Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: Panopeeper on January 17, 2009, 10:48:31 pm
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After having received enough raw files to analyze this issue thoroughly, I can make some statements regarding the usability of ISO steps with the 5D2.
1. When shooting JPEG, using the 1/3 ISOs and 6400, 12800 and 25600 is sometimes justified (if the exposure can not be adjusted as necessary).
However, using ISO 50 is not justifiable.
2. when working with the raw data:
a. ISO 6400, 12800 and 25600 are wasting of the dynamic range by a full stop per ISO stop.
b. ISO 50 is identical to ISO 100, except for the metering. This means: if you meter for ISO 50, then the result is the same as metering for ISO 100 with +1 EV bias.
c. ISO 125, 250, 500, 1000, 2000 and 4000: these are numerical derivatives of the next lower stop. They cut off 1/3 stop from the highlights without enhancing the shadows, i.e. the DR is reduced by 1/3 EV; there is no reason to use these.
d. ISO 160, 320, 640, 1250, 2500, 5000: these are numerical derivatives from the next higher ISO. They do not reduce the DR; they do reduce the number of levels, but the leftover (about 11800 levels) is more than enough. However, when someone switches to 1/3 ISO steps, the "bad" steps are in the way.
Now, to ACR: ISO 160, 320, 640, 1250, 2500, 5000 are even more mistreated by ACR, than the other ISOs: ACR does not recognize the reduced numerical pixel value range, thus it does not notice pixel saturation. The consequence is, that clipped areas turn out with wrong colors. For example if the greens are saturated in the sky, ACR converts that in magenta.
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Thanks for posting your analysis. I had hoped Canon would use a more sensible implementation of "tweener" ISO settings (compared to previous non-1D cameras), but I'm not surprised.
Unfortunately, 1/3 ISO is one of the settings I have to enable for movie mode on my 5d2 in order to get more reasonable "control" (if you can call it that) over the exposure.
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Do you have an analysis of the original 5D, would be much appreciated especially re ACR.
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Do you have an analysis of the original 5D, would be much appreciated especially re ACR.
1. I have only one raw file with 1/3 ISO, at 1250; that appears to be genuine, i.e. not numerically derived.
2. ISO 3200 is fake with the 5D.
3. I can not determine with the available raw files how ISO 50 works. I need a pair of shot of anything fixed scenery with constant illumination, one shot @ 50, one @ 100, either with the identical exposure or ISO 50 with 1 stop higher exposed.
4. I don't think the same problem with ACR's interpretation occurs with the 5D.
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Gabor, unfortunately the reason for this discrepancy is that the original unit that we received from Canon (obviously a preproduction model) had identical white points at all ISOs. We had no way of knowing at the time that the final release would be different in this regard.
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the original unit that we received from Canon (obviously a preproduction model) had identical white points at all ISOs
This is extremely strange. I am not doubting what you are saying, but I would like to see such a raw file; I wonder, how for example ISO 320 has been realized at that time. The ISO 320 pixel values are simply the ISO 400 values, multiplied by 0.8 (after black level correction, and recorrected afterwards). Thus is is natural, that the saturation level goes down, just like with the 40D, 50D, etc.
(http://www.panopeeper.com/Demo/Canon5DMkII_ISO320_fineHist.GIF)
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is there any practical reason for using "in-between" ISOs (or Canon supplying them in the first place)?
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is there any practical reason for using "in-between" ISOs (or Canon supplying them in the first place)?
For JPEG shooters it allows you to get exposure *just* right for a given aperture/shutter and picture profile. For raw shooters, if the "tweener" ISO worked like normal ones, it would allow you to do the ITTR (ISO to the right) with more precision. For movie shooters, it gives you a slight advantage in "manual control" over the exposure. There are probably more reasons.
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For JPEG shooters it allows you to get exposure *just* right for a given aperture/shutter and picture profile. For raw shooters, if the "tweener" ISO worked like normal ones, it would allow you to do the ITTR (ISO to the right) with more precision. For movie shooters, it gives you a slight advantage in "manual control" over the exposure. There are probably more reasons.
it's not possible to control select ISO, shutter speed or aperture in movie mode.
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it's not possible to control select ISO, shutter speed or aperture in movie mode.
Exactly. The trick, obviously, is to cover the lens and slowly uncover it and watch as the camera chooses values and try to lock exposure at *just* the right time. After maxing all three values, the first it will change is 1/f shutter. Then ISO. With 1/3-stop ISO increments, it's easier to watch it slowly decrease so you can lock exposure at the right ISO. Otherwise the increments are too fast and crude to time correctly. (Especially since the procedure must be repeated for every shot.)
