Get Canon 135mm f/4L at B&H Photo.
This aperture series evaluates optical performance at full shift (12+ mm). Some downward angling of the camera was still needed, but the lens is using maximum fall (downward shift).
Full shift is generally a poor situation with most all shift lenses; most break down quite badly in optical performance and require f/11 for acceptable (just) performance. But a 135mm lens is easier to construct to have a larger and high quality image circle than a shorter focal length.
Includes images at sizes up to full camera resolution from f/4 through f/11, and crops also.