Luminous Landscape Forum
Site & Board Matters => About This Site => Topic started by: joedecker on September 22, 2009, 02:08:40 pm
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Nice article. One question, though....
For per-channel histograms, on cameras that create a histogram even for RAW images based on the associated JPG, doesn't the white balance setting affect the relative brightnesses of the channels and therefore affect the accuracy of the histogram as well?
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With Canon (at least on my 1000D) if you put the lens cap on, take a shot using the smallest aperture, lowest ISO and highest shutter speed and then use it to set the white balance, it sets the RGB multipliers to exactly 1, so you shouldn't have that problem.
It looks very ugly on the back of the camera though, and means JPEGs are basically unusable. (but you're not doing this to shoot JPEG anyway)
I really wish you could shoot with settings that gave you a rough approximtion of what your ‘developed’ RAW file will look like on the back of the camera, and for JPEGs, but that the histogram would be based on the RAW data rather than the JPEG preview.
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With Canon (at least on my 1000D) if you put the lens cap on, take a shot using the smallest aperture, lowest ISO and highest shutter speed and then use it to set the white balance, it sets the RGB multipliers to exactly 1, so you shouldn't have that problem.
It looks very ugly on the back of the camera though, and means JPEGs are basically unusable. (but you're not doing this to shoot JPEG anyway)
Yes, reading the introductory sentence on the what's new page, I was just awaiting an UniWB tutorial (http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm) . The tips gaved here are more basic, but maybe more usable too.
To second Michael's conclusion note, it would be great that photographers don't need such tortuous techniques to simply assess what's in the raw data they captured - and going on on this, why no expose-to-the-right matrix metering when in raw mode?
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This contrast hint is interesting. I've now created a user defined setting on my Canon 5D2: Picture Style=Neutral, Sharpness=0, Contrast=-4, Saturation=0, Color tone=0.
The Picture Style seems to influence the in-camera histogram, too. I've selected "Neutral" but and I am not sure about the practical difference between "Faithful" and "Neutral". Which setting do you use if you shoot RAW-only?
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Not sure I agree 100% with the article. Perhaps for applications where you will be putting significant effort into processing each picture it would make sense but when you are going to be applying a set minimum of contrast to your image as default then suddenly your histogram is not so accurate anymore because at zero contrast it is not true to the possibilities within the final image. Give you an example. I'm a wedding shooter. I need to process hundreds of photos with very high contrast/DR. Now if I set at zero contrast and shoot ETTR then when I apply the contrast needed for print I'm going to blow the dress. I of course do have that information and with the Local Adjustment Tool have huge amounts of 'recovery' room however the histogram is not accurate relative to the contrast curve of the final image which I work with when trying to process hundreds of images fast.
I set my camera to 1 click under 'neutral' contrast and I find that it gives me a histogram which accurately reflects the extra RAW highlight information compared to the jpg but doesn't give me an impression of more highlight headroom than I would have WITHOUT having to use dodge and burn tools in the RAW converter to get a finished result without having to enter PS.
I know that my specific circumstances may differ from the majority on this site who concentrate on 'each image will have it's own unique processing start to finish' type of photography but for people like me, the ability to see what your image will look like with the minimum amount of faffing around in post processing, both in the preview and histogram, is very valuable. I would never ever shoot jpg but neither do I shoot weddings with the kind of processing that my landscapes would require either. I have a default set of values in ACR, a custom profile or two, I change the WB and tweak the brightness (and exposure if highlights need recovering) and that is it apart from dodge and burn where absolutely necessary. Batch to jpgs and send to the lab for printing as proofs. ETTR is pretty much accurate exposure for wedding work anyway, make the dress white but holding detail and the face should be pretty much spot on.
Think what I'm trying to say is that if you need to process and get out a lot of prints, without being false to the idea of ETTR, it makes sense to try and get the exposure as spot on as possible in camera to reduce the amount of post work. You can't do that if your histogram thinks that you only want zero contrast.
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Nice article. One question, though....
For per-channel histograms, on cameras that create a histogram even for RAW images based on the associated JPG, doesn't the white balance setting affect the relative brightnesses of the channels and therefore affect the accuracy of the histogram as well?
That is true, and many photographers load a UniWB into the camera so that the white balance multipliers are equal to unity, giving a better evaluation of the actual status of the raw data in the RGB channels. The black and white histogram is usually a luminance histogram and is most sensitive to green and least sensitive to blue; such a histogram may not demonstrate clipping in the red or blue channel. A composite RGB histogram (such as is default in Photoshop) would represent RGB equally and would be preferable in detecting blown red or blue channels. The Cambridge in Color (http://www.cambridgeincolour.com/tutorials/histograms2.htm) site gives a good discussion of this topic. If Adobe RGB is the widest color space that the camera offers, then the histogram may show saturation clipping when none is present. It would help if the cameras offered a wide gamut space such as ProPhotoRGB.