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I made several C1 ICC profiles for the original 5D, and these produced good color on the units I tested, but never worked for customers. I always assumed that it was a batch issue with respect to the sensor, but now I wonder whether it wan't an ISO and conversion effect.
Edmund
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Now my 5D2 has come, and I tried to find out how to expose.
Hardware was a Zeiss Contax Sonnar 85, used via a CameraQuest adapter; 2 Solux® lamps, the ColorChecker card, a Kodak and a Wand gray card.
In-camera settings were "everything neutral". ACR settings were Brightness +50, Contrast +25, Tone Curve Moderate Contrast, everything else zero.
The in-camera lightmeter shows 2/3 EVs lower than the Asahi Pentax spotmeter, i.e. when the Asahi shows EV 9 ~ f/11, 1/8 second, the camera shows 1/5 second. (200 ISO) This is also the value that places the in-camera histogram in the middle of its window.
When opened in Rawnalyze, the DNGs, but not the CR2s, show "+1/3" left of the lightness slider. The Rawnalyze histograms of a gray card however look exactly the same for both CR2s and DNGs.
On the 1/3 f-stop level, there is virtually no difference between the highlight warnings in aRGB and sRGB. It looks like the warning is triggered as soon as the rightmost part of the histogram touches the -1/3 EV mark (in the Rawnalyze histogram). So one can use the first highlight warning as ETTR exposure; no need to go back 1/3 EV.
When I opened the DNG image of the Kodak gray card in ACR (CS3), the peak of the histogram was exactly in the middle of the histogram window, if I chose Adobe Standard as the camera profile. The Camera Neutral profile moved the top 0,2 EVs to the left.
I tried to find out how these different exposures related to the density of the CC 4/4 patch. Once upon a time, there was a densitometer on the Mac. For now, I tried the Color Meter utility, and the color sampler pipette in ACR. Results were different from each other, and different for 2 camera profiles, Adobe Standard and Camera Neutral. (Camera Neutral was used when shooting).
The correct RGB values for ProPhoto according to BabelColor are
101 or 102/102/102. Here are mine:
--these were faulty. New values in post #19. Hening.
So...the only finding that I can immediately use is the highlight warning. The rest is a mess to me. In particular: How does the +1/3 come in, which is shown by the Rawnalyze lightness slider? In which direction does it go? How does ACR handle this? ***How can I achieve an exposure that renders D 0.7 as D 0.7 ?***
Jeez it's certainly not easy to photograph if you have to reverse engineer everything you touch before you can use it...
And why are the RGB values so way off? I am aware of that they are for 8 bit, while I am working in 16 bit. But that could not change the overall density that much? A single set I compared made no difference. And what are the differences between the Mac Color Meter and the ACR pipette about?
Sigh...
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I became confused at post #5: unsure whether it questions the original post, or the analysis of the original 5D.
Am I right in assuming the conclusions for the 5D MkII are to avoid 6400 if possible, and to set the ISO increment to 1-stop to avoid the bad 1/3 stops (or memorize the bad stops in your list c. and avoid them)?
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Some answers.
When opened in Rawnalyze, the DNGs, but not the CR2s, show "+1/3" left of the lightness slider. The Rawnalyze histograms of a gray card however look exactly the same for both CR2s and DNGs.
Rawnalyze blindly shows the exposure adjustment dictated by the creator of the raw file, in this case by the Adobe DNG converter. This adjustment usually reflects the difference between the camera maker's and Adobe's interpretation of the ISO standard.
The raw histogram displayed by Rawnalyze is absolute; it reflects the unadjusted raw pixel values. Only the scale underneath depends on the top saturation level, which is fixed for the camera and the ISO (like 16383).
NOTE: the mapped histogram is only for educational purpose, i.e. for the demonstration of what is happening there, particularly when discussing ETTR. Neither demosaicing, nor color space conversion (from the camera's color space to whatever) took place before calculating that histogram. Accordingly, the effects of the contrast and saturation adjustments are not reflected in the mapped histogram.
So...the only finding that I can immediately use is the highlight warning
You mean the clipping warning, right? That's how it should be.
The entire concept of metering and of headroom in highlights and footroom in shadows are the worthless leftovers of the film era, for the reassurance of dynosaurs. There is no such thing as headroom and footroom with raw processing. The raw data is absolute, and you can put the head and foot wherever you want to. This is particularly relevant with ETTR: you are striving to eliminate the headroom in order to lift out the shadows from the noisy region.
