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Author Topic: Today's DSLR should have another exposure mode  (Read 26232 times)
oldcsar
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« Reply #20 on: May 23, 2006, 02:06:42 PM »
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I've found that I can fudge an expose-to-the-right mode by selecting Aperture priority on my Rebel, and then stepping up my exposure compensation (by a fraction of a stop or a full stop, depending on the shutter speed that the camera decides and by how close it is to where I want it). I've found that my Rebel on Ap. Prior. mode generally makes a reasonable exposure in regards to preventing clipping on highlights (with exp. comp. at baseline), but I've found that the little bit of exposure compensation makes it JUST right for my purposes.
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Serge Cashman
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« Reply #21 on: May 23, 2006, 04:58:41 PM »
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What are you basing that on?
[a href=\"index.php?act=findpost&pid=66342\"][{POST_SNAPBACK}][/a]
If sensors were the best at capturing highlights up to the clipping point you'd be able to pull almost infinite high quality details out of the highlights just before they clip. I'm not an engeneer but it is definitely not what I can observe. The bits are there, the details are not.

"Just don't blow the highlights" exposure is really opposite to trying to shift everything to the right. "Desired" highlights is the key word here - you decide what to clip. I mean - if your image has a  narrow dynamic range then of course just overexpose until highlights are about to blow, it should be technically possible at some point in the future to do it automatically... But that's not the case in a situation like described in the original post - it's not a narrow dynamic range.
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Ray
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« Reply #22 on: May 23, 2006, 10:18:47 PM »
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If sensors were the best at capturing highlights up to the clipping point you'd be able to pull almost infinite high quality details out of the highlights just before they clip. I'm not an engeneer but it is definitely not what I can observe. The bits are there, the details are not.


Almost infinite? I think you mean, 'the maximum detail the system can deliver in the shooting circumstances', don't you?

As I understand, the response of digital imagers is quite different to film in the sense that highlights in film undergo significant compression before total clipping, commonly known as a 'shoulder'.

Digital sensors have a much narrower shoulder. Within half a stop or so, it seems, you can go from a situation of full, uncompressed detail in the highlights to totally blown highlights. There's a much sharper cut-off which presents a major problem for ETTR. It's clearly better to be a 1/2 stop under the correct exposure for ETTR than a 1/2 stop over, if preserving those highlight details is important.

However, what I've just written is an oversimplification (how could it be otherwise. I'm not even sure I know what I'm talking about   ). There's another issue relevant here, which is addressed in another current  thread, 'expanding dynamic range'. It is unlikely that all 3 channels in a digital sensor are going to 'blow out' at at the same point. The red channel might blow out first, followed by the blue channel, leaving the green channel as pure luminance. It seems there is no way around this, other than to use the right type of filter in front of the lens and do a 'custom WB' before taking the shot.

Now, just how precise do you want to be in your photography?
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Serge Cashman
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« Reply #23 on: May 23, 2006, 11:33:40 PM »
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Almost infinite? I think you mean, 'the maximum detail the system can deliver in the shooting circumstances', don't you?
[a href=\"index.php?act=findpost&pid=66415\"][{POST_SNAPBACK}][/a]

I was objecting to a description of contemporary sensors as exceptionally adapt at capturing extreme highlight details. Personally I think the sensors are not  good at that whatsoever. So you seem to agree with me.

Still - the point of ETTR is overexposing as much as possible so you get the most bits possible dedicated to your image information. Not to measure the extreme highlights and push everything else to the left.
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jani
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« Reply #24 on: May 24, 2006, 08:32:32 AM »
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I was objecting to a description of contemporary sensors as exceptionally adapt at capturing extreme highlight details. Personally I think the sensors are not good at that whatsoever. So you seem to agree with me.

Still - the point of ETTR is overexposing as much as possible so you get the most bits possible dedicated to your image information. Not to measure the extreme highlights and push everything else to the left.
This is a misrepresentation of the technique as described here on the LL and on other sites. I can see why you object to following a technique which is obviously in error, but it isn't ETTR you're describing.

It's not about detail, it's about sensor noise and artifacts, and you definitely shouldn't blow the highlights*:

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The simple lesson to be learned from this is to bias your exposures so that the histogram is snugged up to the right, but not to the point that the highlights are blown. This can usually be seen by the flashing alert on most camera review screens. Just back off so that the flashing stops.

