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Author Topic: two different ways to set black- and whitepoint  (Read 22372 times)
Robert-Peter Westphal
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« on: February 02, 2011, 02:54:09 PM »
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Hello,

in the gorgeous LR3 video Michael and Jeff show how to set the white- and blackpoint using the appropiated slider in the Basic panel 
Yesterday I got a smal manual ( 'light & land - landscapes in the digital darkroom' ) by Michael Frye. In this manual he explains to set these two point by moving the gradation point-curve highest anchor-point to the left to set the white-point and lowest anchor-point to the right to set the blackpoint.

Now I'm wondering what real difference these two ways cause, or if there is a difference in fact.
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« Reply #1 on: February 02, 2011, 03:49:27 PM »
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Yesterday I got a smal manual ( 'light & land - landscapes in the digital darkroom' ) by Michael Frye. In this manual he explains to set these two point by moving the gradation point-curve highest anchor-point to the left to set the white-point and lowest anchor-point to the right to set the blackpoint.

Well, if you are talking about moving the point curve in the Tone Curves panel, yes you can clip white or black points...but that's a pretty crude adjustment and will alter (or screw up) the overall tone curve unless you remove all the other points on the curve. However, I view this as suboptimal since you loose the ability to preview the clipping points by holding the Option/Alt key which I think is important. Naw, I really don't think using the point curve to set white and black points is a good idea. You know, just cause you CAN do something doesn't mean you SHOULD do something :~)
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john beardsworth
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« Reply #2 on: February 02, 2011, 03:57:40 PM »
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I guess the chap recommended it because it's the way he's been doing it in another program. Not exactly a good reason for doing it in LR.

Also look at dragging the histogram, and at Shift-double-clicking the words Exposure, Recovery, Blacks.

John
« Last Edit: February 02, 2011, 04:00:16 PM by johnbeardy » Logged

MFRYE
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« Reply #3 on: February 04, 2011, 07:01:48 PM »
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Robbie, thanks for purchasing my Light & Land eBook. I hope you found it helpful. I certainly understand your confusion about this issue: everybody seems to do things differently, and you often hear conflicting information. Remember that there are really no right or wrong answers - these are all just tools, and you should use the tools that work best for you, and that you're most comfortable with.

Jeff, I've long admired your work, and all the excellent and helpful educational materials you've given us over the years. I hope to meet you one day and discuss Lightroom stuff, or maybe just shoot the breeze. Since I live just outside of Yosemite, let me know if you're ever headed this way.

So let me explain why I prefer using the Point Curve to set black and white points in Lightroom. Again, this is a preference, not a case of right vs. wrong.

It's true that with the Point Curve you can't hold the Option/Alt key to see clipping, as you can with the Exposure or Blacks sliders. But you can see clipping by clicking on the triangles in the Histogram, or just pressing the J key. Doing it this way also allows you to see the image as a whole, and judge the effects of setting the black and white points on the image as a whole, as well as seeing the areas being clipped.

And Jeff you're right, of course, that moving the black and white points in the Point Curve will affect the rest of the curve. That's one reason I always set the black and white points first, before placing other points on the curve. But I'd prefer to set black and white points first anyway before adding contrast or doing anything else with the curve, as setting the black and white points provides the foundation for the rest of the tonal adjustments. Also, by making all the tonal adjustments with the Point Curve I can do everything in one place, and see the interaction between the black and white points and the other points on the curve.

But the main reason that I use the Point Curve, and why I'm so happy that Adobe finally added this to version 3.0, is the difference in the image's appearance. Now as far as I can tell there is no difference between using the Blacks slider and pushing the lower-left end of the Point Curve to the right (as long as you keep this point along the bottom of the Curves box).

But there is a difference between using the Exposure slider to set a white point and doing it with the Point Curve. When Lightroom first came out Adobe said that pushing the Exposure slider to the right was the same as setting a white point with Levels or Curves in Photoshop. Maybe Adobe said that to justify not including Levels or a real point curve in early versions of Lightroom. But it’s not the same, and it’s easy to dispel that myth, especially now that Lightroom has a real point curve:

• First, find an image that has some room between the lightest pixels and the right edge of the histogram—in other words, room to move the white point before seeing clipping.

