Luma | a Follow-up
Demystifying that second curve.
This is a quick follow-up to the Archival Dig Compendium post, after some of you expressed confusion regarding luma curves. To be clear, I didn’t write about luma curves in that post but luma range. Both, however, use the luminance (or brightness) of an image to perform their tricks.
LUMA RANGE
As we saw in that previous post, Luma Range is an option found in the Layers panel in Capture One (it’s available in LR, but I’m not familiar with the UI). It lives under layers because it’s a masking tool: it allows us to select a portion of an image by targeting specific luminance values.
LUMA CURVE
The Luma Curve, however, is found in the Curves panel, next to the RGB tab (and right before individual colour channels). If you look at both side by side, you’ll notice the histogram in the background is different: one shows coloured graphs, the other doesn't.
And this is key: unlike the contrast slider or the RGB Curve, the Luma Curve doesn't affect colour. Which is probably the reason it feels strange when playing around with it because we expect contrast to affect saturation. But this can be useful in portraiture, for instance, where adding contrast quickly over-saturates skin tones, requiring further adjustments. With the Luma Curve, it’s possible to make these contrast adjustments without altering the underlying colours in the image. Here’s an example, using the same curve:
For me, it’s always a balancing act: this control over contrast is useful, but there’s a point where it becomes unnatural. Generally, if I decide to use it, it’ll be as a local adjustment.
Another important point to remember is that, although they appear in the same panel, Luma and RGB are TWO SEPARATE tools that can be used in conjunction with each other, even on a shared layer. We can build a curve in the RGB tab and then an entirely different one in the Luma tab—their effects will combine, like everything else in the processing chain.
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That’s the gist of it. Luma isn’t all that esoteric, it simply affects images differently than what we’re used to. Personally, it’s one of those tools I tend to forget unless I run into a specific problem. Even with portraiture, I’m used to applying an RGB curve and then tweaking saturation as necessary (1). Force of habit, really. And I never use Luma as a “Film Curve” either: compressing (fading) blacks quickly results in a weird solarization effect. Something to keep in mind.
I hope this lifts the veil. Hit me in the comments or Slack if you have any further questions.
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Chances are I’m using the Skin Tool in this case which provides even better control.
