_SNAPBACK}][/a]
I am more likely to trust reviews with actual photos than a listing of facts, figures, and speculation in this photo forum :-)
Anyway, I'll have the camera and decide for myself soon enough.
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I am more likely to trust reviews with actual photos than a listing of facts, figures, and speculation in this photo forum :-)
[{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=159017\")
Here's a 14-bit RAW from a 1Dmk3 at ISO 100. This is the lowest 256 RAW levels, untouched, and quantized to 12, 11, and 10 bits. This is dark brown, almost black hair pushed to about ISO 2800 to 3200. All quantized, then white-balanced and interpolated to fill the RGB of all pixels in the RAW (simple demosaicing). All have an extra 7 bits of precision. As you can see, the difference between 14-bit and 12-bit is miniscule, and this is the camera/ISO with the lowest read noise of any 14-bit DSLR. My estimation is that when the minimum (read or blackframe) noise is about 1.3 to 1.4 RAW levels (ADUs), the bit depth is optimum. For this image, the read noise is 1.22 12-bit ADUs, or 4.88 14-bit ADUs.
Blue channel only:
[a href=\"http://www.pbase.com/jps_photo/image/84815836/original]http://www.pbase.com/jps_photo/image/84815836/original[/url]
In color:
http://www.pbase.com/jps_photo/image/76001165/original (http://www.pbase.com/jps_photo/image/76001165/original)
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Reviewers who discuss the benefits of HTP are effectively pointing out how image processing modifications combined with a higher bit-depth in the RAW image data can improve the rendition of highlight details compared to earlier cameras without those features
Those reviewers are repeating slogans instead of going into details.
the 14-bit A/D conversion enhances the effect of HTP because it provides 4 times as many gradations to choose from as the RAW image data from cameras with 12-bit A/D conversion
This too is worthless blathering. One needs to think about what effect exactly the greater bit depth does for HTP. It's easy to show, that it does nothing.
I hoped that the increased bit depth has some visible effect on the images. I am still not ready to accept the quite amateurish "proofs", that the extra bits are only noise.
I invested inordinate amount of time with shooting scenes designed to demonstrate, that the two extra bits do in fact represent graduations and with analyzing the results. Several times I though I had it, but after reviewing, I alway came to the conclusion that my proofs were questionable.
However, independently of the worthiness of these "new" bits, there is an other side of the issue, namely when and how the intermediate levels can be utilized. If at all, then only in situations, when the low end gets boosted several stops or the contrast at the low end gets highly (but very highly) increased. Neither of these are happening with HTP (note, that the advantage of HTP is in recording JPEG).
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Those reviewers are repeating slogans instead of going into details.
This too is worthless blathering. One needs to think about what effect exactly the greater bit depth does for HTP. It's easy to show, that it does nothing.
[{POST_SNAPBACK}][/a] (http://index.php?act=findpost&pid=159157\")
Speaking of worthless blathering, would you (or John) care to demonstrate your findings here? I'd like to see your proof that what Canon has incorporated into these cameras does nothing useful. All the techno-speak does very little for me. Let see some real evidence, rather than just words.
In the mean time, here are two of the places where I saw stuff which seems to suggest that there are demonstrable improvements.
[a href=\"http://www.bobatkins.com/photography/digital/canon_eos_40D_review_6.html]http://www.bobatkins.com/photography/digit...D_review_6.html[/url]
http://www.dpreview.com/reviews/canoneos40d/page20.asp (http://www.dpreview.com/reviews/canoneos40d/page20.asp)
Again, let's see proof guys, not just tech talk.
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Speaking of worthless blathering, would you (or John) care to demonstrate your findings here? I'd like to see your proof that what Canon has incorporated into these cameras does nothing useful. All the techno-speak does very little for me. Let see some real evidence, rather than just words
1. If there is a claim, that the camera has some improvements, then that claim needs to be proven, not the opposite.
2. I have not stated, that it does nothing useful. Just the opposite: I stated on another thread, that HTP is useful for JPEG. Its usefulness for raw files is very limited: the in-camera display is brighter than it would be with 1 stop underexposure, and the embedded JPEG file as well as the thumbnail are better with HTP. One may find it an advantage of HTP, that DPP and ACR automatically corrects the exposure by +1EV, though I don't like such covert actions. DPP adjusts the contrast in the highlights as well, like the in-camera conversion.