• Next, activate the clipping warnings by pressing the J key, or clicking on the right-hand triangle in the Histogram.

• Go to the Point Curve. Push the upper-right end of the curve to the left, making sure to keep it along the top of the box, until you barely see some clipping (with the clipping warning activated, this will show up as red spots).

• Make a Virtual Copy of this image, and reset the Point Curve. Then push the Exposure slider to the right until, again, you barely see some clipping.

• Compare the two versions. You’ll see that while the white points are the same—the amount that’s clipped to pure white in each version is the same—the midtones are not. Using the Exposure slider moves the white point, but also boosts the midtones. With the Exposure slider the image will look lighter overall, and while the brightest pixels in the histogram should be at the same place in both versions, the rest of the histogram will look different.

In some cases boosting the midtones like this is okay, but in many cases it flattens highlight contrast and washes out colors. The difference is more pronounced when the Brightness slider is set to its default of +50. Setting Brightness to 0 minimizes the difference between these two methods—it’s still there, but smaller.

I put a page up on my blog to show an example of the difference:

http://www.michaelfrye.com/landscape-photography-blog/setting-the-white-point-in-lightroom-a-comparison/

I think this example makes the difference pretty clear, but I suggest (to anyone) that you try it yourself.

So Robby, I hope this helps explain the difference, and why I prefer using the Point Curve rather than the Exposure slider. Unfortunately I didn't have room to discuss this esoteric topic in the eBook, but you gave me an incentive to explain it on my blog, so thanks!

Jeff, again, I really appreciate all that you do. Maybe our little discussion here will help shed some light on this whole topic.
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Peter_DL
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« Reply #4 on: February 05, 2011, 02:48:21 AM »
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But there is a difference between using the Exposure slider to set a white point and doing it with the Point Curve.
... The difference is more pronounced when the Brightness slider is set to its default of +50. Setting Brightness to 0 minimizes the difference between these two methods—it’s still there, but smaller.

I put a page up on my blog to show an example of the difference:

http://www.michaelfrye.com/landscape-photography-blog/setting-the-white-point-in-lightroom-a-comparison/

>>So what’s going on? It appears that the Exposure slider in Lightroom does not set a straight linear white point, like the white point in Photoshop’s Levels or Curves, or the white point in Lightroom’s Point Curve. Moving the Exposure slider to the right also boosts the midtones.<<

It is probably explainable,
although following comment was more about negative Exposure setting:

Brian Griffith wrote >>  There's no way to know whether a clipped value is just exactly clipped or may be 100x beyond the clip point or somewhere in between. Assuming clipped values are 1.0 allows you to preserve linear tonal relationships across the full color range when adjusting exposure, as the identical exposure correction is applied to every value, and this really is the technically proper exposure adjustment. The tradeoff for this is [that]clipped white values, that you may want to assume are infinitely white or at least very much brighter than their peer, become dark gray when adjusting exposure significantly down.

As you have found, Adobe's RAW software (ACR and Lightroom) handle exposure differently, more like assuming clipped values are infinite. The problem with assuming clipped values are infinite is that in order to not give an ugly appearance when adjusting exposure down, values just below the clipping point and some range below  cannot have the exposure value fully applied. For example assuming 8bit values again if you had a value of 254 that was not clipped and set exposure to -1, that value would become 127 and say those very bright, but not clipped pixels were from some clouds that were right up against some sky that was clipped at 255 and was assumed to be infinite. You basically would end up with very dark clouds right up against a bright maximum white sky.

So if you assume clipped white values are infinite, in order to give a pleasing appearance you basically have to use a curve or non-linear exposure adjustment. This is essentially what Adobe does with ACR and Lightroom exposure. The exposure slider isn't really a true "exposure" adjustment equally applied to all values at least when using negative values.<<


Peter

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« Last Edit: February 05, 2011, 02:53:23 AM by Peter_DL » Logged
john beardsworth
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« Reply #5 on: February 05, 2011, 03:40:29 AM »
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Again, this is a preference, not a case of right vs. wrong.
That YMMV line is always a bit of an evasion. Michael.

Anyway, it strikes me that there's only validity in this approach if you're dealing with a low contrast image and one where you want to work with one hand behind your back and not use the adaptive tools, recovery and fill light.