Using a low contrast setting on the camera is often recommended and is helpful if the camera histogram gives a conservative estimate of clipping (as most do), and indicates clipping when none is present, allowing a safety margin for the highlights. Advocates of ETTR may not want this headroom. Increasing the contrast setting on the camera applies an S curve to the tonal data, lifting the three quarter tones and lowering the quarter tones. It does not affect the extreme highlights or extreme shadows. At the two extremes of the contrast curve, input equals output. If the camera histogram is conservative, then lowering the contrast will lower the three quarter tones and less clipping will be apparent in the histogtram. If the histogram is dead on, the contrast setting will have no effect. A better way of improving the appearance of the histogram would be to load a custom curve in the camera so that clipping in the raw channels takes place when the histogram is fully to the right. The contrast of the preview image would then be normal.
One should compare the raw histogram to the camera histogram to determine the amount of headroom that the camera gives, and make a corrective custom curve if indicated.
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It would be nice if the rendering of the preview image, displayed on the back of the camera, could be "exposure corrected" when shooting ETTR. When shooting, I've had people ask "was it a good pic?" and my reply is (almost universally) "I'll know when I load it onto a computer." This leaves them a bit puzzled because almost everyone expects the picture on the back to be what you've just taken...
The other by-product of ETTR is that I no longer pay any attention to review comments about back of the camera LCD screen colour accuracy (the image is not exposed correctly, so the rendered colours are going to be meaningless.) The only detail that's important, now, is whether or not I can get 100% zoom to check if it is properly focused.
A software option in the firmware to "correct the exposure" could be interesting...
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Other settings for a more accurate histogram:
- White balance and color space, as others mentioned, that's a big one.
- Auto Lighting Optimizer (or whatever your cameras calls the context-sensitive local brightness adjustment)
- Disable Highlight Tone Priority (or whatever your camera calls the setting that reduces ISO and applies nonlinear EC)
- Tone curve: This is generally the same as contrast.
- Gamma curve: if your camera allows loading a different gamma curve. (Linear is not useful in the general case IMHO, but would be nice to have a one-button push for comparing with normal gamma.)
I'm sure there are more I'm forgetting. All these are dumb workarounds for what all cameras should have already: a raw histogram. RED ONE is an example of the kind of additional tools that would be highly useful. One is a false color mode that shows every raw intensity as a different color, like Predator's infrared vision. That way you can see exactly what part of the image maps to what part of the histogram.
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This forum has a thread that is IMO more interesting and accurate to have the best possible information about the RAW file than this recently appeared article. It is here: Camera's histogram reliable to the RAW data, Specific WB for realistic camera JPEG (http://luminous-landscape.com/forum/index.php?showtopic=22250). The detailed article here: UNIWB. MAKE CAMERA DISPLAY RELIABLE (http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm)
I disagree with michael that the correct WB must be set to have the best knowledge about the RAW, because when applying this WB the camera performs overexposure over the RAW data to generate the JPEG that provides the LCD histogram. Because of this, the camera histogram is pesimistic telling us certain areas are blown that actually remain intact in the RAW file.
So the best way to have a more reliable histogram in the camera is not set the correct WB but cancel the WB. This will give us information about what we can really expect to find when developing the RAW file. The so called by Illiah Borg 'UniWB' is not perfect, but much better than using the correct WB or fiddling with the saturation, contrast or colour profile features of the camera (that anyway can be tuned at the same time as UniWB).
Regards.
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An old trick and I've been doing this for years. I shoot Canon bodies and set the contrast, saturation, and color tone to -4 and sharpness to 0 (the lowest setting). This not only affects the histogram but in the event that you shoot jpegs you get the most control over your images; you can mod them in post as opposed to having the camera do something you may not want.
Same applies to Nikon; they're heavy handed on the saturation, so I'd definitely be turning the sat down at a minimum if I shot Nikon. Why give control to the camera?
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The best setting needs to be figured out for every camera model (and, ideally, for every kind of illumination, but that's not realistic). One should remember, that the neutral contrast and saturation settings are not the lowest values but zero. I find it strange, that some are using the minimum contrast and saturation setting.
Regarding curves: I know of no camera, which allows for loading a gamma curve. The curve one can load is a substitute for the standard contrast curve, or S-curve; a "linear curve" eliminates that transformation, i.e. it moves closer to the raw histograms.
However, the loadable curve can be used to compensate for the gamma curve. I created a custom curve for my 40D, resembling the inverse gamma curve. Note, that the curve never affects clipping, but it does affect the distribution of the intensities (the appearance of the histograms). The closer the displayed histogram resembles the raw histogram, the better one can judge the "exposure lattitude between the currect exposure and clipping.
