I've searched on the web, but I can't seem to find any existing free software that could be used as a base for contrast-based autodetection.

Basically, if you can detect the total contrast of a scene, you play with the focus until you get the contrast to a maximum. That is your focused point. you could do selective focus by using a bounding box around the area you wish to focus...

Does anyone know of a library that supports getting the overall image contrast level?