Lightroom CC: Weird Math?

A slew of updates today from Adobe with the release of Adobe Creative Cloud 2015. I won’t dive into the tons of details many others out there will cover extensively, surely better than I ever could. But I do want to share something I realized this morning—by accident—that I believe may be useful to some of you: brush and filters behaviour.

With this update Lightroom gains the ability to make local adjustments to the white and black points—very useful and quite welcome. One of the things I loved about Aperture was that ANY adjustment could be transformed into a brush, so the more the better as far as I’m concerned. But as I was testing this I noticed there were inconsistencies in how brush values were applied, with the new brushes behaving even more strangely than the others.

Ok, I’m not an engineer so I may be way off on my technical explanation but basically, I’ve always understood that Lightroom adjustments work against each other—either additively or substractively— and if you add +50 exposure globally (using the Basic panel) and then brush in –50 exposure to a portion of the image, the effects will cancel themselves out giving you 0 exposure where you applied the brush. Makes sense right? You give with one hand and take with the other. And this I understood to be true for any localized adjustments including the grad or circular filters—which is why I found the new erase brush options useful but not earth shattering, since it was already possible to “erase” filter effects by doing what I described above and cancelling out those adjustments; it was just easier and more obvious to use an eraser—simpler is better.

But this morning my understanding of the way Lightroom calculates these changes has shifted, initially due to how dramatically the new Whites and Black brushes behave. My head is spinning a little so let’s make this visual… Three images from the XF 56mm APD campaign I worked on awhile back: on #1 exposure is at 0; on #2 I’ve boosted the global exposure to +85; on #3 I’ve added a circular filter with –85 exposure. Click for larger images.

You’ll notice the exposure within the circular filter on #3 is back to the value of #1—we’ve cancelled the exposure boost in that area by reducing it by the equal amount we had first added. Good.

Now let’s do this with shadows, same deal: #1 at 0, #2 at –100 and #3 at +100. This one is harder to see, look for the blacks in Frédérique's blouse.

Again, #1 and #3 are identical within the affected area. All is well in the world.

But when I use this method with the white and black points something very different happens: there’s no cancellation. Instead, the value set in the Basic panel becomes the baseline and the local adjustment is added after. Let's use the same process again: on #1 the Whites slider is at 0; on #2 the Whites slider is boosted globally at +52; on #3 we add the circular filter with Whites at –52. 

Do you see what I mean? The white point inside the filter on #3 is NOT back to the value of #1; it’s just using +52 as the baseline and “adding” –52… Essentially this is like starting with an overexposed JPEG file and trying to bring exposure down to recover highlights: it won’t happen because they’re blown and there’s no data to recover. Except in this case it’s a raw file and the overexposure only exists within my Lightroom adjustments. The data IS there.

This is getting a bit long but let’s do Blacks so we’ve covered all the bases: 0, –100 and +100. 

Ouch. Now we're in crazy-for-Cocoa-Puffs zone. Ok, I honestly thought this was a glitch with the Whites and Blacks adjustments and obviously to a certain extent it is. But let’s do Exposure again, this time with extreme negative values: 0, –4 and +4.

That’s not what I expected to see. It’s not as terrible as it is with the Whites and Blacks but this isn’t at all a return to the exposure value of #1. We’re seeing hot spots and colour shifting as well. 

Again, not an engineer or a programmer… But to me it seems the math is all over the place and nothing is behaving as expected. Of course life is never perfect and there may be no way to avoid this, I don’t know. In any case, I find it useful to know these things in order to better envision what can and cannot be achieved when processing files so I thought I’d share the tests with y’all. The general take away for me is this:

  • Be careful with extreme values. Not everything can be zeroed-out.
  • Know the limitations. Not everything is logical.
  • When using the new white and black adjustments locally it’s better to intensify than to recover. Also better to stay within minimal adjustment values given how they behave.


One more semi-related thing: if anyone knows how to change the black and white points using the new Tone Curve panel in Lightroom Mobile let me know. I’m talking about both ends of the curve here. Right now I’m looking at it and scratching my head… It IS possible right? Adobe didn’t seriously add a curve tool on which we can’t do this?