John
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digitaldog
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« Reply #6 on: February 05, 2011, 11:01:58 AM »
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I guess the chap recommended it because it's the way he's been doing it in another program. Not exactly a good reason for doing it in LR.

The idea of working top down (expect for as most of us have discussed elsewhere, picking a DNG profile) is a sound approach. However, I did get a good tip from George Jardine in terms of using the black (Shadow) slider in Curves instead of using the black slider in Basic to affect the black tone adjustments. It does behave differently and after trying George’s suggestion on a lot of images, I find I prefer this method. Give it a try, see what you think.
« Last Edit: February 05, 2011, 11:04:26 AM by digitaldog » Logged

Andrew Rodney
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« Reply #7 on: February 05, 2011, 04:12:22 PM »
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But the main reason that I use the Point Curve, and why I'm so happy that Adobe finally added this to version 3.0, is the difference in the image's appearance. Now as far as I can tell there is no difference between using the Blacks slider and pushing the lower-left end of the Point Curve to the right (as long as you keep this point along the bottom of the Curves box).

But there is a difference between using the Exposure slider to set a white point and doing it with the Point Curve. When Lightroom first came out Adobe said that pushing the Exposure slider to the right was the same as setting a white point with Levels or Curves in Photoshop. Maybe Adobe said that to justify not including Levels or a real point curve in early versions of Lightroom. But it’s not the same, and it’s easy to dispel that myth, especially now that Lightroom has a real point curve:


It's pretty simple to explain. Both the Blacks and Exposure adjustments change the resulting brightness because the Brightness adjustment is set based the middle point between the black clipped point and the white clipped point (and the Blacks do indeed have the same impact on the halfway point, it's just less visible in the darker tones.

By adjusting the Exposure, it's implied that the movement of the clip point should have an impact on the Brightness as well. The primary differences between ACR/LR's Exposure, Brightness and Blacks adjustments and Levels in Photoshop are that you are working on a linear gamma image. The Levels adjustment in Photoshop is working on a non-linear gamma color space unless you've created a custom working space–I actually have a linear gamma ProPhoto RGB profile that allows me to work in a linear gamma in Photoshop.

There's a difference in the behavior of the Brightness adjustment in ACR/LR as well. Brightness in ACR/LR won't clip. The middle adjustment in Levels in Photoshop is can.

So, do you think Thomas was wrong when designing the relationship of Blacks, Brightness and Exposure?

I don't...it all comes down to understanding the functions of the adjustments and how to use them to arrive at the final tone curve for the image. Generally, I even prefer using the Parametric Curves over the Point Curves editor...the only reason I like the Point Curves editor is for very accurate tone curves of the extreme upper highlights to tease the best of highlight texture.
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DeanSonneborn
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« Reply #8 on: February 05, 2011, 04:24:32 PM »
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And when do you adjust white balance? Before or after you adjust the white-black point? I find that adjusting the white balance is sometimes more important than getting the perfect white-black point. I wish LR had a more direct way to balance the colors. I'm not always happy just clicking around with the white balance eye dropper hoping to fine just the right spot that give me what I'm looking for. How are other balancing their colors?
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bjanes
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« Reply #9 on: February 05, 2011, 08:25:38 PM »
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The idea of working top down (expect for as most of us have discussed elsewhere, picking a DNG profile) is a sound approach. However, I did get a good tip from George Jardine in terms of using the black (Shadow) slider in Curves instead of using the black slider in Basic to affect the black tone adjustments. It does behave differently and after trying George’s suggestion on a lot of images, I find I prefer this method. Give it a try, see what you think.

One situation where adjustments with Curves is helpful is in photomicroscopy with low contrast images. Shown below is an image of diseased brain tissue taken with a Nikon D3 attached to a Zeiss microscope equipped with top of the line planapochromatic objectives. The top screen capture is ACR with the default settings. The image is short scale and needs major adjustments.

The middle image was adjusted with the exposure and black sliders to just short of clipping and brightness was adjusted to give normal appearing midtones. Since contrast was still low, I cranked it up to maximum.

In the bottom image, I left the ACR settings at default and brought up the curves dialog and set the tone curve to linear to get rid of the anchor points. I then used the histogram to set the black and white points to just short of clipping and then added an S curve for contrast. IMHO, this method gives the best results and is easiest. Comments and suggestions are welcome.

