For Canon EOS 1D/MkII/MkIIN/MkIII and 1Ds/MkII/MkIII digital SLR cameras, the Canon Ec-A Focusing Screen (Microprism) is a matte focusing screen with a microprism focusing spot in the centre which allows you to preview the framed image in your viewfinder.
The Canon Ec-A is used for general photography and particularly suitable when using f/5.
6 aperture or brighter.