Now of course when you look at the RAW file in your favourite RAW processing software, like Camera RAW, the image will likely appear to be too light. That's OK. Just use the available sliders to change the brightness level and contrast so that the data is spread out appropriately and the image looks "right". This will accomplish a number of things. The first is that it will maximize the signal to noise ratio. The second is that it will minimize the posterization and noise that potentially occurs in the darker regions of the image.

If reading Michael's article isn't enough, maybe this article by Roger Cavanagh can help.

*There are few rules without exceptions, and I'd just like to point out that there are some highlights that you might desire to blow, such as specular highlights.
« Last Edit: May 24, 2006, 08:33:42 AM by jani » Logged

Jan
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« Reply #25 on: May 24, 2006, 10:47:53 AM »
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There's no doubt that ETTR is the right thing to do for the best quality image. The problem is getting it right. Backing off till the 'blown highlight' warning stops flashing can result in underexposure in my experience. However, this result might be due to RAW converters getting better. I recall when I first started using the D60, I could recover about 2/3rds of a stop in BreezeBrowser. With the latest versions of ACR it seems to be about one and 2/3rds stops.

Consider the following image of a sunrise taken about 4 years ago on one of the rare occasions that I arose before the sun did. It's being converted into a very wide color space, ProPhoto RGB. Brightness and contrast have been taken to a minimum yet it looks as though I have seriously blown the red and green channels.

[attachment=599:attachment]

However, if I apply minus 1.5 EC, the apparently blown red and green channels are clearly not blown, as can be seen in the image below.

[attachment=600:attachment]


But what about the centre of the sun? That white spot is bigger than a mere specral highlight. Whatever the setting in ACR, it's 255,255,255, even with -4EC, which is okay by me. I'd expect the centre of the sun to be a blown highlight, but I was curious as to what a linear conversion would reveal and was very surprised to find that even that centre white spot does not seem to be blown. It's just a neutral white. The image appears to be actually underexposed by about 1/4 of a stop. That's close enough for me   .

[attachment=601:attachment]
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Serge Cashman
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« Reply #26 on: May 24, 2006, 07:38:57 PM »
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This is a misrepresentation of the technique as described here on the LL and on other sites. I can see why you object to following a technique which is obviously in error, but it isn't ETTR you're describing.

It's not about detail, it's about sensor noise and artifacts, and you definitely shouldn't blow the highlights*:

[{POST_SNAPBACK}][/a]

Well - reread the article. Pay extra attention to the rationale for the technique. It's about bits dedicated to the image data.

It's an overexposure technique geared towards heavy post-processing that benefits from extra bits dedicated to image information, not a guide to correct exposure.

"Don't blow highlights" is a disclaimer, so to speak. "We all know (or at least should by now)" that it's only realistic on narrow dynamic range images. Some highlights are born to be blown (as you've mentioned in your note) .

Quote:

"...Now of course when you look at the RAW file in your favourite RAW processing software, like Camera RAW, the image will likely appear to be too light. That's OK. Just use the available sliders to change the brightness level and contrast so that the data is spread out appropriately and the image looks "right"..."

[a href=\"http://www.luminous-landscape.com/tutorials/expose-right.shtml]http://www.luminous-landscape.com/tutorial...ose-right.shtml[/url]
« Last Edit: May 24, 2006, 08:08:18 PM by Serge Cashman » Logged
Serge Cashman
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« Reply #27 on: May 24, 2006, 07:57:06 PM »
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It's just a neutral white. ...
[a href=\"index.php?act=findpost&pid=66459\"][{POST_SNAPBACK}][/a]

Which means "blown" in colloquial English.
« Last Edit: May 24, 2006, 08:01:21 PM by Serge Cashman » Logged
John Sheehy
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« Reply #28 on: May 24, 2006, 10:38:00 PM »
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I was objecting to a description of contemporary sensors as exceptionally adapt at capturing extreme highlight details. Personally I think the sensors are not  good at that whatsoever. So you seem to agree with me.

You seem to be reading things from me that I didn't write.  I said that the range just below clipping of the RAW data is the highest quality recording range.
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John Sheehy
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« Reply #29 on: May 24, 2006, 10:45:33 PM »
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If sensors were the best at capturing highlights up to the clipping point you'd be able to pull almost infinite high quality details out of the highlights just before they clip. I'm not an engeneer but it is definitely not what I can observe. The bits are there, the details are not.

What are you basing this on?

Every experiment I've ever conducted shows that the upper ranges are the best; the noise is highest there, but the signal is even higher, so the S/N ratio is higher.  There is less posterization of your upper tones, as well.

