The "look" of a particular TV network

I’ve always noticed that of the three networks in the US, ABC, NBC, and CBS seem to have different “looks” to their shows. Mainly I’m talking about prime-time dramas, but it applies to sitcoms and other shows as well.

For the most part, ABC shows seem to have a sharp, very clear image, while NBC and CBS shows often seem to have a softer look to them. For example, think of the hazy look of “The West Wing” and the sharper look of “Alias” or “The Practice”. Are there different standards in equipment used by these networks?