Auto Whites and blacks in Lightroom -- what is behind this adjustment?

[adann]

When I Shift+double-click the white or black slider in Lightroom, what is Lightroom optimizing for?  I'd like to understand what is behind this adjustment so I can explain it to a student.

In The Digital Negative, Jeff Schewe says the white and black sliders are for managing white and black clipping, which makes sense.

However, when I shift+double-click the Black slider for the image below, I see I'm actually introducing clipping.  So what is Lightroom doing with this adjustment shortcut?

Here's an unadjusted image:

Here is the adjusted image after Shift+double-click on the Black slider:

The black adjustment of -20 seems to have introduced clipping on the lower part of the branch.

I like the results I get with the auto-white and auto-black adjustments but I think they are doing something other than managing clipping.

[Rafael Aviles]

I believe that the advise given in many places regarding adjusting the black and white points manually using the sliders in LR is to do it until you “just begin to see a little bit of clipping”. If that is the case, the auto-black adjustment you show is doing the same thing.

[adann]

Thanks for your answer.  I'd agree that's the correct way to do a manual adjustment, but if you look at the screen shots in my question, you'll see that the auto-black adjustment added clipping that wasnt there previously.  So it doesnt work the same. 

[Rafael Aviles]

I am not sure I understand you. Say we both begin with a picture that has no black clipping. I manually adjust the black slider until I just begin to see some clipping, and I stop there. So, I arrive at the point where I have adjusted the black level as recommended. And yes, I have introduced a little bit of clipping that wasn’t there before, but that was the target. In your case, you shift-double-click the black slider, and it automatically moves to the point where you see just a tiny bit of clipping, which wasn’t there before. How are these two processes not the same?

Of course, this implies that having a tiny bit of clipping is OK, which is very often the case, especially when it happens in unimportant areas (no detail is lost). And, it ensures that you have stretched the histogram to its limit, without going overboard.

[adann]

Rafael, I think you are answering a different question.  My question is not about manual adjustment of the slider.

[Rafael Aviles]

Right. Your question is “what is the auto adjustment trying to accomplish?”. And I answered: "it seems to be doing the same thing one would do manually”. That is, move the black point to the left, until one sees some clipping.

[Rafael Aviles]

I forgot to add, and that is the expected direction to move the black slider when adjusting a picture that has no black clipping.

[Jeff Schewe]

The -20 Blacks that result in a blue channel clip is actually to be expected. And while it's correct that the Whites and the Blacks settings "manage clipping" that's not to say you should NEVER clip, just be aware of it :~)

Sometimes intentional clipping helps an image...

[adann]

Jeff, very kind of you to answer this one.  I've really enjoyed The Digital Negative.

To follow up, I'm not sure why the -20 black clip when I shift+double-click the black slider would be expected.  What is this "auto" adjustment trying to accomplish?  Said a different way, what is the algorithm triggered by this keystroke optimizing for?

[Jeff Schewe · Accepted solution]

adann  wrote

Said a different way, what is the algorithm triggered by this keystroke optimizing for?

It's optimizing the black point according to Thomas Knoll's opinion of what a black point should be. It's not always 100% correct, more like 80/20 and sometimes does something a bit less than optimal, but it usually is in the correct direction. From the result, season to to your own personal taste :~)

[adann]

Thanks, Jeff.  That's very helpful.

[Todd Shaner]

adann  wrote

However, when I shift+double-click the Black slider for the image below, I see I'm actually introducing clipping.  So what is Lightroom doing with this adjustment shortcut?

One thing I can tell you is that simply SHIFT clicking on the Whites and Blacks sliders will rarely produce proper results. This is because the PV 2012 Tone controls are interactive and should be adjusted in a top-down direction starting with the Exposure control and ending with the Blacks control. You can download a PV 2012 Basic Tone panel adjustment tutorial I created here:

https://www.dropbox.com/s/lsl4jfswwoc1yo4/Lightroom%20PV2012%20Basic%20Tone%20Control%20Adjustments....

If you use SHIFT click Whites on an image that has raw data highlight clipping before adjusting any other Tone controls, it will most certainly go to the extreme -100 setting. Obviously this is not a proper setting. In addition if you SHIFT click on the Whites or Blacks controls after using the 'Auto' button to automatically set all of the Tone controls the numbers will most likely change. With an image that has extensive raw data highlight clipping the Whites control will again most likely go to the -100 setting. The SHIFT click Whites clipping threshold appears to be very near 0.00% pixels (i.e. no clipped pixels) while the Auto Tone button Whites clipping threshold is in the range of 0.01-0.10% pixels (i.e. some clipped pixels). The Auto Tone and SHIFT click Blacks clipping thresholds are different, but only slightly so still usable. For more details on setting these controls please refer to the tutorial.

(Auto Tone Button Whites Setting +7)

[adann]

I dont think you are right about the adjustment to -100.  When I shift-click the white slider in my example (after reset), it goes to -34.

I understand that all the sliders in the Basic panel impact each other.  And I'd agree that the sliders in basic are usually best worked top to bottom.  I disagree however that shift-click always yields a bad result because I've found plenty of cases where that was not so.

You don't have to believe me.  Read Scott Kelby's article in Layers Magazine: http://layersmagazine.com/setting-your-white-point-and-black-point.html

I still don't have an answer to my question, though.

I'd like to understand what Lightroom is optimizing for when I shift-click the slider.

[Todd Shaner]

adann said

I dont think you are right about the adjustment to -100.  When I shift-click the white slider in my example (after reset), it goes to -34.

It is image dependent and you will only see this behavior (SHIFT click -100 Highlights) if the image has raw data highlight clipping (i.e. over-exposed in-camera).

Using the image I posted (IMG_0049.CR2) SHIFT click on Whites consistently produces a -100 setting regardless of the other Tone control settings. This is because the Auto Tone button and SHIFT click on Whites use different clipping thresholds and the SHIFT click on Whites is calculated as if all Tone controls are set at 0. IMHO the SHIFT click Whites behavior is simply an incorrect implementation (i.e. bug)! Whites clipping levels should only be calculated with all of the other Tone control settings applied. It is not doing that and I can't think of any situation where it would be useful. Please feel free to download the raw image file I posted and try it for yourself. I'm sure you can find some of your own as well that exhibit the same behavior.

Dropbox - IMG_0049.CR2

As Jeff Schewe mentioned and I agree, "Sometimes intentional clipping helps an image." How to arrive at the proper settings is explained in the tutorial I provided. Please let me know if you have any further questions.

https://www.dropbox.com/s/lsl4jfswwoc1yo4/Lightroom%20PV2012%20Basic%20Tone%20Control%20Ad justments...

Peace