How does the +1/3 come in, which is shown by the Rawnalyze lightness slider? In which direction does it go? How does ACR handle this?
The 1/3 reflects ACR's rendering. +1/3 indicates, that ACR will apply a +1/3 EV adjustment to the "exposure" slider (but it is not shown on the slider!!).
***How can I achieve an exposure that renders D 0.7 as D 0.7 ?***
I know it is not nice to say so, but the question itself is wrong. The goal of raw conversion is not to render D 0.7 as D 0.7 (if you are expecting that, then go with JPEG). Initially, it should show how the shot is on its own, i.e. without faking exposure, ISO, etc. From that point, you should decide what to do. Adobe's misunderstanding of ACR's role is becoming apparent here: they think they are supposed to "correct" the shot automatically, instead of helping you in processing it
If you underexposed the shot, then it is underexposed. Basta. If you have blown the highlights, then they are blown. Basta.
The raw processor is not supposed to make an experienced photographer from someone, who does not understand his DSLR. P&S cameras are supposed to do that.
And why are the RGB values so way off?
Pls describe it better, what you mean; this is an important issue.
Sigh...
The questions indicate, that this is not so bad.
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I became confused at post #5: unsure whether it questions the original post, or the analysis of the original 5D
Neither, nor. Eric gave an explanation for the discrepance.
Am I right in assuming the conclusions for the 5D MkII are to avoid 6400 if possible
To avoid generally. There is no reason to use it when working with raw: you can achieve the very same in PP without reducing the DR. ISO 6400 is for JPEG only.
and to set the ISO increment to 1-stop to avoid the bad 1/3 stops (or memorize the bad stops in your list c. and avoid them)?
No need to memorize. Half of the 1/3 EV stops are bad, the other half are not different from the full stop. Thus, there is no need to use any.
You did not mention ISO 50: drop it from the menu.
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Panopeeper,
The guys here are photographers, not scientists. So, exposure to them is A PRIORI exposure, meaning a decision which is taken on the ground prior to making a shot, not A POSTERIORI terms which you as a scientist use to analyze the resultant pixel map.
In this context your remarks are misleading the poor pragmatic men and women in this forum who are trying to earn a living with their cameras. Exposure for them is something to be done with respect to an expected reflectance in a given lighting, with precautions for headroom for speculars (forehead, nose), tableware or in-image lights (lamps in architecture), and acceptable shadows.
Exposure is a term of the art that is very much alive. As a rule of thumb, I'd recommend one stop headroom over the white of a Colorchecker.
Ronald
Some answers.
You mean the clipping warning, right? That's how it should be.
The entire concept of metering and of headroom in highlights and footroom in shadows are the worthless leftovers of the film era, for the reassurance of dynosaurs. There is no such thing as headroom and footroom with raw processing. The raw data is absolute, and you can put the head and foot wherever you want to. This is particularly relevant with ETTR: you are striving to eliminate the headroom in order to lift out the shadows from the noisy region.
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Exposure is a term of the art that is very much alive.
Agreed. The two most significant artistic choices in exposure are:
1. What portion of the dynamic range of the scene to capture.
2. How to display that dynamic range in a print.
#1 is deciding, for example, that everything outside the window will blow out in order to have enough usable range for the indoors.
#2 is the gamma, tone curve, and contrast adjustments made in post processing (or in-camera JPEG).
Maybe in #1 you decided to underexpose to capture some of the detail outside the window. Then in #2, use a film-like tone curve so the highlights blow out gradually and gracefully.
After and between the artistic choices there is a lot of room for technical excellence. After choosing how much dynamic range to capture, for example, a technical mistake could lead to capturing less than that, or the wrong part of the range. Or, a technical mistake in the camera settings or raw converter could cause the image to not produce the desired effect.
The way I see it, the art happens in the mind; technical excellence is just trying to get that from the mind into real life.
The guys here are photographers, not scientists. So, exposure to them is A PRIORI exposure, meaning a decision which is taken on the ground prior to making a shot, not A POSTERIORI terms which you as a scientist use to analyze the resultant pixel map.
In this context your remarks are misleading the poor pragmatic men and women in this forum who are trying to earn a living with their cameras. Exposure for them is something to be done with respect to an expected reflectance in a given lighting, with precautions for headroom for speculars (forehead, nose), tableware or in-image lights (lamps in architecture), and acceptable shadows.
