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I tried to find the answer to this on the web, but no success so far. My question is very simple, when creating bump and roughness maps I'm finding that as I adjust the levels of my greyscale image that the histogram is not a solid graph anymore, but the greyscale values have empty pixel spaces between, vertical line, then maybe 2 or 3 pixels bland, then vertical line, etc. Every histogram so far that I've looked at in the MXM library (not that many I've looked at yet) are all solid, which means a full range of those values from one end to the other.
Is this something I should be concerned about? And if so, how do I "fix" that to include all the values in that range?

Use 16 bit images and apply a small radius blur (repeat as necessary). It's what I do for displacement maps. I imagine it would also be good for bump maps.