This Eg series Super Precision Matte Eg-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.
★★★★★ Makes manual focusing so much easier in low light with my fast lenses. The increased contrast around the focus point is a huge help, and installation with the included tool was straightforward. Exactly what I needed for my 5D Mark II.