Another important issue is the color space. This has to be tested with every camera model. I found, that my Canon 40D has to go with sRGB, otherwise the displayed histograms show lower exposure than the raw data itself. One could say it is logical, for AdobeRGB "compresses" the pixel values more, but it is not so simple. I suggest trying it out.
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An old trick and I've been doing this for years. I shoot Canon bodies and set the contrast, saturation, and color tone to -4 and sharpness to 0 (the lowest setting). This not only affects the histogram but in the event that you shoot jpegs you get the most control over your images; you can mod them in post as opposed to having the camera do something you may not want.
Same applies to Nikon; they're heavy handed on the saturation, so I'd definitely be turning the sat down at a minimum if I shot Nikon. Why give control to the camera?
I fail to see the logic in this reasoning. Negative contrast and saturation setting is just as a change of the image data as a positive value.
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I created a demonstration of ETTRing with my Canon 40D. I made two shots of the very same scenery (shrubs in my yard), in manual mode, same aperture, shutter and ISO, only a few seconds apart. The first one was a picture style changed to high contrast and high saturation, white balance "daylight"; the second one was made with neutral settings and a neutral custom WB (some call it UniWB), and with the inverse gamma curve.
I photographed the in-camera displays (sometimes it's good to have the old 20D in reserve):
(http://www.panopeeper.com/Demo/ETTR_InCameraHist_HighContrastSunny_Canon40D.GIF)
(http://www.panopeeper.com/Demo/ETTR_InCameraHist_NeutralSetup_Canon40D.GIF)
The first one shows the sky as mostly clipped (see the black at the top left part?). The second one does not show any clipping by flashing, and the green channel's histogram shows a small clipping.
Now, the fact: clipping in all three channels, but very small (the y axis is logarythmic), only between the leaves; there flashing clipping indication on the sky is totally incorrect.
(http://www.panopeeper.com/Demo/ETTR_Sample_Hist_Canon40D.GIF)
The following capture shows most of the clippings; magenta indicates green clipping, the red shows green and blue clipping and black shows, that all three channels clipped:
(http://www.panopeeper.com/Demo/ETTR_Sample_Clipping_Canon40D.GIF)
This shows, that the special setup reflects the real (raw) exposure very closely, while the other setup is useless for ETTR, because it suggests reducing the exposure.
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I created a demonstration of ETTRing with my Canon 40D.
This shows, that the special setup reflects the real (raw) exposure very closely, while the other setup is useless for ETTR, because it suggests reducing the exposure.
An excellent demonstration, Gabor. I have done a few studies with histograms on my Nikon D3. For scenes without saturated colors and without saturation clipping imposed by white balance, the camera histogram wtih the standard picture control does a good job. Here is a Stouffer wedge with the green channel slightly below clipping:
(http://bjanes.smugmug.com/photos/415568835_GUxuh-O.png)
And the camera histogram is also just short of clipping. Accurate enough for most work.
(http://bjanes.smugmug.com/photos/415568824_PHcTE-O.png)
With a saturated yellow flower, the situation changes. The raw histogram shows the red channel at clipping and the green channel just short of clipping:
(http://bjanes.smugmug.com/photos/319807614_3NUTu-O.png)
The camera histogram with the camera set to aRGB demonstrate strong clipping in green and red, but the luminance histogram appears OK. The green clipping occurs because the green does not fit into aRGB. The extreme red clipping is due both to the white balance multiplier and color space limitation. UniWB would have been helpful, but I was not using it at that time.
(http://bjanes.smugmug.com/photos/319807676_L8euB-O.jpg)
Reducing exposure will eliminate the green clipping when the green comes into the gamut of aRGB, but there will still be massive red clipping because of the red multiplier. This image shows decreasing exposure from top to bottom:
(http://bjanes.smugmug.com/photos/319807719_Km7uz-O.jpg)
The screen shot below shows a decent rendering into ProPhotoRGB. Negative exposure is needed, since the original exposure was fully to the right. In this case, UniWB would have been helpful, but without a wider space than used for the aRGB preview, there would have been underexposure of the green.
(http://bjanes.smugmug.com/photos/319807586_AqhqF-O.png)
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This shows, that the special setup reflects the real (raw) exposure very closely, while the other setup is useless for ETTR, because it suggests reducing the exposure.
Real or just closer? I’ve tried all kinds of settings on my Canon, some do produce a closer approximation of the Raw data but its pretty far off (calling it approximation is being kind).
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The screen shot below shows a decent rendering into ProPhotoRGB. Negative exposure is needed, since the original exposure was fully to the right. In this case, UniWB would have been helpful, but without a wider space than used for the aRGB preview, there would have been underexposure of the green.