In fact, a super-low-contrast scene exposed with +2 to +3 EC at ISO 400 or 800 has less noise and better subject detail than an ISO 100 shot with 0 EC.  All the time; every time.  No exceptions noted.

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"Just don't blow the highlights" exposure is really opposite to trying to shift everything to the right.

It's not the opposite; it's a different paradigm.  It's a matter of what it is that you want to put all the way to the right.
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Serge Cashman
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« Reply #30 on: May 24, 2006, 11:11:22 PM »
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It's not the opposite; it's a different paradigm.  It's a matter of what it is that you want to put all the way to the right.
[a href=\"index.php?act=findpost&pid=66517\"][{POST_SNAPBACK}][/a]

I can agree with that. But the objective of not clipping highlights pushes everything else to the left. The objective of exposure to the right demands an intelligent compromise.

And obviously both of these methods are post-processing oriented.
« Last Edit: May 25, 2006, 12:04:41 AM by Serge Cashman » Logged
Ray
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« Reply #31 on: May 25, 2006, 03:43:38 AM »
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Which means "blown" in colloquial English.
[a href=\"index.php?act=findpost&pid=66501\"][{POST_SNAPBACK}][/a]

Well perhaps I should have been less colloquial. In the linear conversion, the centre of the sun appears to be a neutral pale grey with values of 214,214,214, indicating that I could have given a fraction of a stop more exposure.

Technically there are no whites, only shades of grey. The palest shade of grey within our 24 bit color system is represented by the numbers 255,255,255. Isn't that correct?
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bjanes
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« Reply #32 on: May 25, 2006, 07:19:12 AM »
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But what about the centre of the sun? That white spot is bigger than a mere specral highlight. Whatever the setting in ACR, it's 255,255,255, even with -4EC, which is okay by me. I'd expect the centre of the sun to be a blown highlight, but I was curious as to what a linear conversion would reveal and was very surprised to find that even that centre white spot does not seem to be blown. It's just a neutral white. The image appears to be actually underexposed by about 1/4 of a stop. That's close enough for me   .

[attachment=601:attachment]
[a href=\"index.php?act=findpost&pid=66459\"][{POST_SNAPBACK}][/a]

Ray,

So that we can evaluate your findings, what software did you use for the linear conversion, and what settings did you use. Was white balance applied? What do you mean by linear conversion?
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Ray
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« Reply #33 on: May 25, 2006, 09:33:43 AM »
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Ray,

So that we can evaluate your findings, what software did you use for the linear conversion, and what settings did you use. Was white balance applied? What do you mean by linear conversion?
[a href=\"index.php?act=findpost&pid=66534\"][{POST_SNAPBACK}][/a]

Bill,
I used the linear conversion option in BreezeBrowser. WB as shot would have been applied. All other settings at default. Not accurate enough?
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bjanes
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« Reply #34 on: May 25, 2006, 11:22:22 AM »
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Bill,
I used the linear conversion option in BreezeBrowser. WB as shot would have been applied. All other settings at default. Not accurate enough?
[a href=\"index.php?act=findpost&pid=66546\"][{POST_SNAPBACK}][/a]

I downloaded the trial version of BreezeBrowserPro. Obviously, I don't know the program, but I can not easily get it to display the actual raw file with no white balance, tone curve, or levels applied. My overexposed color checker looks overexposed in the preview but normally exposed in the conversiion.

I can use manual levels in postprocessing, but I am unable to determine how to display the actual data numbers in the raw file. Personally, I prefer DCRaw for this purpose. Have you tried it?

Bill
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Serge Cashman
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« Reply #35 on: May 25, 2006, 07:44:08 PM »
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In the linear conversion, the centre of the sun appears to be a neutral pale grey with values of 214,214,214,
[a href=\"index.php?act=findpost&pid=66528\"][{POST_SNAPBACK}][/a]

Oh. Then it's not blown  Sorry for my remark.

I obviously assumed that "white" meant all 255s.
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Ray
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« Reply #36 on: May 25, 2006, 10:56:15 PM »
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I downloaded the trial version of BreezeBrowserPro. Obviously, I don't know the program, but I can not easily get it to display the actual raw file with no white balance, tone curve, or levels applied. My overexposed color checker looks overexposed in the preview but normally exposed in the conversiion.


I also had to download the trial version because my older version, which I haven't used for years, would not of course support 5D RAW images. Intitially I could not get the preview window to display the linear image as it should appear and often I would just get a black screen, so I assumed there was a minor bug or system incompatibility somewhere. Oddly enough, it appears to be working today as it should. Don't know whether the system just required a reboot or whether a 'ticking and unticking' of the 'tagged' box did it. Whatever, it's now fine. Below is a screen shot of the BB window.

