Using eyedroppers from Levels?

Question

After setting white, mid, and dark points with the eye droppers in the levels:
1. Why does the histogram looks different, with broken lines. What does this mean?
2. Is setting these points a way to remove color casts?
Thanks.

Answer 1

The "gray point" dropper is indeed one way to help remove a color cast.
The broken lines are because you're working with 8 bits/channel data, and because you're moving the black and white points in from the ends you're causing the 8 bit values that used to be spread across a limited range to be spread across the full range from 0 through 255.  With 8 bit math, some resultant values are simply impossible to get, hence the "gaps" in the histogram.
I really don't think I'd use the Levels function to bring the black and white points in, myself.  Personally I prefer curves and to never actually move the black and white points, but to adjust things so the values "roll off" near the ends.  This avoids clipping things to black or white.
But even more basic than that:  Perhaps someone here can suggest an entirely better workflow if you'll let us know what kind of data are you opening, to start with...  Are you opening photos from your camera, JPEGs, web graphics, or what?
For example, you may want to consider doing your editing on 16 bits/channel data for your editing to increase the accuracy of operations. This implies you'll want to open raw photos from a digital camera at a higher bit depth (e.g., from raw files using Camera Raw into 16 bits/channel), and set your black and white points and correct color casts early, in Camera Raw.
-Noel