In film, #1 and #2 above were always commingled, due to the nonlinear response. But now that response is linear and we can make more choices about #2 after the fact, I think it's beneficial to understand the full range of possibilities.
You can get great results with a simpler system (e.g. your white+1 stop is a great one), but with the time and inclination to learn more complicated possibilities (boring technical stuff), one is able to photograph with greater precision (less mistakes) and more ambition (subjects that were too heretofore too difficult to render in the way you desired).
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You did not mention ISO 50: drop it from the menu.
I never turned the ISO expansion on, so not an issue for me. On the 6400, that may explain why when using Auto ISO, it only runs up to 3200. I have set the camera to full stops to avoid the tweener ISOs.
Re: Art vs. Science comments, it is useful to know how something impacts your art, and if in-between ISO settings reduce the dynamic range, I want to know that.
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Hi Gabor. Thank you very much for your detailed reply. -
Yes, by "higlight warning" I meant "highlight clipping warning". I agree that it is like it should be. I just thought it might also have been so that when the warning starts, highlights *are* already clipped. In this case, I would have needed to "step back" 1/3 EV from the 1st warning. So I found it useful to see in Rawnalyze what actually is going on.
>The entire concept of metering and of headroom in highlights and footroom in shadows are the worthless leftovers of the film era, for the reassurance of dynosaurs. There is no such thing as headroom and footroom with raw processing. The raw data is absolute, and you can put the head and foot wherever you want to.
Well if the clipping warning starts at -1/3 EV from saturation, then at this point you have 1/3-x EV of "headroom" left. This is just semantics.
>This is particularly relevant with ETTR: you are striving to eliminate the headroom in order to lift out the shadows from the noisy region.
This is exactly my understanding of ETTR.
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>>***How can I achieve an exposure that renders D 0.7 as D 0.7 ?***
>I know it is not nice to say so, but the question itself is wrong.
Well if the question is wrong it's wrong and it must be OK to say so, preferably in a nice way, as you did :-) For the remainder of the paragraph, we either misunderstand each other or disagree.
>The goal of raw conversion is not to render D 0.7 as D 0.7 [...]. Initially, it should show how the shot is on its own, i.e. without faking exposure, ISO, etc.
This is exactly my idea. But doesn't this imply that if I measure and shoot a gray card, the image should have D 0.7 after all *standard* processing is done? (Presupposed my light meter is correct) Isn't this exactly "showing the shot on its own"? Well there could be other sources of error, the sensor itself,... and I agree that the raw conversion should not try to cover up anything. But what I asked for was an *exposure* with a predictable result, not for a raw processing that faked anything. What I actually meant was an overall *standard* procedure, of which exposure is the first step, and the raw conversion the next.
>From that point, you should decide what to do. [...]
Yes. And in order to have a guideline for the subsequent processing, I make what I call my reference shot. In this shot, I blow on highlights and shadows ;-) It's based on the gray card (in the future to be replaced by my palm, which turns out to have the same reflectance), and its only purpose is to aid my memory in remembering how dark/light the scene actually was; in technical terms: show correct mid-tones; which ETTR does not. In this sense, I do not think that metering is obsoleted by our being able to see the histogram at shooting time. (In case your phrasing "The entire concept of *metering* and of headroom in highlights and footroom in shadows are the worthless leftovers of the film era" meant something like that).
My above color values are wrong, I'll try to delete them from the post. I made new ones, which show a somewhat more understandable pattern, yet leave some questions. DNGs opened in ACR with my standard settings as noted above.
RGB values for CC patch 4/4 in ProPhoto. Correct values according to BabelColor:
101 or 102/102/102
[attachment=11018:RGB_valu...ProPhoto.tiff]
So the answer to my question seems to be: middle gray is rendered as middle gray when using external light metering (or adding 2/3 EV to the camera metering) and the Adobe Standard camera profile - if the ACR pipette is correct. This is with ACRs 1/3 EV added. Question left: What causes the difference between the Mac Color Meter and the ACR pipette?
Next step, I will make my own DNG profile and figure out how I have to expose with that to achieve correct tone values.
Getting closer, bye and bye... :-)
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Hening,
When using Apple's DigitalColor Meter, you need to set it to Lab readouts instead of RGB which derives those numbers NOT from the color space of the file within the imaging software but from how the monitor profile is used to adjust the preview in a color managed system. You'ld have to convert to your monitor profile within Photoshop to get the same readouts as the ColorMeter. This of course will not work within ACR and Lightroom.