3. The 40D does have improvements (although it does not have "better colors" in raw). For me the difference between the 20D and the 40D was enough to but it (now, what to do with the former? I think I will make an IR camera from the 20D).
Furthermore, I am still not fully convinced, that the increased bit depth is really only noise. However, that needs to be proven, and I have not seen any proof of that, and I was unable to prove it convincingly (although I found some indications, that it is not or not always noise).
The review you cited from Bob Atkinson is an excellent demontration to the shallow blathering. the similar function in the EOS 1D MkIII is said to provide up to an extra stop of dynamic range in the highlight regions is an extraordinary nonsense (although he expresses this cautiously, "is said to"). This part of the review is restricted to guess, might, would.
DPReview did not go into details about the exact effect of HTP on raw data.
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... I'd like to see your proof that what Canon has incorporated into these cameras does nothing useful...
Nobody has ever said HTP "does nothing useful" in JPEG. Where we differ is RAW.
Perhaps we are differing in semantics here, so lets get that out of the way first. When people say (myself included) that HTP has no effect on RAW, they are actually saying: "HTP has no effect on RAW other than underexposing it by one stop".
Both sides further agree that HTP does affect how a RAW file is displayed on the camera's screen and later on in a RAW converter. Where both sides of the debate really differ is in how an HTP RAW shot affects highlights. My side of the debate says: HTP does not affect highlights any better then underexposing RAW file by one stop does.
The other side of the debate (i.e, Bob Atkins and very few others) seem to believe that HTP does more than that, i.e., that it somehow manages to preserve highlight detail better (i.e., better than underexposure does). If that would be true, i.e., preserving more highlight details, that would simply mean that we would have a one stop better dynamic range of the sensor. Even Canon does not claim that.
But let us suppose for a moment it is true (i.e., that HTP does increase the DR of the sensor). Why would then Canon offer cameras with HTP as an option only? Why would then anyone choose to cripple the sensor and blow highlights by not using HTP?
And this is why the belief in the HTP RAW "virtues" is dangerous: it makes people keep HTP on all the time (and why not, they say, who would not want a better DR?). By keeping it on all the time, they are underexposing every shot they take, even those not in danger of highlights being blown. Instead of "exposing to the right" they are consistently doing the opposite: exposing to the left, i.e., losing details and adding noise in mid-tones and shadows.
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I posed a similar question on the Adobe forums regarding the value of HTR using RAW - see Thomas Knoll's reply http://www.adobeforums.com/webx?14@@.3bb6a869.3c0330d0/0 (http://www.adobeforums.com/webx?14@@.3bb6a869.3c0330d0/0)
That reply is incorrect (sloppy). Not the exposure gets reduced but the ISO. This is the reason, why HTP is not available with ISO 100. It does not need any reverse engineering to find it out.
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The more I read and analyse about the newly come 14-bit cameras, the more I think Canon and Nikon reached some agreement with all the manufacturers of memory cards, hard disks and storage devices in general. We are starting to deal with 25MB RAW files per photograph, that's beginning to be insane.
Lots of Mpx is probably fine for certain applications (large printings basically), but I am starting to question myself if going beyond 12 bit RAW files is of any use to improve image quality. At least meanwhile noise remains being the limiting factor to DR.
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That reply is incorrect (sloppy). Not the exposure gets reduced but the ISO. This is the reason, why HTP is not available with ISO 100. It does not need any reverse engineering to find it out.
[a href=\"index.php?act=findpost&pid=159302\"][{POST_SNAPBACK}][/a]
I think the confusion here is the idea that HTP at say ISO 200 is the same as one stop underexposed at ISO 100. That implies, or could imply depending on how you interpret it, that using HTP with a particular exposure at ISO 200 is the same as dropping down to ISO 100 and dialing in -1 EV, when in fact it's the same as dialing in -1EV at ISO 200.
Whether the underexposure is acheived by dialing in the -1EV at ISO 200, or by the camera dropping down to ISO 100 but using the same exposure that has been auto-metered at ISO 200, is immaterial. The same number of photons arrive at the sensor, except, and it appears to be a big exception, higher ISO's can handle underexposure better, at least in Canon DSLRs.
