Optimizing Black and White Conversions

... or, "why do one black and white conversion when you can do two?"

With Photoshop CS3's black and white layer, it is now fairly trivial to convert different parts of an image to black and white differently. For this image, I processed everything above the horizon one way, and everything below it another. Each black and white layer has a mask that is the inverse of the other. Since the whole image is going black and white, you would have to make some pretty extreme adjustments for the dividing line between the two layers to be visible.

This could obviously get even more complicated with more layers or gradient masks, etc., but it's very common for one conversion to work well for sky and clouds and something entirely different to be optimal for ground and trees and so on.

You could even apply different tints that way, though that could yield some hideous results. (This image was tritoned as a whole after the black and white conversions)