can some one explain about this please?

[Robbo]

just seen this for sale

EF-s Focusing screen

how does it work exactly?

cheers.

 

[Kemble3]

Canon Focusing Screen EF-S - Exclusively for the EOS 40D and 50D, this focus screen is optimized for wide-aperture lenses from f/1.8 thru 2.8. Areas that are slightly out of focus appear more out of focus, making it easier to tell when focus is right-on. It is ideal for users who frequently manually focus in dim light with fast lenses.