I've verified this myself with the 5D. An image at ISO 100, say 1 stop underexposed, will have more shadow noise than it will if the ISO is bumped up to ISO 200, keeping the exposure the same. This apparently, (I just read what makes sense to me) is because at ISOs above base ISO, the analog signal is boosted prior to A/D conversion resulting in lower read noise. Same thing applies at all the other ISO's except ISO 3200. Comparing an image that's been underexposed by 4 stops at ISO 100 with the same image at literally the same exposure but at ISO 1600, really brings out the differences in noise in the mid-tones, lower mid-tones and shadows.
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Whether the underexposure is acheived by dialing in the -1EV at ISO 200, or by the camera dropping down to ISO 100 but using the same exposure that has been auto-metered at ISO 200, is immaterial
You are over-complicating this very simple issue. It has nothing but absolute nothing to do with metering, EV bias, etc.
The camera (at least my cameras) displays the shutter speed, the aperture (and only these determine the exposure, not ISO), as well as the ISO. When shooting, that very exposure will be used, i.e. the indicated shutter and aperture, and the ISO gets "manipulated".
It is not subject to interpretation, that HTP leaves the exposure unchanged but reduces the ISO.
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Your reply is (sloppy) I am fully aware that this is the reason why only 200 ISO is available, it doesn't take a rocket scientist to work this out, but the end result is 1 stop of underexposure which ever way you look at it.
IMO you need to study the difference between having to say something and having something to say.
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You are over-complicating this very simple issue. It has nothing but absolute nothing to do with metering, EV bias, etc.
The camera (at least my cameras) displays the shutter speed, the aperture (and only these determine the exposure, not ISO), as well as the ISO. When shooting, that very exposure will be used, i.e. the indicated shutter and aperture, and the ISO gets "manipulated".
It is not subject to interpretation, that HTP leaves the exposure unchanged but reduces the ISO.
[a href=\"index.php?act=findpost&pid=159350\"][{POST_SNAPBACK}][/a]
Everything is subject to interpretation, my friend, even the most objective scientific theories.
The confusion is in the language. I notice that John Sheehy has amended his explanations a couple of times.
So let me rephrase what I understand is going on here, and you can correct me if I'm wrong.
(1) Activating HTP does not change the actual and real exposure (shutter speed). It just drops the ISO setting down a stop. In relation to the 'hidden' lower ISO setting that the camera has surreptitiously used, the shot now has the equivalent of one stop less exposure. Whether or not the shot is actually underexposed as a result, will depend on other circumstance.
(2) If the shutter speed at the higher ISO is correct for a perfect ETTR (and one is shooting RAW), then activating HTP will be disadvantageous and produce more noise in the shadows.
(3) If the shutter speed at the higher ISO would have been correct for an ETTR if one had been shooting RAW but one happens to be shooting in JPEG mode, then one is protected from blown highlights by using HTP.
This is the purpose of HTP.
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You also said -
"IMO you need to study the difference between having to say something and having something to say."
Nick,
You've got the wrong person. It was Panopeeper who said that.
There is no confusion in the language as HTP results in one stop less of exposure at the time of recording the exposure. The shutter speed and aperture selected are not altered by HTP otherwise it wouldn't work using manual shutter and aperture speeds. Therefore the only means left available for HTP to work is by dropping the sensor to 100 ISO to reduce the exposure. To meet Canon's HTP feature demands a minimum setting of 200 ISO to enable this to happen.
The above paragraph quoted from you is riddled with confusion.
Because I'm in a pleasant mood at the moment (because my beautiful photographic assistant is returning to me from Bangkok in a few days) I'll explain, phrase by phrase.
...HTP results in one stop less of exposure at the time of recording the exposure.
It doesn't. Exposure is unaltered. Only ISO setting is altered.
The shutter speed and aperture selected are not altered by HTP
Exactly true. You've just contradicted yourself in the space of one breath.
Need I say more .
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Everything is subject to interpretation, my friend, even the most objective scientific theories
You just have been resisting Nick's desperate attempts to "interpret" the definition of exposure.
(1) Activating HTP does not change the actual and real exposure (shutter speed). It just drops the ISO setting down a stop. In relation to the 'hidden' lower ISO setting that the camera has surreptitiously used, the shot now has the equivalent of one stop less exposure
(emphasis by me)
Not so. One stop lower exposure and one stop lower ISO are not the same. If you believe, that it is nitpicking, then you have not seen the related details yet..
