What would be the most efficient way to only capture at each new frame in a camera feed?

I want to measure every frame of a webcam stream, but no more than once.

Is there a good way of going about this that opencv deals with naturally, or am I forced to do image comparisons? It feels like a healthy thing to ask someone who's dealt more with opencv than I have before I go about the endeavor.

In my case, I'd like to limit the capture at 25 FPS for example.