If you know the Lab measurements of your CC chart, you can trust the ColorMeter within ACR and Lightroom. I've used this procedure tweaking within ACR my own CC chart to calibrate my camera getting very accurate results once opened in Photoshop which will show the same Lab numbers as the meter.
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[quote name='tlooknbill' date='Jan 21 2009, 04:24 AM' post='253509']
Thank you for this very useful tip!
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A new discovery concerning light metering on the 5D2:
At EV 4, I found a gross inconsistency with the Asahi, whose output was confirmed by the Nikon D200 and the Metered Light Pocket Spot meter. So I set up a little experiment: From the starting point at EV 4 at available daylight, I illuminated the gray card with 2 Solux lamps, whose voltage I regulated. Metering method on the 5D2 was "partial", = 8% in the center of the image.
[attachment=11076:Asahi_vs...ering__1.jpg]
When I looked at the results, I found them so incredible that I took a second run:
[attachment=11077:Asahi_vs...ering__2.jpg]
This shows at least a more transparent pattern.
The camera had been left in the "studio", but switched off, while I looked at the results of the 1st take, and maybe it had acclimatized. The temperature in the "studio" is about +15° Celsius, vs. ca. 22° in my living room. (In the meantime, daylight had gone down so much that I needed the Solux lamps all the way.)
Conclusion: Apart from the 2/3 EV offsett that results from the ISO cheating, this gap increases to 1/1 EV below EV 6 (5 and 4).
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The conversion to DNG adds 1/3 EV to Canon raws, according to the indikation in Rawnalyze. However, if I expose a ColorChecker according to the Asahi spot meter and open the DNG in ACR, the row 4/column 4 patch shows RGB readings of 98/98/98, which is close to the correct value for Pro Photo (101/102). How come?? (Exposure 0, Brightness +50, Contrast +25, medium Tone Curve).
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The guys here are photographers, not scientists. So, exposure to them is A PRIORI exposure, meaning a decision which is taken on the ground prior to making a shot, not A POSTERIORI terms which you as a scientist use to analyze the resultant pixel map.
In this context your remarks are misleading the poor pragmatic men and women in this forum who are trying to earn a living with their cameras. Exposure for them is something to be done with respect to an expected reflectance in a given lighting, with precautions for headroom for speculars (forehead, nose), tableware or in-image lights (lamps in architecture), and acceptable shadows.
Exposure is a term of the art that is very much alive. As a rule of thumb, I'd recommend one stop headroom over the white of a Colorchecker.
I wonder why a photographer could not be interested in knowing what's going on with the different ISO settings allowed by his camera. If you are a RAW shooter, Gabor's findings are very valuable and help maximise the quality of results of any photographer (i.e. a person who takes photographs).
One stop over the white of a Colourchecker as a general rule can easily be non-optimum, leading to loose DR in the shadows or in the highlights. I, as arquitecture and interiors photographer, would check my histogram instead, and make sure I got it exposed to the right. And would do additional overexposed shots to improve DR in the shadows if needed in medium or high DR situations.
BR
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The conversion to DNG adds 1/3 EV to Canon raws, according to the indikation in Rawnalyze. However, if I expose a ColorChecker according to the Asahi spot meter and open the DNG in ACR, the row 4/column 4 patch shows RGB readings of 98/98/98, which is close to the correct value for Pro Photo (101/102). How come??
ACR applies the adjustment as "exposure correction", but it does not appear on the slider. This is the worse aspect of it: the vast majority of users don't know about the automatic adjustment and are misled. For example if the adjustment is positive, the users will tend to expose lower than ideal.
ADDED:
if you want to see this demoed, pls upload a raw file from your brand new 5D2.
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The guys here are photographers, not scientists. So, exposure to them is A PRIORI exposure, meaning a decision which is taken on the ground prior to making a shot, not A POSTERIORI terms which you as a scientist use to analyze the resultant pixel map
Edmund, the subject of this thread is a camera of the price $2700. You are talking about cameras, which cost ten times more.
1. The 5D2 shows an RGB histogram, which can be turned into a more or less raw channel histogram.
2. The 5D2 can make three exposure bracketed shots within a second, even without flapping the mirror in between. One can review the clipping indications and the histograms immediately and fire another triplet (it is the biggest idiocy from Canon to limit the bracketing so primitively. Why on earth are Japs allowed in function design, when they have no idea of photograpy?)
3. You can afford to be less vigilant with a P45+ because of the large DR than with a 5DMkII (or a lesser one). If I am sloppy with my 40D, then I burn the clouds or push the dark side of the mountain in the night.