The difference is not only in the "quality" of the pixels (noise). You probably have not heard of the fact, that the clipping levels of the 40D depend on the ISO.
The rounded values are:
ISO Clipping
---- --------
100 13820
125 16383
160 12740
200 16220
250 16383
320 12740
400 16220
500 16383
640 12740
800 16220
1000 16382
1250 12740
1600 16220
3200 16383
Note, that the 1/3 stop ISOs are not native, they are the result of a multiplication of the 1/3 lower ISO respectively a divison of the values from the 1/3 higher ISO.
Now, look at ISO 100 and 200: if you are using ISO 100, you get almost 20% less levels than with ISO 200. (ISO 100 uses only 13.6 bits from the 14.)
I don't have enough data from the 1DMkIII; I see only, that the green clips at 15300 with ISO 100.
You may see this difference as irrelevant, I don't. Of particular interest is, that ACR does not know the camera good enough, it thinks that everything over 13600 is clipped, independently of the ISO.
(Note, that the above fact makes all reviews relating the DR invalid, which use ACR for this purpose.)
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Addendum:
this weakness of ACR not to recognize the true clipping point has another side: it is impossible to create a shot with ISO 160, 320, 640 and 1250, which would indicate raw clipping in ACR (note the difference between raw clipping and the RGB clipping indication).
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The confusion is in the language. I notice that John Sheehy has amended his explanations a couple of times.
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There are a lot of slippery slopes talking about this stuff. There are a lot of issues of context, especially as far as absolutes and relatives are concerned. The policy I try to stick to is to use relative descriptions as default, and talk about absolutes with qualification. For example, in DPReview, I made a comment about something having less shot noise, and someone (who also happens to be on this board) replied that lower shot noise is not a good thing. What he meant is that a lower absolute, linear amount of shot noise means that less photons are collected, but in general writing, I think we are more concerned with S:N than absolute noise, so the situation that in one context has "more absolute shot noise", has lower shot noise in the sense that we can't see it as well.
For HTP, the camera seems to meter exactly the same if you have HTP on or off; you get the same Av and Tv values in the same situation. What is different, is that even though you have the camera set to a specific ISO setting, *internally*, the camera hardware is processing the capture as if were 1/2 the stated ISO; the gain the camera uses at ISO 200 with HTP is exactly the same gain as it uses with ISO 100 and no HTP. If you shot with the same manual exposure for two images, one with ISO 200 and HTP, and the other with ISO 100 and no HTP, the RAW DR, noise, IQ, etc, are all the same. They are basically the same RAW capture (as similar as any two ISO 100s or any two ISO 200s w/HTP).
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Someone said:
...HTP results in one stop less of exposure at the time of recording the exposure.
And Ray answered:
It doesn't. Exposure is unaltered. Only ISO setting is altered.
I think the discusion about this would come to an end if you just define what exposure is.
- If we call exposure to the optical exposure that photocaptors receive and that still has to be amplified by ISO gain, then exposure doesn't change with ISO and therefore HTP doesn't affect exposure.
- If you refer as exposure to the electronic exposure with which the ADC is fed, then exposure changes with ISO so the HTP mode would reduce exposure.
You are both right, it's simply about language. But I think you already know.
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You may see this difference as irrelevant, I don't. Of particular interest is, that ACR does not know the camera good enough, it thinks that everything over 13600 is clipped, independently of the ISO.
(Note, that the above fact makes all reviews relating the DR invalid, which use ACR for this purpose.)
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It's a real shame that converters don't have the capability to learn individual cameras with a series of test shots. Bad pixels could be declared worthless, or their errant sensitivities taken into account. Sensor non-linearities near saturatio could be relinearized; permanent dust/scratch shadows on the sensor could be corrected; real clipping points could be learned for different rows and columns of pixels; different amplifications of rows or columns of pixels could be noted and corrected, etc, etc.
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Panopeeper,
You (Panopeeper) are over-complicating this very simple issue (to use your own words). This thread is about HTP and your last posts are not relevant to it, being nothing else but petulant nitpicking and hairsplitting over definitions and language usage.
As they say about pornography: hard to define, but you will know it when you see it. The same with underexposure: any photographer but absolute beginners understands perfectly well what underexposure means, regardless of how you define "exposure".
