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Author Topic: Can you expose too far to the right even if not clipping?  (Read 17354 times)
AFairley
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« on: December 09, 2010, 11:36:33 AM »
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I am aware of "shooting to the right" and the thinking behind it.  But I did run into a situation where the main subject matter was dark to dark brown tones, and my exposure was way to the right (but not clipped).  Naturally, in ACR the image looked quite washed out as shot.  No matter what I tried in ACR (the current version) to the RAW file I just could not get same look I believe I would have gotten from a normally exposed RAW.  It was like the tones were just dead when I brought them down to where they should be; fiddling with all kinds of combinations of tone sliders, constrast, vibrance, saturation, etc. just could not get me to where I knew the shot would look if I had exposed "normally" (i.e., a la modified zone system, metering off the target and adjusting exposure to place it where I wanted on the tone scale).  I'm no ACR guru, but I've gotten pretty good at knowing how to get to the look I want with adustments in ACR.  Not this time. 

So I wonder if you can expose too far to the right for a given subject even though you're not cllpping.  Thoughts?
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pegelli
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« Reply #1 on: December 10, 2010, 12:47:44 AM »
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Interesting observation and I had a similar "hunch" that got further solidified when reading this thread on getDPI.

Even though it's not the main subject being discussed what I learned is that the problem with high DR sensors (especially increased highlight headroom) ETTR can lead to putting your whole picture in a narrower gamut at the bright side of the histogram, and then by bringing the EV down in the raw converter you can't get all you would have gotten at a lower exposure.

Since I value colour in a picture much more than the absence of shadow noise I've stopped using ETTR, and am more exposing such that my pictures are "correctly" exposed when using the default raw conversion and am much happier with the colours I'm getting from this tactic.
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pieter, aka pegelli
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« Reply #2 on: December 10, 2010, 02:55:18 AM »
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So I wonder if you can expose too far to the right for a given subject even though you're not cllpping.  Thoughts?

Unquote

Yes I think you have a good point. This is where subjectivity comes into play. I have in the past used the Uni WB, but I don't now. However it helps me judge my exposure. The Uni WB in my experience was that I could get about an extra 2/3 to 1 stop more exposure. In ACR I found that though the highlights weren't technically blown out lowering the exposure slider didn't give me a pleasing result. Using a Nikon camera if I get slight blinking on the camera highlight warning then I am happy with the results though I know there is nearly a stop more. This is using raw and I know about the jpeg rendering of the image to give me a histogram reading. Experience will guide you and I think you are on the right track.  Smiley
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NikoJorj
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« Reply #3 on: December 10, 2010, 05:21:18 AM »
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ETTR can lead to putting your whole picture in a narrower gamut at the bright side of the histogram
I would have thought that this problem of reduced gamut at max luminance really does mean that you're clipping a channel on that high-luminance zone.
In the link you mention, the phrase (message #4 from hardloaf) :
Quote
This means that brightest stop of the camera range has most of colors gone forever and they cannot be restored with negative exposure compensation.
makes me think of the old mistake of not taking into account the 3 color channels for ETTR (discussion just goes on about exposing grays), or much more probably the old chore of not having a raw histogram to judge it.

That said, it's just an hypothesis and I'd like to hear more on the subject.

So I wonder if you can expose too far to the right for a given subject even though you're not cllpping.  Thoughts?
That might indicate some non-linearity in the upper part of the histogram...
I've heard some claims about it with some cameras (including IIRC the 10D), but couldn't get evidence of something like that with my good'olRebel (almost identical sensor as the 10D) : with a set of upwards bracketed exposures, set to "match total exposures" within LR, colors are spot on up to the point where they clip (LR clipping warning, spike in the histogram).
Though, there is a gray zone where I see the clear effects of clipping, and LR displays the clipping warning in the histogram, but no on-screen clipping overlay is shown. But it's really something like a third stop or so, much less than the error given by evaluating exposure with a jpeg-based histogram.

