The image on the DVD has width Dw and height Dh. The movie itself occupies a width of Mw and a height of Mh. When scaled for display, the image from the DVD is D′w wide and D′h high and the movie itself occupies M′w wide and M′h high. The display aspect ratio is D′a. These values are linked by:
From this you easily get:
The values here are for -vop scale=M′w:M′h,crop=Mw:Mh.
You may find this article (also available here) more easy to understand.