One stop lower exposure and one stop lower ISO are not the same
Regardless of the accuracy of the above statement and regardless of specific qualities of each ISO step (in terms of noise and dynamic range), one stop lower ISO is always going to result in underexposure (all other factors being equal).... and that fact (underexposure) is all it matters for the subject of HTP.
I also see you are hell-bent on teaching Thomas Knoll a thing or two about the language of photography... good luck with it... something like lecturing the Pope on the proper language of his prayers.
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(emphasis by me)
Not so. One stop lower exposure and one stop lower ISO are not the same. If you believe, that it is nitpicking, then you have not seen the related details yet..
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Well, let this be an exercise in interpretation.
Following is your emphasis on my quote:
(1) Activating HTP does not change the actual and real exposure (shutter speed). It just drops the ISO setting down a stop. In relation to the 'hidden' lower ISO setting that the camera has surreptitiously used, the shot now has the equivalent of one stop less exposure.
and below is a different emphasis by me on my own quote.
(1) Activating HTP does not change the actual and real exposure (shutter speed). It just drops the ISO setting down a stop. In relation to the 'hidden' lower ISO setting that the camera has surreptitiously used, the shot now has the equivalent of one stop less exposure.
John Sheehy has now suggested a way we might clarify this issue further with his 'shot noise' analogy which is quite interesting. The statement that high shot noise is a good thing to have, and low shot noise is something to be avoided, at first seems the wrong way round. All noise should be avoided as much as possible.
However, in an absolute sense the statement is quite true. Because shot noise is inextricably a fundamental property of the behaviour of light and varies with the square root of the number of photons arriving at the sensor, low shot noise (in terms of photon count) is indicative of a low signal level and high shot noise is indicative of a high signal level.
For a good quality image, it's always better to have a higher signal level than a lower signal level (within reason of course...no need to point out that overexposure does not necessarily result in a good image ).
With this in mind perhaps we could distinguish between two types of exposure.
(1) Absolute exposure defined by the total number of photons arriving at the sensor.
(2) Relative exposure defined by the camera's processing of absolute exposure in relation to its ISO parameters.
Does this shed light on the matter?
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Sorry for jumping in so late, but there's one point in the ISO peeping that seems to have been missed:
ISO Clipping
---- --------
100 13820
125 16383
160 12740
200 16220
250 16383
320 12740
(...)
Note, that the 1/3 stop ISOs are not native, they are the result of a multiplication of the 1/3 lower ISO respectively a divison of the values from the 1/3 higher ISO.
You seem to have ignored the divergent clipping point for 125 ISO. This seems to indicate that there isn't simply a multiplication going on, but something else.
In every other case (250, 500, 1000 etc.), the difference between clipping point from the full stop is merely 1%, but for 100 -> 125 ISO, the difference is 18.5%.
I'm not sure whether this discrepancy is significant.
But I do hope that you've reported the ACR bug to Adobe.
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You seem to have ignored the divergent clipping point for 125 ISO
I have not ignored it. It's easy to analyze the images with my raw analyzer, but it is quite laborous to create a demonstration with proofs (which requires lots of co-ordinated screen shots). As the in-between ISOs are interesting only for JPEGs, I did not find it worthy to work a lot with that. To boot it, the ISO100-125 issue is particularly messy.
This seems to indicate that there isn't simply a multiplication going on, but something else.
In every other case (250, 500, 1000 etc.), the difference between clipping point from the full stop is merely 1%, but for 100 -> 125 ISO, the difference is 18.5%.
I'm not sure whether this discrepancy is significant
Sure, the ISO 100 - ISO 125 relationship is different from the others.
1. Please note, that one has to substract the black level from all values (it is a constant additive) before calculating any ratios. Its average is or close to 1024.
2. The ratio between the ranges of ISO200 and ISO160 close to 1.3. The ratio between the ranges of ISO125 and ISO100 is 1.2.
Both ratios are quite far from 1/3 stop, which is 1.26 . Accordingly, these ratios represent no important aspect.
3. The histograms of an ISO200 and ISO160 shots are virtually equal. Note, that the histograms are relative to the actual range of pixel values, though not exactly, for example where the value range is 16220, the histogram range is 16384.
The ISO 160 values are derived from the ISO 200 values by simply a division, it appears linear. The histograms of the mapped values are virually identical (they are useful, because the black level correction and the value range are considered in those).