Of course, for saturated colors the clipping arises before high luminances, hence the traditional problem to render a sunrise/sunset on print : it's difficult to get both the proper saturation and the proper brightness.
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Nicolas from Grenoble
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BartvanderWolf
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« Reply #4 on: December 10, 2010, 06:06:26 AM »
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Interesting observation and I had a similar "hunch" that got further solidified when reading this thread on getDPI.

Even though it's not the main subject being discussed what I learned is that the problem with high DR sensors (especially increased highlight headroom) ETTR can lead to putting your whole picture in a narrower gamut at the bright side of the histogram, and then by bringing the EV down in the raw converter you can't get all you would have gotten at a lower exposure.

Hi Pieter,

Don't believe everything you read Wink .

At capture time, there is no color space (with its particular gamut hull) defined. The mostly linear sensitivity of our sensor arrays just records photons converted to electrons in R/G/B filtered sampling positons. Then a Raw converter needs to convert that data into RGB colors at each sample position (=demosaicing), and a number of calculations is performed before mapping those RGB coordinates to some RGB coordinate system. One of the operations, after demosaicing, is a linear scaling of the RGB channels to perform White Balancing and Exposure correction. Both should be done in linear gamma space.

Only after all these operations is the data set mapped to a color space (a coordinate system), that is why we effectively get a wider gamut by e.g. choosing Adobe RGB instead of sRGB, and ProPhoto RGB can squeeze a bit more out of our files (although most coordinates in the PP RGB are left unused in a real image).

So it depends on the Raw converter if it scales the ETTR exposure down to the proper/required level before mapping it to a colorspace. Only then, with a colorspace assigned, will the change of exposure potentially impact the saturation due to the remapping model in the given colorspace coordinate system.
 
Therefore the answer to the OPs question can only be given for a specific Raw converter, because we have no guarantee that the processing is done for optimal quality rather than speed. In fact, quality probably requires to do most of the processing in floating point math, which is slower than integer math. I saw some Raw conversion samples by Iliah Borg, indicating that (while slower) floating point math during Raw conversion indeed produces superior results compared to the current LibRaw/DCRaw conversions.

The best proof is therefore to shoot a scene (or a ColorChecker card) at different exposure levels, making sure that there is no single channel clipping (!), and pulling the exposure at Raw conversion to a common brightness level. Then a comparison of color differences will reveal if there is a problem with the Raw processing engine or not.

Cheers,
Bart
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sandymc
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« Reply #5 on: December 10, 2010, 06:45:35 AM »
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I am aware of "shooting to the right" and the thinking behind it.  But I did run into a situation where the main subject matter was dark to dark brown tones, and my exposure was way to the right (but not clipped).  Naturally, in ACR the image looked quite washed out as shot.  No matter what I tried in ACR (the current version) to the RAW file I just could not get same look I believe I would have gotten from a normally exposed RAW.  It was like the tones were just dead when I brought them down to where they should be; fiddling with all kinds of combinations of tone sliders, constrast, vibrance, saturation, etc. just could not get me to where I knew the shot would look if I had exposed "normally" (i.e., a la modified zone system, metering off the target and adjusting exposure to place it where I wanted on the tone scale).  I'm no ACR guru, but I've gotten pretty good at knowing how to get to the look I want with adustments in ACR.  Not this time. 

So I wonder if you can expose too far to the right for a given subject even though you're not cllpping.  Thoughts?

Yes, you can - with pretty much any modern raw processor you get color shifts, exactly as you found. If you want all the gory detail, I went through the technical reasons why in a blog post here some time ago.

Sandy
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NikoJorj
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« Reply #6 on: December 10, 2010, 08:18:24 AM »
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Yes, you can - with pretty much any modern raw processor you get color shifts, exactly as you found.
The hue shift you expose in your post is interesting, but... would you expect to detect it in a real-world image? I couldn't see any of these shifts with my test.
Btw, are you also sure that no channel clipped in the green color you show?

