Luminance masking is a special masking mode that works on the brightness of different parts of the image. Basically, you can set a threshold where areas that are lighter (or darker) than your chosen value will not be affected by your adjustments.
One good example is where you want to use a graduated mask to darken a sky in an outdoor shot but there’s a tall building sticking up into the sky too.
Normally, this building would also be darkened by your graduated effect, and while you can get away with that up to a point with careful positioning and toned-down adjustments, it’s obviously not ideal.
So here’s the thing. Maybe it’s just me, or the kinds of images I work on, but out of all the (many) times I’ve used luminance masking to try to refine and restrict a mask to specific areas, I’d say it achieves a useful result maybe one time in five.
What you really need is a mask that affects the sky but shields the building, and that’s what – potentially – luminance mask can do. You can use the difference in brightness between the building and the sky to protect it from your sky adjustment.
It’s probably easiest to explain this with an example – and also to show the limitations of luminance masks. It’s not something you’re doing wrong, simply a limitation in what they can do.
This is a shot of the extraordinary Hallgrímskirkja cathedral in Reykjavík on an icy February afternoon. That sky does not look as intense as it did in real life, and I want to add some darkness and drama without affecting the cathedral, which is dark enough already.
1. Adding a graduated filter effect
So here’s a ‘straight’ sky adjustment using a graduated mask and some curves adjustments. The sky is fine, but the cathedral tower is a bit of a disaster and makes my adjustment look obvious and clumsy.
2. Adding a luminance mask
What I need to do is add a ‘luminance mask’ to the gradient mask so that my adjustments apply only to the sky, which is lighter, and not the cathedral, which is darker.
I’m working in Capture One, which lets you adjust the ‘luma range’ for any mask (it’s a slightly different term for the same thing). Other programs have their own versions. In the latest versions of Lightroom you would need to ‘intersect’ your mask with a luminance range.
The names and controls are different but the principle is the same. I have a black-to-white bar with sliders for setting the black/dark cutoff point and the white/light cutoff point. Capture One offers a pair of markers for setting the blend points, as does Lightroom (if you alt-drag the markers).
3. Shielding the dark tones
First, I’ll enable the mask overlay so that you can see what’s happening more clearly. At the moment the mask covers the building as well as the sky, but if I drag the left-most marker to the right, it progressively shields the darker tones in the picture and you can see the mask overlay has disappeared from the cathedral tower.
4. So what’s the problem?
Well, you’ll see it’s also disappeared from the top right corner of the sky. It turns out that this is as dark as some of the tones in the tower, and if I move the slider to shield the tower completely, some of the sky is affected too – and if I drag the slider back to keep the sky fully masked, too much of the tower is affected.
This is not because I haven’t used the controls with enough precision or care, it’s simply because the sky tones are not fully separated from those in the tower – there is too much overlap.
This is the fundamental weakness of luminance masking; it can only work when the conditions are right. I generally find that’s not very often.
5. Do the feathering/blending sliders help?
Sometimes, but not here. These are designed to smooth out abrupt tonal transitions created by your luminance mask. The trouble is, they won’t stop this cathedral tower being darkened, they will only help disguise the areas where it’s happened.
6. Swapping to the right tool for the job
Sometimes the masking tool you actually need isn’t the one you think. This image looked a prime candidate for a gradient/luminance mask to darken the sky alone. In fact, the solution was to select the building, which let me tweak the tones for this first, then invert the mask so that it applied to the sky, and enhance it that way. That proved both quicker and more effective than endless fiddling with a luminance mask.
So here’s the thing. Maybe it’s just me, or the kinds of images I work on, but out of all the (many) times I’ve used luminance masking to try to refine and restrict a mask to specific areas, I’d say it achieves a useful result maybe one time in five.
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