(http://bjanes.smugmug.com/photos/319807586_AqhqF-O.png)
Question: You aren't printing in ProPhotoRGB (can any printer do all of ProPhoto?) so I assume that when you convert to a smaller space some colours will start clipping again. As such would it not make more sense to try and equalise the histogram towards nearer towards the output colour space to reduce problems?
Or when you normalise the exposure from the original ETTR will you be anyway pulling the clipped colours back 'into the fold'?
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Real or just closer?
There is no "real", for there are transformations involved, which we can not universally counter. Already the demosaicing causes shifting between the raw channels, but the biggest item (if the WB issue is solved) is the color space transformation. As this results in complex shiftings between the raw and the RGB channels, it can not be balanced (countered) by the minimum of adjustments available in-camera.
I can imagine, that different, specific not neutral WB templates, depending on the scenery (representing the prevailing source colors) and on the illumination would help in creating histograms more closely resembling the raw data, but the involved labour is probably enormous - and that would have to be repeated with every camera model, because that depends on the spectral characteristics of the sensor.
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would it not make more sense to try and equalise the histogram towards nearer towards the output colour space to reduce problems?
The need to reduce the "exposure" in raw processing excludes automatic raw conversion, but I don't see any other "problem". Though I do realize, that this aspect can be serious for those, who are creating huge number of raw files and converting them in batch.
On the other hand, the higher real exposure reduces the noise, compared to a non-ETTR exposure, supposed that the DR of the scenery is large enough to justify ETTR in the first place.
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Question: You aren't printing in ProPhotoRGB (can any printer do all of ProPhoto?) so I assume that when you convert to a smaller space some colours will start clipping again. As such would it not make more sense to try and equalise the histogram towards nearer towards the output colour space to reduce problems?
Or when you normalise the exposure from the original ETTR will you be anyway pulling the clipped colours back 'into the fold'?
Mapping the colors that are out of gamut for the printer or display device from ProPhotoRGB is another matter. Reducing the luminance will bring some colors back into gamut with a narrower output space, since the gamut for saturated colors is greatest at mid-luminances in an RGB color space, but that may not the best approach. The aim of the raw capture is to capture as much of the scene luminance and color as possible. You may not be able to print saturated colors at high luminance and a reflection print can not show the DR of the capture, but at least you can map those values to best advantage. Besides, output devices are improving all the time.
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Real or just closer? I’ve tried all kinds of settings on my Canon, some do produce a closer approximation of the Raw data but its pretty far off (calling it approximation is being kind).
Do Canon cameras permit rendering into a wider space than aRGB by the camera?
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Do Canon cameras permit rendering into a wider space than aRGB by the camera?
Adobe RGB yes. But its not much help.
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I've been doing some more experimenting with this now because I thought I had it all figured out before, but hadn't considered saturation clipping and the fact that Lightroom is using Melissa RGB with the camera using Adobe RGB for its preview.
What I've found is that I actually don't want to have contrast all the way down at -4. If I'm using a ‘neutral’ white balance (the RGB multipliers at 1) and go by the clipping indicators on the camera, I end up with bad looking highlights.
I use a custom DNG profile with my camera in Lightroom and have most of the picture controls at zero by default, using a linear tone curve. From my testing, it seems that if I have to go below around -0.22 on the exposure correction (before adjusting white balance) I may be able to recover textural detail, but it's badly posterised/discoloured.
So while there may be more highlight detail there in the RAW file, and -4 contrast might be a better indicator of that, there doesn't seem to be any more useful detail above that point. (note: after changing the white balance, I can go much lower than -0.22, due to the way Lightroom seems to change it)
I don't know if other RAW converters may be able to get more useful detail out of the highlights without ending up discoloured, but that seems to be the limit with my 1000D in Lightroom at least.
To get the highlight clipping indicators on the camera to match that point, the neutral picture style at zero contrast is closest.
I then find that -4 saturation gets the rest of the histogram to be a close match to Lightroom's. (due to it using a wider colour space)
It's not a perfect match, especially very near black (the RAW file has more usable shadow detail than the camera preview) but it is at least useful for knowing where your highlights are going to clip badly.
Maybe if you're setting the white balance in the camera rather than putting it to a neutral position, or shooting in greyscale, -4 contrast is a good idea to help avoid clipping a single channel, but with my camera at least, and when using a neutral white balance, it's actually making me over-expose images.
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Adobe RGB yes. But its not much help.
Isn't aRGB Adobe RGB?
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Nice article. One question, though....
For per-channel histograms, on cameras that create a histogram even for RAW images based on the associated JPG, doesn't the white balance setting affect the relative brightnesses of the channels and therefore affect the accuracy of the histogram as well?
Yes it does. That is why UniWB helps--it sets the WB multipliers to 1 so that the resulting histograms which are derived from the JPG give a better indication of the raw channels. Saturation clipping into a small color space such as aRGB or sRGB can still occur.