Regards,

Bill

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David Eichler
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« Reply #10 on: February 06, 2011, 01:53:27 AM »
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Schewe: "Brightness in ACR/LR won't clip."  Well, you know a hell of lot more about this stuff than I.  However, this seems to contradict what I see on the screen. Not infrequently, when raising the brightness level in Lightroom, I do see the red clipping indicator showing increased clipping. Please clarify.

Thanks.
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David Eichler
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« Reply #11 on: February 06, 2011, 01:58:35 AM »
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The idea of working top down (expect for as most of us have discussed elsewhere, picking a DNG profile) is a sound approach. However, I did get a good tip from George Jardine in terms of using the black (Shadow) slider in Curves instead of using the black slider in Basic to affect the black tone adjustments. It does behave differently and after trying George’s suggestion on a lot of images, I find I prefer this method. Give it a try, see what you think.


I find myself using this method quite often, and seldom use the black slider to increase clipping, except for an occasional bump of 1 or 2 points after doing all of my other curve and contrast slider adjustments.

However, I do often use the black slider to decrease black clipping from the default setting of 5, and then fine tune the blacks with the curve. Maybe should just change this to a default of zero, but I am kind of used to just leaving it at 5.
« Last Edit: February 06, 2011, 02:00:36 AM by David Eichler » Logged

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« Reply #12 on: February 06, 2011, 05:55:11 AM »
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Quote Schewe

The Levels adjustment in Photoshop is working on a non-linear gamma color space unless you've created a custom working space–I actually have a linear gamma ProPhoto RGB profile that allows me to work in a linear gamma in Photoshop.

Unquote

Your friend Martin Evening in his Photoshop Lightroom 2 book states.

"but a linear gamma space is not so good when it comes making direct image edit adjustments"

page 572

Are you at odds with him? Wink
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Peter_DL
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« Reply #13 on: February 06, 2011, 11:53:31 AM »
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One difference between +Exposure setting vs. White point setting in the Point Curve tab can be seen with colors that are driven into single-channel clipping. The point curve approach better preserves the hue angle (see example below; for reference the starting image is at the left).

Otherwise we have yet to find a systematic difference between +Exposure, +Blacks settings compared to corresponding Point Curve settings (at least @ Brightness, Contrast = 0, and using the baseline matrix profile). Although, the resulting appearance across the ColorChecker can vary.

Situation is different with minus-Exposure setting which does not turn 255 white down to gray like with a corresponding White point setting in the Point Curve tab and a strictly linear implementation in general.

Peter

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Schewe
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« Reply #14 on: February 06, 2011, 01:10:15 PM »
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Your friend Martin Evening in his Photoshop Lightroom 2 book states.

"but a linear gamma space is not so good when it comes making direct image edit adjustments"

page 572

Are you at odds with him? Wink

Nope...I use ProPhoto RGB (with a normal 1.8 gamma) for real editing...I have the ProPhoto RGB gamma 1.0 as a working space for testing linear processing. In Photoshop with the algorithms designed the way they are, using a linear gamma is not optimal. Camera Raw/Lightroom was designed for working with linear images. Photoshop was not.
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bjanes
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« Reply #15 on: February 06, 2011, 03:00:46 PM »
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Schewe: "Brightness in ACR/LR won't clip."  Well, you know a hell of lot more about this stuff than I.  However, this seems to contradict what I see on the screen. Not infrequently, when raising the brightness level in Lightroom, I do see the red clipping indicator showing increased clipping. Please clarify.

Thanks.

Brightness may appear to clip in ACR, but that is due to 8 bit rounding errors. For example, here is an image of a Stouffer wedge exposed so that step 1 is just short of clipping. The image was demosaiced with Iris and the green channel was used for the histogram. Due to the bell shape of the histogram imposed by shot noise, clipping begins when the right tail of the histogram reaches saturation.



The raw image was opened in ACR and rendered into 16 bit Adobe RGB with the default tone curve and negative exposure was needed because of the BaselineOffset. Step 1 has an 8 bit pixel value of 254.



Increasing brightness to 150 causes the rightmost Channels to appear blown:



On examining the pixel values in Photoshop (15+1 bits), the pixels are very light but not clipped. A brightness adjustment layer was used to that the image could be edited without changing the original file values.



