Using Lens Hoods to Improve Contrast
Many people never bother with lens hoods unless they're shooting into the sun, figuring that they otherwise don't really need them. Yet a lens hood does more than just prevent flare; they also will help improve contrast, even when the sun is behind you.
The way we see things is based on the fact that the reflect light. That light, however, is not directed solely towards you. In fact, from a typically irregular surface, the light is reflected in virtually every direction. As such, this stray reflected light is coming at you and your lens from everything you're not photographing that is anywhere in front of you. If you could draw a line from the front of your lens to something, the reflected light from it is hitting your lens. While this light is likely not bright enough to cause visible flare, the sum total of it from all sources is sufficient to produce a kind of "visual white noise" that will lessen the contrast of what you are trying to photograph.
So, the moral of this little tale is to always use the proper lens hood for each lens, even when there's no "obvious" reason for doing so. The improvement in contrast can vary, but it always there to some degree.