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Isn't aRGB Adobe RGB?
Presumably it is. I don’t use the “term” aRGB, but I suspect anyone who does means Adobe RGB (1998).
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What I've found is that I actually don't want to have contrast all the way down at -4. If I'm using a ‘neutral’ white balance (the RGB multipliers at 1) and go by the clipping indicators on the camera, I end up with bad looking highlights
1. The advice to set the contrast to anything non-neutral (i.e. positive or negative) is misguided. Positive contrast increases the highlights and decreases the shadows, thereby causing raw conversion clipping. Negative contrast does the opposite, thereby hiding present raw clipping.
2. It plays no role, how the highlights - or the shadows - look in an ETTRed raw image without appropriate adjustments. The goal of ETTR is to capture as much light as possible without raw clipping (sometimes with a tolerable amount of raw clipping). The "look" of the shot is the question of raw conversion.
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I've been doing some more experimenting with this now because I thought I had it all figured out before, but hadn't considered saturation clipping and the fact that Lightroom is using Melissa RGB with the camera using Adobe RGB for its preview.
What I've found is that I actually don't want to have contrast all the way down at -4. If I'm using a ‘neutral’ white balance (the RGB multipliers at 1) and go by the clipping indicators on the camera, I end up with bad looking highlights.
I use a custom DNG profile with my camera in Lightroom and have most of the picture controls at zero by default, using a linear tone curve. From my testing, it seems that if I have to go below around -0.22 on the exposure correction (before adjusting white balance) I may be able to recover textural detail, but it's badly posterised/discoloured.
Andrew, you don't say what camera you are using, but are you aware of the baseline exposure offset that Lightroom and Camera Raw uses. It varies with the camera. For My Nikon D3, the offset is +0.5 EV meaning that highlights that are not blown in the raw file may appear to be blown in ACR/Lightroom unless one takes this offset into account. For this camera, if I set the Exposure in ACR to -0.5 EV and set the other sliders all to zero and the point curve to zero, a so called linear TRC even though it is in the gamma of the working space, I get good correlation between the RGB histograms in Rawnalize and ACR for the green channel, which has no WB multiplier. Saturation clipping in aRGB may occur in some images, but is not that common.
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Andrew, you don't say what camera you are using, but are you aware of the baseline exposure offset that Lightroom and Camera Raw uses. It varies with the camera. For My Nikon D3, the offset is +0.5 EV meaning that highlights that are not blown in the raw file may appear to be blown in ACR/Lightroom unless one takes this offset into account. For this camera, if I set the Exposure in ACR to -0.5 EV and set the other sliders all to zero and the point curve to zero, a so called linear TRC even though it is in the gamma of the working space, I get good correlation between the RGB histograms in Rawnalize and ACR for the green channel, which has no WB multiplier. Saturation clipping in aRGB may occur in some images, but is not that common.
It's a Canon 1000D, which has a +0.25 correction, if I remember correctly. When using the custom DNG profile I made with a ColorChecker, -0.22 seems to work better.
Maybe I wasn't too clear with what I was trying to say though. -0.25 exposure is the lowest I can go in Lightroom and get useful data back. I can still go lower and bring back more detail, but it's badly discoloured.
If I set the camera to the neutral preset with 0 contrast, the clipping indicator on the camera matches up nicely with Lightroom at -0.25.
If I set contrast as low as possible (-4) as recommended in this article, the clipping indicator only shows when the RAW file is entirely clipped—the point where I can get nothing back at all, rather than the point where I can't recover any more useful information. This isn't really very helpful, as you're potentially moving information you want to keep into an area where you can recover textural detail, but not colour when exposing to the right.
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1. The advice to set the contrast to anything non-neutral (i.e. positive or negative) is misguided. Positive contrast increases the highlights and decreases the shadows, thereby causing raw conversion clipping. Negative contrast does the opposite, thereby hiding present raw clipping.
This is not universally true; the meaning and range of JPEG adjustment parameters varies widely, depending on the brand and model of the camera. The only way to know how a particular camera setting affects the relation between RAW and JPEG values is to test it for yourself.
2. It plays no role, how the highlights - or the shadows - look in an ETTRed raw image without appropriate adjustments. The goal of ETTR is to capture as much light as possible without raw clipping (sometimes with a tolerable amount of raw clipping). The "look" of the shot is the question of raw conversion.
I agree with this, and find it amazing how many photographers, even supposedly master-level photographers, who haven't figured this out yet. It's the root of much of the "MFDB vs. DSLR" debate.
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I agree with this, and find it amazing how many photographers, even supposedly master-level photographers, who haven't figured this out yet. It's the root of much of the "MFDB vs. DSLR" debate.
