Advice about Lightroom masks and "artefacts"
I have a beginner question about using the mask tool in Lightroom 4 on a mac.
When i apply an auto mask using the brush and apply some basic exposure changes etc, I sometimes see a light band or series of white "artefacts" around contrasty edges. This is quite obvious when I review the image afterwards.
Subsequent mask attempts don't seem to correct this effectively. In fact the artefacts appear to be almost permanent and unmovable.
Does anyone have any practical advice for correcting or avoiding this issue in the future?
Many thanks in advance!
If you turn on the "automask" option of the local corrections brush, you are very vulnerable to enhancing any digital artefacts in the underlying image, especially but not only, along such edges. This makes a harsh boundary with blocky irrregularities. Especially this happens when working on a JPG image, or one that has at some previous point been JPG compressed. JPG compression tries to hide its artefacts so they are below the level of our normal attention, but Automask can "see" them.
One answer is to turn off automask for doing certain edges, which allows you to paint using a smooth feather. Different edges can be done in a more or less soft / hard manner depending on the brush softness, size and flow - and this allows convincing edits to be made even within defocused parts of a picture.
Another method that works well, is to deliberately paint across / beyond the edges in a smooth manner (without automask), and then to use the Erase option of the same brush, to cut back to the edge from the other side - with or without Automask as the occasion dictates.
Doing it this way, Automask is (in my experience) a lot more useful. Any imperfections or artefacts will be occuring in areas where the adjustment effect is being applied at zero vs low percentages, when you are Erasing - rather than (as your example shows) showing distracting variations between full vs partial percentages, when painting positively. Also you can correct erasing as much as you want without getting an uneven result... for example, you can make a hard edge first, and then come back with a larger radius erasing brush and automask turned off, to "feather" it back a little. Correcting and feathering positive painting, is always more difficult to do in a smooth manner.
Another way to look at that: you could be making use of the relatively even colour/tones of the non-sky items, to discriminate where the sky edge is, rather than using the highly variable colour/tone values that sky edges tend to accidentally exhibit.
Looking again at your image (which I could not view while posting) it seems you may have painted your adjustment onto the darks, up to the sky - rather than muting down the sky with negative exposure as I had at first assumed. So some of my comments need to be reversed - in a case where you are raising exposure due to backlighting, you may prefer to use the better-exposed area (the sky) as your masking guide rather than the darker area.
It depends whether the underlying "problem" is darkening the light areas (with blown pixels causing an issue) or lightening the dark areas (with noise artefacts causing an issue). And depending how you do it, it may be worse to go across an edge, or to stop short, but the ideal always is of course, to coincide with the edge using the appropriate degree of softness.
But: there is a really convenient option since LR4 which makes that less critical: a local "highlights" adjustment applies principally to bright areas and has little (unwanted) effect on dark areas, even if your brush has strayed into those. And vice versa, if you are applying a "shadows" adjustment it is comparatively unimportant if your brush has strayed into light areas (on which "shadows" adjustment will have little or no effect).
If you use the single Exposure correction, however, then that has a clear effect on both dark and light areas - and you will therefore need to avoid any such imprecision.