Get Canon 135mm f/4L at B&H Photo.
This aperture series evaluates optical performance on an outdoor scene using tilt to make a sharp scene near to far along a rushing creek. In some lucky cases, the subject matter fits into a plane or something close to a plane. In these cases, shooting wide open at f/4 with tilt or swing can yield sharpness near to far for the desired area.
With a 135mm lens, where would one focus for this scene? No matter what, most of the water would be blurred until stopped well down, and there is just no way even at f/16 to make sharp water from the bottom of the scene to the bend in the creek near top right. Moreover water blurred by motion is quite beautiful, but it still has motion-blur detail/character—but if it is blurred by being out of focus, the effect is not at all the same.
In this case, even stopped down where the water is more blurred (from motion), tilt is of great value in delivering only the motion of the water, not focus blur erasing the streaks of motion.
Includes images at sizes up to full camera resolution from f/4 through f/16, and crops also.
Below, consider that this was shot at f/4 with a 135mm lens. Think about how shallow depth of field is at 135mm—this kind of shot is impossible without tilt.