Film culture I think. Those master-level photographers never faced a media with such linear properties as a digital sensor, and now tend to extrapolate their past know how. I am tired of hearing people talking about changes in hue and saturation depending on digital exposure.
Regards
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1. The advice to set the contrast to anything non-neutral (i.e. positive or negative) is misguided. Positive contrast increases the highlights and decreases the shadows, thereby causing raw conversion clipping. Negative contrast does the opposite, thereby hiding present raw clipping.
There is a great deal of confusion regarding positive and negative contrast settings for the camera JPEG rendering. On the Nikon D3, normal contrast (contrast = 0 in the picture control) applies a rather strong S curve to the raw data, lightening the quarter tones and darkening the three quarter tones. A contrast setting of -3 applies a slightly less strong S curve to the data, but it is still a positive contrast curve where the light quarter tones are lightened and the dark quarter tones are further darkened. The highlight clipping point is not affected by a contrast curve, since input equals output at an 8 bit value of 255. If the camera histogram is conservative and indicates clipping in the JPEG conversion when there is none in the raw file, then a lower contrast curve will lower the quarter tones and decrease clipping in the histogram.
Shown below are TRCs for the Nikon D3 with normal contrast (picture control contrast = 0) on the top and reduced contrast (picture control contrast = -3) on the bottom. These TRCs were produced from the same raw file by varying the picture control settings in CaptureNX, which would closely approximate the in camera settings.
[attachment=16896:NxContrasts.gif]
The effects of curves can be shown in ACR. This Stouffer wedge demonstrates clipping in Step 1 with a linear tone curve.
[attachment=16897:LinearCurve.png]
The clipping is unchanged with a positive contrast curve. The quarter tones are raised as shown. The value of 194 in the file is raised to 251. If the camera histogram were conservative and shows clipping at 251, then the histogram would show clipping when there is none.
[attachment=16898:PositiveContrast.png]
A true negative contrast curve will decrease the quarter tones as shown below. A value of 193 is reduced to 176. A conservative camera histogram would show less clipping.
[attachment=16899:NegContrast.png]
However, true clipping can not be eliminated by a contrast curve, since at the right end of the curve, input is equal to output. To eliminate the clipping, the curve has to decrease the highlights slightly as shown below. If the camera histogram is showing clipping when there is none, the best solution would be to upload a custom curve to the camera that decreases the highlights.
[attachment=16900:DecreaseHighs.png]
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There is a great deal of confusion regarding positive and negative contrast settings for the camera JPEG rendering. On the Nikon D3, normal contrast (contrast = 0 in the picture control) applies a rather strong S curve to the raw data, lightening the quarter tones and darkening the three quarter tones. A contrast setting of -3 applies a slightly less strong S curve to the data, but it is still a positive contrast curve where the light quarter tones are lightened and the dark quarter tones are further darkened
I guess the curve can be loaded with the D3 as well. Thus the effects of the "contrast" setting and the curve should be separated. One should load a linear curve, or even better, an inverse sRGB (or Adobe RGB) curve, to balance the non-linear mapping associated with the color space.
The highlight clipping point is not affected by a contrast curve, since input equals output at an 8 bit value of 255
I suggest you to test it with any raw file in ACR:
1. reset contrast to 0,
2. adjust the exposure so, that it is close to but under clipping,
3. increase the contrast; clipping will occur,
4. reset the contrast again and increase the exposure, so that some clipping occurs,
5. decrease the contrast (i.e. go into negative contrast): the clipping will disappear.
I don't think the in-camera JPEG creation differs from the above.
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I recall experimenting with my 5D in Hanoi several years ago, shooting the same scene out of the hotel window and changing the contrast settings of the Landscape Picture Style for each shot. The minimum contrast setting seemed to produce the most accurate histogram with regard to ETTR. At least it corresponded most accurately to the ETTR rendition of ACR after an appropriate amount of negative EC.
However, I have since experienced feedback from Gabor, with regard to some of my images I've sent him, that what I've experienced in ACR as an accurate ETTR, is in fact slightly clipped in the extreme highlights according to his Rawnalyze program.
The issue then becomes, 'does it really matter to the practising photographer if an image appears to be correctly exposed to the right when by certain technical standards it's actually very slightly overexposed?'
I remember on the same occasion in Hanoi experimenting with another method of achieving a good ETTR. Set camera to spot meter mode. Focus on the brightest part of the image, whether a bright cloud or a white napkin in a restaurant. Take note of the exposure reading, then increase exposure by 3 stops. You should get a good ETTR with the 5D without even looking at the histogram.
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I guess the curve can be loaded with the D3 as well. Thus the effects of the "contrast" setting and the curve should be separated. One should load a linear curve, or even better, an inverse sRGB (or Adobe RGB) curve, to balance the non-linear mapping associated with the color space.