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Hi Gabor
thank you for your answer.
How can I upload a raw? The max. single upload size is 2 MB.
>ACR applies the adjustment as "exposure correction", but it does not appear on the slider.
Yes that's how I understood it. But what I still do not understand is that this does not show in the density of the CC middle gray patch.
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How can I upload a raw? The max. single upload size is 2 MB
Use yousendit.com, it is still the best and less circumstancial of these services:
- you don't need any registration up to 25MB per file. You don't even have to use your own email address or the recipient's address, you can use fictional ones. After uploading you receive a URL and if you post that, everyone can download it 7 days long. Of course you can send it to my address directly.
- you need a free registration up to 100 MB per file
Anyway, if you want to measure the intensity ("density"), you must not apply brightness, nor contrast, nor saturation, and use "linear" instead of the curve.
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Gabor, I have no quarrel with any of your experimental findings, as you know very well. However, as always we have differences over the way such findings should be internalized to make *pratical* photographics decisions *when imaging real scenes*. I am warning people who read your charts tht they had better get their heads around their meaning before they start doing dumb things like ETTR in real scenes. Burning the cloud detail or losing nose skin is a typical price you'll pay for listening to the ETTR sirens. As to the significance of in-camera histograms, when you relaize they are computed on sRGB previews you also understand their lack of worth!
Edmund
Edmund, the subject of this thread is a camera of the price $2700. You are talking about cameras, which cost ten times more.
1. The 5D2 shows an RGB histogram, which can be turned into a more or less raw channel histogram.
2. The 5D2 can make three exposure bracketed shots within a second, even without flapping the mirror in between. One can review the clipping indications and the histograms immediately and fire another triplet (it is the biggest idiocy from Canon to limit the bracketing so primitively. Why on earth are Japs allowed in function design, when they have no idea of photograpy?)
3. You can afford to be less vigilant with a P45+ because of the large DR than with a 5DMkII (or a lesser one). If I am sloppy with my 40D, then I burn the clouds or push the dark side of the mountain in the night.
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Gabor, I have no quarrel with any of your experimental findings, as you know very well
Edmund, be assured, that I was not reading any imaginary statements from your post :-)
Most of the differences come from the different usage of different cameras. I am shooting exclusively with a total neutral setup, which renders the embedded JPEGs practically worthless except for the recognition of the scenery, for that is helpful with my targets (landscapes and city scapes) and with my camera, which is lacking a few stops of DR (compared to my preference). This may not be the prevalent position on LL, but I am among the vast majority of DSLR owners with such problems.
Re the usefulness of in-camera histograms: the cameras need a good portion of prodding to create a histogram, which resembles the raw histogram despite demosaicing, color space conversion, white balancing and non-linear mapping, but it can be done with high reliability, at least with those cameras we have tried.
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Gabor,
have you looked at the noise characteristics of the different ISO setting and noise?
The tradeoff at long exposures would also be interesting - higher iso vs longer exposure etc?
Cheers
Murray
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Hi Gabor,
a raw file of my ColorChecker is uploaded to Yousendit.
When I take the CC image that was exposed according to the Asahi reading, open the DNG in ACR with everything zero and linear, chose the same camera profile that was used when shooting (Camera neutral), the CC 4/4 patch shows RGB values of 66. When I move the Exposure slider until the densitometer reads 100, the exposure slider has been moved to +1,10.
The Asahi exposure is 2/3 EV lower than the camera meter. So if the DNG conversion adds 1/3 EV, + 0,33 in Exposure should do the trick.
In the same image, exposed according to the camera meter, i.e. 2/3 EVs higher, the middle gray patch has RGB values of 86. Achieving 101 requires Exposure + 0,45. If the DNG converter adds +1/3, it should require -1/3.
So it looks like middle gray is middle gray AFTER application of a particular tone curve. I thought the D 0.7 was the pivoting point for the rest.
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In the meantime, I have found this thread:
"Exposing to the right"
http://luminous-landscape.com/forum/index....24354&st=40 (http://luminous-landscape.com/forum/index.php?showtopic=24354&st=40)
post #45 by Bill Janes
>
With the saturation method, an exposure of an 18% gray card based on a standard light meter reading will result in an image with a grey level of 18%/√2 = 12.7% of saturation, which corresponds to a pixel value of 100 in a gamma 2.2 space. The square root of 2 is to allow 0.5 EV of headroom for specular highlights. Since the standard light meter is calibrated on the basis of the equivalence of 12% reflectance, you have to add 0.5 EV of exposure if you want the gray card image to have a pixel value of 118 in the gamma 2.2 file.
