I'm sure many of you have seen this. When you turn down the highlights and turn up the shadows, a glow effect appears at the boundary of these two areas. What causes this?
When you turn down the highlights and turn up the shadows, a glow effect appears at the boundary of these two areas. What causes this?
I don't mean to be flippant, but turning down the highlights and turning up the shadows causes it. I think what you really want to know is why doing that causes a "glow." I haven't looked into it beyond messing around with some of my own images for a few minutes, but I think the basic reason is that you're manipulating the image in a way that makes areas near edges stand out. There are a number of reasons that edges look different, including:
reflection: The "edges" of a curved surface (like a cylinder) can reflect light in a way that makes the "edge" of the object in the photo have a very different color.
focus: Out of focus edges blend the edge of an object with the background, creating a different color.
lens effects: Distortions like chromatic aberration can create a visible line that follows edges, again resulting in a different color.
In each of these cases the resulting color change may react to your adjustments differently than the rest of the object or the background does. Here's an example image:
Dropping the highlights and increasing the shadows gives this:
You can now see a blue outline along the upper/left edges of the flower petals, and a pinkish outline along the lower/right edges. Those pixels exist in the unmodified version, but they look the way you expect them to so you don't notice them. Changing the levels in extreme ways changes the colors in those areas in ways that don't look natural.