I suggest you to test it with any raw file in ACR:
1. reset contrast to 0,
2. adjust the exposure so, that it is close to but under clipping,
3. increase the contrast; clipping will occur,
4. reset the contrast again and increase the exposure, so that some clipping occurs,
5. decrease the contrast (i.e. go into negative contrast): the clipping will disappear.
I don't think the in-camera JPEG creation differs from the above.
Gabor,
As requested, more or less. Here is ACR with a linear TRC, contrast = 0, minimal clipping in the highlights as shown in red. I am rendering into a 16 bit space.
[attachment=16974:SlClip_contrast0.png]
Increasing the contrast all the way to 98 causes only a very minimal difference:
[attachment=16975:SlClip_contrast98.png]
My original assertion stands. Why would contrast affect clipping when a curve does not affect values at the extreme right end of the curve? The situation is analogous to clipping with the brightness curve. As Jeff Schewe states in his ACR book, setting brightness to 100 looks like clipping, but if you check the 16 bit file you will find that the values have been pushed to max, but there is no clipping. The same applies to contrast.
Regards,
Bill
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As requested, more or less. Here is ACR with a linear TRC, contrast = 0, minimal clipping in the highlights as shown in red. I am rendering into a 16 bit space.
Bill,
you almost caused me a heart attack. Before I posted my previous message, I loaded a raw image in ACR without any particular selection and did just what I suggested, and the effect was just what I described. Now, I wanted to repeat that, and I picked a raw file - and it did not work. Then I pickd another one - it did not work. I tried about six raw images, until I found one, which reacted just like I described.
The first capture is with exposure +0.90; this is the maximum intenity increase without clipping; the contrast is 0 (tone curve: linear in all samples):
Exposure +0.9 Contrast 0 (http://www.panopeeper.com/Demo/Contrast_vs_Clipping_E0.9C0.jpg)
Now the same exposure, but contrast +100; it obviously created lots of clipping:
Exposure +0.9 Contrast 100 (http://www.panopeeper.com/Demo/Contrast_vs_Clipping_E0.9C100.jpg)
Next, the intensity increased by 1.1; this causes clipping. The contrast is 0:
Exposure +1.1 Contrast 0 (http://www.panopeeper.com/Demo/Contrast_vs_Clipping_E1.1C0.jpg)
Finally, the same intensity with contrast -50, and the clipping is gone:
Exposure +1.1 Contrast -50 (http://www.panopeeper.com/Demo/Contrast_vs_Clipping_E1.1C-50.jpg)
This effect is natural to me; particularly, decreasing the contrast must pull the higher intensity pixels back, i.e. away from clipping. Now, I'd like to know why this is not working that way with all images...
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My original assertion stands. Why would contrast affect clipping when a curve does not affect values at the extreme right end of the curve? The situation is analogous to clipping with the brightness curve. As Jeff Schewe states in his ACR book, setting brightness to 100 looks like clipping, but if you check the 16 bit file you will find that the values have been pushed to max, but there is no clipping. The same applies to contrast.
I'm getting a bit confused here, Bill. It's always possible to apply curves which increase contrast in certain parts of the image but not others. The standard 'brightness/contrast' tool in earlier versions of Photoshop would very easily blow highlights. In CS3 and beyond, those highlights are protected and the tool is now much more useful.
The issue of getting the camera's histogram to accurately reflect exposure for a full ETTR has to do with that flashing highlight-warning, does it not?
With the Canon 5D, setting contrast to zero (in one of the picture Styles) results in the histogram providing misleading information in respect to a degree of highlight-warning flashing. As a consequence, one tends to reduce exposure when it may not be necessary. Setting the contrast to -4 (on the 5D) fixes this problem. It's as simple as that, isn't it?
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With the Canon 5D, setting contrast to zero (in one of the picture Styles) results in the histogram providing misleading information in respect to a degree of highlight-warning flashing
There is no point in trying to coax the camera into displaying raw-like histograms with only a subset of the related adjustments. If the saturation is not 0, if the white balance is not neutral (i.e. the coefficients are not close to 1.0), then the contrast setting will not make up for all those. A certain contrast setting may "repair" the histogram in one setting, but not in another, particularly because of the changing WB.
(The tonal curve changes the appearance of the histograms, but it does not cause or eliminate clipping.)
It's as simple as that, isn't it?
Well, good for you.
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There is no point in trying to coax the camera into displaying raw-like histograms with only a subset of the related adjustments. If the saturation is not 0, if the white balance is not neutral (i.e. the coefficients are not close to 1.0), then the contrast setting will not make up for all those. A certain contrast setting may "repair" the histogram in one setting, but not in another, particularly because of the changing WB.
(The tonal curve changes the appearance of the histograms, but it does not cause or eliminate clipping.)
Well, good for you.