And yes, ETTR is of no use past base ISO and is to be applied to that specific situation (in which I personally am 95% of time). Wink
To rephrase ETTR, it is a simple way to adapt the exposure thinking from film (exposing middle grays at the middle between the shoulder and the toe of the emulsion) to digital (no shoulder but a progressive toe and saturation).
So it's not very different to simply taking care on what is clipped (generally quite bad), and what is drowned in the shadows (generally not as bad as clipped but to be avoided).
For low-contrast subjects I fully agree that ETTR does not bring much (as long as you don't clip anything important).
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Nicolas from Grenoble
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sandymc
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« Reply #7 on: December 10, 2010, 09:03:50 AM »
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The hue shift you expose in your post is interesting, but... would you expect to detect it in a real-world image? I couldn't see any of these shifts with my test.
Btw, are you also sure that no channel clipped in the green color you show?

Oh yes. Although usually you only see the issue if you aggressively post process. E.g., If you use a lot of "recovery" in Lightroom while using the newer camera profiles. There are a bunch of threads in the various Adobe forums on this, and the issue led to dcpTool getting its ability to make profile "invariate" or "untwisted", etc

And no, the green channel wasn't blown.

Sandy
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digitaldog
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« Reply #8 on: December 10, 2010, 09:10:01 AM »
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Recovery in LR and ACR need some work... If you clip one of the three channels and Recovery comes into the scene, I’ve seen color shifts (interestingly enough, Raw Developer does a much cleaner job). So yes, use ETTR but do not blow out any channel data that needs to be reconstructed. That can lead to ugly color results:
http://digitaldog.net/files/RD.jpg
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Andrew Rodney
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NikoJorj
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« Reply #9 on: December 10, 2010, 09:22:18 AM »
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If you use a lot of "recovery" in Lightroom [...]
Oh, I'd think hue shifts with Recovery are another, different issue... I've seen even much more than what Andrew shows here.
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Nicolas from Grenoble
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eleanorbrown
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« Reply #10 on: December 10, 2010, 09:23:14 AM »
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Simply put, in my experience in deciding on exposures, as long as I have good separation in my shadows, I get better results with a correct exposure as seen, rather than moving my whole histogram far to the right when I don't need to. I shoot with a 65 plus phase and a Leica m9. Eleanor

Interesting observation and I had a similar "hunch" that got further solidified when reading this thread on getDPI.

Even though it's not the main subject being discussed what I learned is that the problem with high DR sensors (especially increased highlight headroom) ETTR can lead to putting your whole picture in a narrower gamut at the bright side of the histogram, and then by bringing the EV down in the raw converter you can't get all you would have gotten at a lower exposure.

Since I value colour in a picture much more than the absence of shadow noise I've stopped using ETTR, and am more exposing such that my pictures are "correctly" exposed when using the default raw conversion and am much happier with the colours I'm getting from this tactic.
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FranciscoDisilvestro
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« Reply #11 on: December 10, 2010, 09:49:41 AM »
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As Bart explained, the limitations are in the current Raw converters. They are simply not expecting or designed for ETTR. From a signal processing point of view, there is absolutely no reason why an ETTR image will have a compromised color rendition (as long as there is no clipping or saturation in any of the channels)

I guess that the next generation of Raw converters will treat exposure in a different way, since cameras are getting better (where noise is determined by shot noise), so ISO could become metadata. For this you need to be able to adjust exposure more than the +/- 4 EV allowed by some current RAW converters, and it should be done before demosaicing (the same way white balance is treated today, just a scalar multiplication of the values)

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Chris_Brown
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« Reply #12 on: December 10, 2010, 09:49:56 AM »
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Simply put, in my experience in deciding on exposures, as long as I have good separation in my shadows, I get better results with a correct exposure as seen, rather than moving my whole histogram far to the right when I don't need to.