4. The comparable histograms of ISO100 and ISO125 shots don't behave so nicely; the ISO 125 is more stretched. Detailed analysis indicates, that the ISO 125 is not a truely linear transformation of the ISO 100 values.
5. There is another clear indication, that the +1/3 step ISOs are derived from the full-stop ISOs: there are "holes" in the ISO 125 values: red and blue do not occur with all possible values (but their holes are not together). The following seuence represents consecutive pixel values:
RGB, RGB, GB, RG, RGB, RGB, RGB, GB, RG, RGB, RGB, GB, RG, ...
There are no holes in green; I suspect, that the two different greens would show holes, but I don't care enough to go after that.
A layered TIFF can be donwloaded with the screenshots of histograms of eight shots, pairwise fromHERE (http://www.panopeeper.com/Demo/ISO100-125-160-200_histograms.tif)
But I do hope that you've reported the ACR bug to Adobe.
Right. I posted it on the respective Adobe forum, but no-one cared to answer it.
The problem of misrepresenting the clipping point is not new, it occurs with several cameras. However, now with the 14-bit raw files and huge pixel value ranges this is more of a problem.
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Sorry for jumping in so late, but there's one point in the ISO peeping that seems to have been missed:
You seem to have ignored the divergent clipping point for 125 ISO. This seems to indicate that there isn't simply a multiplication going on, but something else.
In every other case (250, 500, 1000 etc.), the difference between clipping point from the full stop is merely 1%, but for 100 -> 125 ISO, the difference is 18.5%.
I'm not sure whether this discrepancy is significant.
But I do hope that you've reported the ACR bug to Adobe.
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100 clips lower than you'd expect with the remaining pattern, probably because the sensor goes into saturation, or at least non-linearity, before it would reach the clipping point of 200, 400, etc. The 20D had the same problem, but the 20D went up to 4095 in the RAW data (3967, after black point subtraction) at every ISO, and for 100, the upper highlights were somehow stretched to make them reach 4095. You'll see figures (such as Roger Clark's) that suggest that the 20D sensor wells collect 51K photons, but the highest that gets digitized in the RAW data at ISO 100 is about 44K.
Why they just couldn't start the camera at ISO 115, or at 125 to keep things somewhat standard, is beyond me. This obsession with 100, 200, etc does nothing but compromise cameras. Perhaps it is marketing; people might think that not having ISO 100 is a bad thing (the fact is, an ideal digital camera with current well depth technology would have a minimum ISO of about 250).
Anyway, back to the 40D, the clipping points can vary from camera to camera, and can vary throughout the image based on pixel row and column, because, apparently, some offset correction is made based on the blackpoint by line and/or column after the data is clipped in th e highlights. I tried to chart the relevant info about the 40D ISOs based on someone's RAW samples, and at first I tried to use the lowest clipping point in the file, but that was sometimes the value of a single line that waqs far below the mean and median, so I redid it with the mdian clipping value, instead. Here are the details ("adjusted" refers to subtracting the blackpoint of 1024 from all of the values):
(http://www.pbase.com/jps_photo/image/87419422/original.jpg)
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im trying to figure out what you are doing here
i still dont get it how you can evaluate things like well-depth without using some sort of calibrated light sorce to fill the sensor with a specific rate of photons ...
And John: if i understand right, you say it would make more sense to put the sensors low/base-sensitivity to a value where its filled to the max. ok, but fill it to the max at what light-levels and exposure-time? to me that variable needs to be defined as well, so what is the point im missing here?
thanks
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I have not ignored it.
Okay, but you didn't mention the divergence in your post, so it did seem that way.
Thanks for the clarification, though; it helps!
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100 clips lower than you'd expect with the remaining pattern, probably because the sensor goes into saturation, or at least non-linearity, before it would reach the clipping point of 200, 400, etc. The 20D had the same problem, but the 20D went up to 4095 in the RAW data (3967, after black point subtraction) at every ISO, and for 100, the upper highlights were somehow stretched to make them reach 4095. You'll see figures (such as Roger Clark's) that suggest that the 20D sensor wells collect 51K photons, but the highest that gets digitized in the RAW data at ISO 100 is about 44K.
Why they just couldn't start the camera at ISO 115, or at 125 to keep things somewhat standard, is beyond me. This obsession with 100, 200, etc does nothing but compromise cameras. Perhaps it is marketing; people might think that not having ISO 100 is a bad thing (the fact is, an ideal digital camera with current well depth technology would have a minimum ISO of about 250).