>
This seems to fit quite good with some of my findings of post #19. For the ACR Camera Profile Adobe Standard, I found that the exposure according to the Asahi meter gave RGB values of 98; with +1/3 EV (if not 1/2): 118.
This is after the DNG conversion has applied its +1/3 EV and with 50/25 brightness/contrast and medium tone curve; but it is something like this I will use in the end. And Bill Janes' post refers to a gamma 2.2 color space, not linear data.
This exposure is 1/3 EV below camera metering. The latter however is the one that places the camera histogram in the middle of its window. So this should be the exposure to choose according to Thom in "Meters don't see 18% gray", http://www.bythom.com/graycards.htm (http://www.bythom.com/graycards.htm).
Oh wait: with the camera exposure and without the DNG bonus of 1/3 EV, i.e. with the Exposure slider in ACR set to -0.33, everything would come out right? - Indeed! quite close, 116!
Getting closer, bye and bye...
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a raw file of my ColorChecker is uploaded to Yousendit
In turn, you can download http://www.panopeeper.com/Download/Hening_LL_0153+3EV.dng (http://www.panopeeper.com/Download/Hening_LL_0153+3EV.dng) and http://www.panopeeper.com/Download/Hening_LL_0153-3EV.dng (http://www.panopeeper.com/Download/Hening_LL_0153-3EV.dng), only as a demo that ACR does apply the adjustment w/o telling you a word about it.
Bill Janes' post refers to a gamma 2.2 color space, not linear data
There are two "curves" involved here.
1. the mapping in sRGB or Adobe RGB or ProPhoto RGB. This is the "gamma 2.2" if you select Adobe 98 RGB.
2. the "tone curve", often mentioned as "S curve". This is, what you can set to "linear".
Anyway, IMO all this is unnecessary if you can afford pursuing true ETTR. Expose as high as you can afford in the given setting without raw saturation and adjust it in raw processing. Keep in eye: reducing the intensity in raw processing does not create noise; just the opposite, it reduces the noise.
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have you looked at the noise characteristics of the different ISO setting and noise?
The tradeoff at long exposures would also be interesting - higher iso vs longer exposure etc?
I'm sorry, but I don't have any long exposure shots; the longest is I think 1.6sec, and that's only one shot
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Hi Gabor,
thank you for your files. If I understand it right, what they demo is that somebody who knows how to do it can manipulate the DNG file by just any amount of exposure without this being reflected in the Exposure indication in ACR.
Yes I can see that all this is irrelevant for ETTR. But as I wrote earlier, alongside with the ETTR-exposures, I want to make one exposure for the mid-tones as my reference for how dark/light the scene was.
Also, when making the camera profile in the DNG Profile Editor, I need a reference for middle gray in the visual space.
Happy I got this far with my workflow, at last.
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According to John Sheehy, the in-between ISO's on the 5D2 are implemented as they are on the 40D/50D, via software multiplication after the sensor signal is quantized (this is different from the 5D, which implemented intermediate ISO's via analog amplification, as in the 1 series). This observation means
1) That there is no difference between using ISO 125 vs underexposing ISO 100 by 1/3 stop and compensating with +1/3 EC during RAW conversion, except that by using ISO 100 one retains an extra 1/3 stop of highlight data. Similarly for 250 (use 200 underexposed 1/3 EV), 500 (use 400 instead), 1000 (use 800 instead), etc.
2. That there is no difference between using ISO 160 vs overexposing ISO 200 by 1/3 stop and compensating with -1/3 EC during RAW conversion. However, by using ISO 200 and overexposing, one is explicitly aware that one is doing ETTR by 1/3 stop at the time of the exposure, rather than having the camera lie to you about it.
These two points also apply to the 40D/50D, which implement the intermediate ISO's in the same way. Note that there is a custom function on all these cameras which allows one to disable the intermediate ISO's.
Furthermore, as long as we are on the subject of what ISO's it makes sense to use for RAW, any ISO above 1600 on Canons (or any other current DSLR, for that matter) has zero noise advantage compared to underexposing at 1600 and applying exposure compensation during RAW conversion. As in point (1) above, underexposing at 1600 has the advantage of retaining additional stops of highlight headroom that are thrown away by using a higher ISO.