+1   Wink
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bjanes
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« Reply #13 on: December 10, 2010, 10:11:05 AM »
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Recovery in LR and ACR need some work... If you clip one of the three channels and Recovery comes into the scene, I’ve seen color shifts (interestingly enough, Raw Developer does a much cleaner job). So yes, use ETTR but do not blow out any channel data that needs to be reconstructed. That can lead to ugly color results:
http://digitaldog.net/files/RD.jpg

Of course, recovery and exposure in ACR are entirely separate entities. Recovery is for recovering clipped highlight tones or toning down the highlights. Its effects are constrained to the highlight tones. Exposure is global and is the tool that should be used for ETTR images that are not clipped. As SandyMC and others have noted it does have some limitations in terms of hue-shifts and other problems.

Regards,

Bill
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RFPhotography
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« Reply #14 on: December 10, 2010, 11:40:48 AM »
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Isn't the very concept behind ETTR flawed?  It tries to mash a non-linear construct (the doubling or halving of light and effective 'stops') into a linear system.  While more nominal data (electrons) will be captured in highlights than shadows, it's not the exponential increase as posited by many; including the article on the main LL site.  Is it?  In a linear system (digital sensors) if it takes 1000 electrons to move from one brightness level to another in the shadows then it takes 1000 electrons to move from one brightness level to another in the highlights.  The idea that exponentially more data is contained in highlights than shadows runs counter to the idea of the linear nature of digital sensors.  Doesn't it?
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FranciscoDisilvestro
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« Reply #15 on: December 10, 2010, 11:48:32 AM »
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The concept behind ETTR is not flawed and it has nothing to do with the linear response of the sensor. The idea is that shoot noise is proportional to the square root of total photon count. So the higher the photon count, the higher the Signal to noise ratio.

The idea is to "fill" the sensor with as many photons as possible to achieve the highest SNR possible.

The problem with the linear response of the sensor vs the nonlinear nature of perception of light is that ETTR poses a high risk of blowing the highlights.
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AFairley
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« Reply #16 on: December 10, 2010, 12:10:09 PM »
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Thank you for your replies.  I do want to emphasize that none of the color channels were clipped when I opened the RAW file in ACR 6.2, so I did not need to do any highlight recovery, though I obviously wanted to bring the overexposed high tones down.
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RFPhotography
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« Reply #17 on: December 10, 2010, 12:11:53 PM »
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Doesn't Sandy's article basically refute that point, Francisco as far as the noise levels?

And if the concept behind ETTR isn't flawed (and I'm not convinced given evidence of it I've seen) then perhaps it's the communication of the concept that's flawed.  

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NikoJorj
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« Reply #18 on: December 10, 2010, 12:21:40 PM »
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In a linear system (digital sensors) if it takes 1000 electrons to move from one brightness level to another in the shadows then it takes 1000 electrons to move from one brightness level to another in the highlights.
The sensor is linear, wheras our perception of brightness (and the exposure count by stops based on it) is logarithmic. So the brightness levels you quote are not equal to our eye and so not equal in the rendered print.
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Nicolas from Grenoble
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FranciscoDisilvestro
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« Reply #19 on: December 10, 2010, 12:50:25 PM »
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Doesn't Sandy's article basically refute that point, Francisco as far as the noise levels?

And if the concept behind ETTR isn't flawed (and I'm not convinced given evidence of it I've seen) then perhaps it's the communication of the concept that's flawed.  



Well, I may interpret it the wrong way (since English is not my first language), but I don't see contradictions regarding noise. The point in Sandy's article accepts there is a reduction of Noise at the expense of degraded color rendition and reduced DR. I claim that the problem with degraded color is due to the way the current Raw converters work and not because of the data itself.

Another point in Sandy's article is that ETTR only deals with sensor noise but creates other problems in the rest of the chain. I completely agree here, with the same comment as before: Because how the rest of the components / softwares are designed today.

ETTR only makes sense at base ISO - Yes. I think this will eventually be settled in the future when ISO will become metadata.

ETTR is not necessary today with modern cameras - I agree up to a point. For example, under incandescent light (2000K) if you expose normally, you will get a rather noisy and underexposed blue channel. ETTR or even using a color compensating filter will improve that.

I do agree that the communication of the concept may be flawed.
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