This is quite interesting, since the 20D's "100 ISO" setting is really equivalent to 125, while the 40D's "100 ISO" is at least very close to 100.
It would appear then, that the 20D ought to have started at 160 and the 40D at 125.
What you're pointing at may explain why Nikon usually only offers standard sensitivities from 200 ISO and upward, with 100 ISO as an extension.
Anyway, back to the 40D, the clipping points can vary from camera to camera, and can vary throughout the image based on pixel row and column, because, apparently, some offset correction is made based on the blackpoint by line and/or column after the data is clipped in th e highlights. I tried to chart the relevant info about the 40D ISOs based on someone's RAW samples, and at first I tried to use the lowest clipping point in the file, but that was sometimes the value of a single line that waqs far below the mean and median, so I redid it with the mdian clipping value, instead. Here are the details ("adjusted" refers to subtracting the blackpoint of 1024 from all of the values):
Unless I'm misreading the numbers, using 160 instead of 200, 320 instead of 400, 640 instead of 800 and 1250 instead of 1600 may be quite acceptable in terms of S/N and DR. Fascinating.
It would be interesting to see how this compares to the 1D(s) MkII, MkIII, 5D and 30D.
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This is quite interesting, since the 20D's "100 ISO" setting is really equivalent to 125, while the 40D's "100 ISO" is at least very close to 100.
It would appear then, that the 20D ought to have started at 160 and the 40D at 125.
Well, the quantum efficiencies of the two cameras are very close, much closer than that, so there must be some other factor.
What you're pointing at may explain why Nikon usually only offers standard sensitivities from 200 ISO and upward, with 100 ISO as an extension.
It might explain the D3, but there are also Nikons with relatively low quantum efficiency that start at 200, so those might have a small fill factor but a large microlense.
Unless I'm misreading the numbers, using 160 instead of 200, 320 instead of 400, 640 instead of 800 and 1250 instead of 1600 may be quite acceptable in terms of S/N and DR. Fascinating.
It would be interesting to see how this compares to the 1D(s) MkII, MkIII, 5D and 30D.
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The 1D cameras and the 5D use an analog amplifier, so all their ISOs go into the ADC proportional to the ISO, and they don't have the clipping variations of the 30D and 40D (but they are always amplified from the lower ISO, so the 160/320 group have the peak read noises).
The 30D is similar to the 40D in approach, but the 160 group have a DR advantage over the 100 group a little bit greater than with the 40D.
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im trying to figure out what you are doing here
i still dont get it how you can evaluate things like well-depth without using some sort of calibrated light sorce to fill the sensor with a specific rate of photons ...
The number of photons at full-well is not directly related to exposure. Depending on the camera, only a certian percentage of photons can be captured, and others reflected or turned to heat.
And John: if i understand right, you say it would make more sense to put the sensors low/base-sensitivity to a value where its filled to the max. ok, but fill it to the max at what light-levels and exposure-time? to me that variable needs to be defined as well, so what is the point im missing here?
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That's a characteristic of the sensor. Of course it needs to be calculated. The point is, the camera should have a low ISO that combines maximum photon capture with minimum read noise relative to max photons. Many cameras fail to bring these two qualities together to do this.
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Although I think that reading the article is clear that HTP is nothing more than shifting ISO down by 1 f-stop and shooting at the same shutter/aperture, I have a real sample: 2 shots with camera settings:
- Up: ISO200+HTP, f8 1/40s
- Down: ISO100 no HTP, f8 1/40s
Both images have been developed with neutral and identical parameters using DCRAW, and the final curve applied was also the same for both images.
This was the scene: same appearance
(http://img165.imageshack.us/img165/6030/comp1as3.jpg)
These are demosaiced but non-WB histograms: both display same exposure
(http://img165.imageshack.us/img165/5989/histsqj3.gif)
These are 100% crops: twins (except obviously for the noise distribution)
(http://img411.imageshack.us/img411/4780/comp2ih8.jpg)
The usefulness of HTP is only in JPEG, where the camera will ensure a correct exposure in the mid tones (not underexposed because of the ISO reduction) but prserving the highlights. Nothing that could not be achieved shooting RAW with 1 f-stop ISO down and same shutter/aperture params and a proper post processing.