Using a brightness of -150 restores the original values, more or less.



Regards,

Bill





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MFRYE
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« Reply #16 on: February 06, 2011, 03:08:30 PM »
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From Peter_DL:

>>As you have found, Adobe's RAW software (ACR and Lightroom) handle exposure differently, more like assuming clipped values are infinite. The problem with assuming clipped values are infinite is that in order to not give an ugly appearance when adjusting exposure down, values just below the clipping point and some range below  cannot have the exposure value fully applied. For example assuming 8bit values again if you had a value of 254 that was not clipped and set exposure to -1, that value would become 127 and say those very bright, but not clipped pixels were from some clouds that were right up against some sky that was clipped at 255 and was assumed to be infinite. You basically would end up with very dark clouds right up against a bright maximum white sky.

>>So if you assume clipped white values are infinite, in order to give a pleasing appearance you basically have to use a curve or non-linear exposure adjustment. This is essentially what Adobe does with ACR and Lightroom exposure. The exposure slider isn't really a true "exposure" adjustment equally applied to all values at least when using negative values.

Thanks for this Peter. As you say, this seems to apply to negative exposure adjustments, but maybe the same design affects positive ones as well.

From Peter_DL:

>>Otherwise we have yet to find a systematic difference between +Exposure, +Blacks settings compared to corresponding Point Curve settings (at least @ Brightness, Contrast = 0, and using the baseline matrix profile). Although, the resulting appearance across the ColorChecker can vary.

>>Situation is different with minus-Exposure setting which does not turn 255 white down to gray like with a corresponding White point setting in the Point Curve tab and a strictly linear implementation in general.

Thanks again Peter. All this fits with my experience. There seems to be little difference between setting a white point with Exposure. vs. the Point Curve when Brightness is set to zero. And indeed, a minus Exposure setting can recover highlight detail that the Point Curve can’t. I usually use Recovery for this, but in extreme cases will use Exposure.

From Schewe:

>>By adjusting the Exposure, it's implied that the movement of the clip point should have an impact on the Brightness as well. The primary differences between ACR/LR's Exposure, Brightness and Blacks adjustments and Levels in Photoshop are that you are working on a linear gamma image. The Levels adjustment in Photoshop is working on a non-linear gamma color space unless you've created a custom working space–I actually have a linear gamma ProPhoto RGB profile that allows me to work in a linear gamma in Photoshop.

Thanks for this explanation. While linear vs. non-linear gamma is beyond my level of technical expertise, this at least gives me some framework for understanding the difference.

For me, as for most people, it’s all about getting my photographs to look their best, and communicate my original vision. I’d been using Photoshop for many years when I first started using Lightroom. In Lightroom I naturally looked for a way to set a white point, and read that I was supposed to use the Exposure slider. But it didn’t take long to discover that this didn’t look right - that setting a white point with Exposure gave an extra boost to the midtones, resulting in flat, washed-out highlights.

So I want back to using Photoshop, then eventually found a workaround - setting various white points with the Point Curve in ACR, then importing those images into Lightroom, and saving those curves as Presets. Cumbersome, but it worked, and allowed me use Lightroom more and Photoshop less. But now that Lightroom 3 has a real Point Curve I no longer need this workaround.

From Schewe:

>>So, do you think Thomas was wrong when designing the relationship of Blacks, Brightness and Exposure?

Not necessarily. Let me say that I admire Thomas’s software engineering skills, love Photoshop, and have grown to love Lightroom perhaps even more. I’ve turned many of my workshop students into enthusiastic Lightroom users.

Personally I would have been fine with the Blacks, Brightness, and Exposure tools if from the beginning Adobe had also included another way to set a white point that was like the white point in Levels or the point curve. I imagine that excluding the Point Curve was a marketing decision, not Thomas’s decision - but you probably know more about this than me.

Perhaps Exposure should have been a one-way tool that you could only use to darken the image, not lighten it, with another tool for setting the white point. Again, I would have been happy just to have a point curve, but for those unfamiliar with curves this arrangement might have prevented washed-out mid- to upper-tones in their photographs.

