What you see is not what you get

Rangefinder: What you see in the viewfinder has infinite depth of field. The resulting image will always have less depth of field than what you see.

SLR: What you see in the viewfinder has the smallest possible depth of field. The resulting image will always have as much or more depth of field than what you see.

Any camera: What you see in the viewfinder can never exactly equal what the camera records at the moment of exposure.