[attachment=613:attachment]

What's interesting here is that the image is not nearly as red as my previous screen shot of a linear conversion, shown earlier in the thread, which was captured within Photoshop. I'm guessing here as to the reason. The linear conversion does not have an embedded profile. My working space is ProPhoto RGB, an extremeley wide gamut space. I did not assign a profile when opening the image in PS but for the purpose of posting on the net, did a conversion to sRGB. The redder image represents how the unchanged numbers, without profile assigned, would look in the sRGB space. The yellower image shows how the adjusted numbers look in sRGB. Does that sound right?

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I can use manual levels in postprocessing, but I am unable to determine how to display the actual data numbers in the raw file. Personally, I prefer DCRaw for this purpose. Have you tried it?


I recently did a Google search on DCRaw and even downloaded something. But I couldn't get the program to work and decided it was beyond my expertise and/or probably not worth the hassle of trying to figure it out. I prefer programs with a user-friendly interface   .
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Dennis
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« Reply #37 on: May 26, 2006, 06:46:10 AM »
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Personally, I prefer DCRaw for this purpose. Have you tried it?
Could you tell us your option settings for linear conversion?

I am experimetning with

-v -3 -r 1 1 1 1

The problem with this set is, that it's still scaled, you can't tell for sure, if a channel is blown or not. Should I include

-o 0

to prevent converting it into sRGB, and thus leave the RGB values as they are? Could be a good idea.
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bjanes
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« Reply #38 on: May 26, 2006, 09:59:39 AM »
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But what about the centre of the sun? That white spot is bigger than a mere specral highlight. Whatever the setting in ACR, it's 255,255,255, even with -4EC, which is okay by me. I'd expect the centre of the sun to be a blown highlight, but I was curious as to what a linear conversion would reveal and was very surprised to find that even that centre white spot does not seem to be blown. It's just a neutral white. The image appears to be actually underexposed by about 1/4 of a stop. That's close enough for me   .

[attachment=601:attachment]
[a href=\"index.php?act=findpost&pid=66459\"][{POST_SNAPBACK}][/a]


Just because the conversion is linear, that does not mean it has not been scaled. The raw file is 0..4095, and the converted linear file should be is 0..255. To convert from the 12 bit raw to 8 bit linear output, the raw pixel value is divided by 16 (a 4 bit right shift in integer math).

I find it difficult to believe that values of 255 in ACR with -4EC do not represent blown channels. Since the linear outupt is around 214, I think it is likely that the divisor is closer to 19 than 16. Since the image is white balanced, we know that multipliers are involved and some scaling has been performed. An analagous situation would be to use the output sliders in Photoshop so that maximum output would be at 214 rather than 255--clipping now occurs at 214 instead of 255.

The easiest way to investigate this possibility is to perform a series of bracketed overexposures and see what the maximum value in the channels is when clipping occurs.

At base ISO most digital cameras set the gain so that sensor saturation results in a data number near full scale in the analog to digital converter. For example, with my Nikon D200, sensor saturation results in RGB values of 254, 249 and 253 expressed as 8 bits and 4064, 3984, and 4048 in 12 bits as determined by conversion with DCRaw with demosaicing and conversion to an RGB image. In this case, clipping occurs in the sensor. Full scale of 4095 is not reached at base ISO, but AD overflow with a maximum data numberof 4095 would occur at higher ISOs and the clipping would be in the AD converter at 4095.
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bjanes
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« Reply #39 on: May 26, 2006, 10:11:45 AM »
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Could you tell us your option settings for linear conversion?

I am experimetning with

-v -3 -r 1 1 1 1

The problem with this set is, that it's still scaled, you can't tell for sure, if a channel is blown or not. Should I include

-o 0

to prevent converting it into sRGB, and thus leave the RGB values as they are? Could be a good idea.
[a href=\"index.php?act=findpost&pid=66621\"][{POST_SNAPBACK}][/a]

The latest version of CDRaw has a -D switch, which causes totally raw output with no scaling. There is no demosaicing and the output is gray scale 0..4095. If you want RGB output, the above switches with -o 0 should do the trick. The output is scaled by a factor of 16 to convert from 0..4095 (12 bit) to 0..65535 for display at 16 bits. The 12 bit output is very dark in Photoshop 15+1 display (0..32768).
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