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Furthermore, as long as we are on the subject of what ISO's it makes sense to use for RAW, any ISO above 1600 on Canons (or any other current DSLR, for that matter) has zero noise advantage compared to underexposing at 1600 and applying exposure compensation during RAW conversion.
This is one of the most interesting conclusions I made from your SNR curve plots. But Emil, is this an empirical rule you concluded from those noise measurements, or there is some physical limit that makes this statement be true on any present or future camera?
(http://img99.imageshack.us/img99/2696/66403125zy4.gif)
(the yellow part of the SNR improvement curve when rising ISO becomes flat, i.e. no improvement for rising ISO)
And another question: I never understood very well what Roger N. Clark used to call 'unity gain' ISO, i.e. the ISO value for which every extra converted photon would account for one extra ADU in the encoded RAW file. He claimed going beyond that ISO value is nonsense in terms of SNR. But I never understood how he extrapolated this no matter which bitdepth had the sensor when there should be a big difference in that ISO value for 12, 14 or 16 bit encodings. Has the 'unity gain' ISO any relation with what you said?
BR
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This is one of the most interesting conclusions I made from your SNR curve plots. But Emil, is this an empirical rule you concluded from those noise measurements, or there is some physical limit that makes this statement be true on any present or future camera?
It has entirely to do with the noise of the ISO amplifier, which is the limiting factor in low ISO DR. If the signal processing circuitry downstream of the photosite array had as much DR as the photosite array itself (what Roger Clark calls sensor DR, as opposed to camera DR), then there would be little distinction between underexposing at a fixed ISO, vs applying EC during RAW conversion. The only reason (apart from jpeg generation) for the camera to have variable gain is the benefit in SNR that it confers at lower EV. The rest of the electronics has less DR than the sensor itself, therefore that lower limit forces one to choose the ISO amplification to decide what part of the sensor DR to access. If the rest of the electronics had a DR that met or exceeded the sensor DR, then the all that the sensor captured would be available starting at the lowest ISO, and so all that raising the ISO would do is remove parts of highlight headroom without adding any more range at the lower end.
Current Canon and Nikon offerings leave anywhere from 1-2.5 EV of DR unrealized due to shortcomings in the DR of the electronics that processes the photosite signal. It is that which limits the utility of changing analog ISO amplification in the camera; once the read noise is dominated by the photosite noise rather than the amplifier/ADC noise, there is no further advantage to raising the ISO. Empirically this happens to be at about ISO 1600 on current Canons, and the D3/D700 from Nikon. I haven't crunched the numbers, but the lower amplifier noise due to the parallel ADC architecture in the D300/D3x probably means that those cameras benefit little or none from raising the ISO past 800.
And another question: I never understood very well what Roger N. Clark used to call 'unity gain' ISO, i.e. the ISO value for which every extra converted photon would account for one extra ADU in the encoded RAW file. He claimed going beyond that ISO value is nonsense in terms of SNR. But I never understood how he extrapolated this no matter which bitdepth had the sensor when there should be a big difference in that ISO value for 12, 14 or 16 bit encodings. Has the 'unity gain' ISO any relation with what you said?
Unity gain is explained by Clark as follows: "Unity Gain ISO is the ISO of the camera where the A/D converter digitizes 1 electron to 1 data number (DN) in the digital image. Further, to scale all cameras to equivalent Unity Gain ISO, a 12-bit converter is assumed. Since 1 electron (1 converted photon) is the smallest quantum that makes sense to digitize, there is little point in increasing ISO above the Unity Gain ISO".
This would be fine if there were no noise in the image processing chain. Noise means that unity gain is merely the point where on average one electron captured raises the raw level by one unit. That does not mean that the sensor is counting electrons, which is what would be required for there to be no advantage to raising the gain above the unity gain ISO. But the noise is much more than one raw level at the unity gain ISO, washing out the accuracy of the raw levels relative to electron counts.
Indeed, because the electronics downstream of the photosite array still has a substantial contribution to read noise at unity gain ISO in many cameras, raising the ISO beyond unity gain decreases their contribution relative to signal (since their contribution to noise stays fixed while the signal is amplified), and SNR improves. Unity gain for the 1D3 is about ISO 500 if one takes the literal definition of one electron per raw level (Clark instead fudges and rescales 14 bit data to 12 bits to avoid the bit depth issue you raised), but my SNR plot at fixed exposure shows a definite improvement from ISO 400 to 800 to 1600:
http://theory.uchicago.edu/~ejm/pix/20d/te...DRwindow1d3.png (http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/DRwindow1d3.png)