This Ec series Super Precision Matte Ec-S Interchangeable Focusing Screen from Canon provides easier manual focusing through your viewfinder on subjects in low light when using lenses with a wide aperture between f/1.8 to f/2.8.
This "S" type focusing screen works by decreasing the focus of the slightly out-of-focus areas around your in-focus subject for a sharper contrast.
Compatible with Canon EOS 1D Mk II N, 1D Mk III, 1D Mk IV, 1Ds Mk III, and 1Dx
Not recommended for ordinary zoom lenses with maximum aperture slower than f/2.8 as the screen will appear dark and grainy
Focusing screens are delicate and should only be handled using the included gripping tool for installation