Gabor,
I take it you are referring to the one setting in one particular 'Picture Style' not being suitable for all situations. This is true. After setting the contast to minimum in the 'Lanscape' picture style on the 5D, the histogram tends to be accurate only for landscape-type shots. It's not accurate for shots in a dimly lit theatre using flash, for example. I haven't experimented with all combinations.
Well, good for you.
Thank you. I like to keep it simple.
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With the Canon 5D, setting contrast to zero (in one of the picture Styles) results in the histogram providing misleading information in respect to a degree of highlight-warning flashing. As a consequence, one tends to reduce exposure when it may not be necessary. Setting the contrast to -4 (on the 5D) fixes this problem. It's as simple as that, isn't it?
Ray, I think that it is not that simple. My thesis is that a contrast curve affects the quarter tones and has no effect at highlight saturation where input equals output. If the camera warning is conservative and indicates clipping when there is none in the raw file, then lowering the contrast will decrease those highlights in the quarter tones that were causing the clipping. However, if the highlight warning is perfectly accurate, then contrast will have no effect.
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My thesis is that a contrast curve affects the quarter tones and has no effect at highlight saturation where input equals output. If the camera warning is conservative and indicates clipping when there is none in the raw file, then lowering the contrast will decrease those highlights in the quarter tones that were causing the clipping. However, if the highlight warning is perfectly accurate, then contrast will have no effect.
I do realize, that different cameras may apply contrast differently, although I have a hard time to believe, that a negative setting causes positive contrast adjustment.
I made a test again with my camera, Canon 40D, and I found, that the flashing clipping indication as well as the appearance of the histogram does depend on the contrast setting. Negative contrast "hides" clipping, while positive contrast increases it. I don't believe, that the accuracy of highlight warning has to do with the way the contrast adjustment works. This accuracy is our special subject, the camera's firmware does not give a damn for it, IMO.
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I don't believe, that the accuracy of highlight warning has to do with the way the contrast adjustment works.
Why not? In my experience with the 5D, taking a series of shots of the same landscape scene with lots of blue sky, using the same exposure for a correct ETTR according to ACR, a maximum contrast setting will result in the highlight warning covering the whole sky. With a zero contrast setting, about half the area of sky will flash. With a -4 (minimum) contrast setting, just a small portion of the brightest part of the sky will flash.
What's the problem? Are you guys just bored with simple solutions?
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The flashing highlight warning starts before JPEG clipping occurs; on my 1Ds and 1D-II it seems to start at about 245 or so. Anything above that will flash even if it isn't clipped in the JPEG. So by adjusting the JPEG contrast setting, you can change the RAW level where the JPEG clipping indication begins. If you have contrast set too high, then you'll find yourself unnecessarily reducing exposure in situations where the RAW isn't clipping at all, just because you see a lot of blinking highlights.
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If you have contrast set too high, then you'll find yourself unnecessarily reducing exposure in situations where the RAW isn't clipping at all, just because you see a lot of blinking highlights.
Exactly my experience too, Jonathan.
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So by adjusting the JPEG contrast setting, you can change the RAW level where the JPEG clipping indication begins. If you have contrast set too high, then you'll find yourself unnecessarily reducing exposure in situations where the RAW isn't clipping at all, just because you see a lot of blinking highlights
My goal is to make ETTR more reliable by making the histogram and the clipping indication more closely reflecting the raw data. I don't care the least for the clipping in the embedded JPEG; I am comparing the raw clipping in Rawnalyze with the clipping indication in the camera (the in-camera histogram is good only for a coarse judgement). With a neutral WB and neutral other settings (including contrast=0), they match within 1/3 EV. If I reduce the contrast to the minimum, i.e. I apply negative contrast, the camera hides some of the raw clipping.
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If I reduce the contrast to the minimum, i.e. I apply negative contrast, the camera hides some of the raw clipping.
I don't doubt that this is true of YOUR camera, but is definitely not true of ALL cameras. You are generalizing the behavior of your particular camera to all cameras, which is demonstrably wrong. It's ridiculous to assume that every camera manufacturer uses the same RAW-to-JPEG processing and highlight indication algorithms in every model of camera they've ever made. It's not even true that every camera model has the same JPEG adjustment parameters; the adjustment range and the effect a particular setting value has vary widely from camera to camera.
As I've said before several times now, the only way to know for sure what the relationship between your camera's JPEGs and RAW is to shoot a series of RAW+JPEG frames containing both neutral and saturated colors, making careful notes of which frames show JPEG clipping in-camera and comparing that data with RAW clipping (or lack thereof) shown by Rawanalyze or an equivalent tool. That test will tell you what settings work best for your camera, but generalizing your results to a different camera model or especially a different camera brand is simply idiotic.
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Sorry, subject moved to another board.
Jonathan