How does the mechanics of a Matte box work if it is attached to the rails? My 50mm lens breathes in and out, so wouldn't the matte box have to move as well with it?

Hi David, do you mean that the lens barrel itself physically moves when changing focus? I know that some zoom lenses which breathe will do this and others won't... depending on the lens, the slight change in focal length brought about when changing the focal plane may or may not result in a physical movement of the lens barrel.

If it does, it shouldn't affect your matte box since the box itself is mounted on rails. If enough clearance is left between the outer diameter of the lens barrel and the inner diameter of the matte box, and the connection between the box ring and the lens ring is made by a flexible bellows or a cloth shroud, then a slight movement of the lens barrel shouldn't affect the matte box.

Great thanks. Yes, my canon 50mm breathes.