But then again, since the difference between setting a white point with Exposure vs. the Point Curve is negligible when Brightness is set to zero, maybe the problem lies with that default Brightness setting of +50.
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Schewe
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« Reply #17 on: February 06, 2011, 04:09:49 PM »
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But then again, since the difference between setting a white point with Exposure vs. the Point Curve is negligible when Brightness is set to zero, maybe the problem lies with that default Brightness setting of +50.

The Brightness default of +50 and the Tone Curve set to Medium Contrast are designed to normalize raw captures. It is what it is...the Defaults in ACR/LR can be changed by users if they so choose. I think the bottom line here is that you've brought a technique process from Photoshop and tried to force the technique into an ACR/LR toolset. The lack of a point curve editor in Lightroom until LR 3 was not a marketing decision, it was a usability decision. Camera Raw got a point curve editor early on. The only issue bringing the point editing to Lightroom was more of a UI issue–that and Mark Hamburg, the founding engineer of Lightroom designed the parametric curve editor and didn't see the need for point editing.

If you understand the fundamentals and know how to use the toolset, I really think the primary need to a point curve editor is really for the precision of highlight editing–which was my argument for including it in Lightroom 3. The vast majority of tone adjustments I make can be made directly in the Basic panel of Lightroom. It's very quick and easy to adjust Exposure, Recovery, Fill Light and Blacks. Once those are set, a slight adjustment in Brightness can usually bring the image into the global tonal range I want. I rarely touch Contrast on the Basic panel because if the above adjustments aren't sufficient, then I go into the parametric Tone Curve. If that doesn't solve the issues, rather than go into the point curve editor I'll use the Adjustment Brush to localize the tone controls.

In my experience, spending a lot of time fiddling with global adjustments can often be better spent treating different areas of the image differently.

But I have no problem with the current "Default" settings in ACR/LR...the only thing that would prompt me to alter the initial default settings for my raw captures is in the case of my P65+ back...I've had to create a custom dual-illuminant DNG profile that I use by default for those captures from that camera.
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« Reply #18 on: February 06, 2011, 07:50:48 PM »
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From Peter_DL:

From Schewe:

>>So, do you think Thomas was wrong when designing the relationship of Blacks, Brightness and Exposure?

Perhaps Exposure should have been a one-way tool that you could only use to darken the image, not lighten it, with another tool for setting the white point. Again, I would have been happy just to have a point curve, but for those unfamiliar with curves this arrangement might have prevented washed-out mid- to upper-tones in their photographs.

I don't think so. Exposure does what its name implies. +1 is the same as increasing the camera exposure by one stop and -1 is the same as decreasing the camera exposure by one stop. The control is linear and +1 exposure is the same as multiplying all the channels by 2. Negative exposure can correct overexposed images, but can only be carried out so far if the RGB channels are clipped. Because of white balance, the green channel usually clips first and the blue and red channels may have enough data to recover the blown channel(s). However, when all channels are blown, recovery is not possible. In this case, ACR leaves the blown channels at white and uses a tone curve to adjust the remaining channels.

In the screen captures shown below are a sequence of ColorCheckers exposed in one stop increments. The limits of negative exposure are shown. In the bottom row of neutral patches of the most overexposed shot, the left patches can not be lowered to less than 255. Some of the colored patches are similarly affected.

Positive exposure is the tool to correct for underexposure. What would we do without it?

Regards,

Bill



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Peter_DL
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« Reply #19 on: February 07, 2011, 09:18:42 AM »
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Exposure does what its name implies.
... The control is linear and +1 exposure is the same as multiplying all the channels by 2.

Not sure.
Exposure in terms of linear scaling should not affect color saturation, at least not without further adjustments on the top.
But it does at times:

Below example shows a crop of a sunset shot. For reference, the starting rendition (linear; everything zero-ed; baseline matrix profile) is given at the left. The mid image shows a +Exposure setting barely before clipping of the red channel in the upper right corner. The right image is based on a corresponding WP setting via the Point Curve tab – precisely adjusted in ACR to the same R value of 250 of the same measuring point in the upper right corner. Finally the right image remains somewhat darker compared to the mid one, but then the mid image apparently also shows a fading of colors.

Peter

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Note: while the screenshot may have suffered from conversion to sRGB (of course after assigning the monitor profile), the reduction of saturation can also be seen and concluded from the numbers in ACR i.e. R:B = 250 : 184 (mid) vs. 250 : 161 (right).
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