I bought one of these little add-ons off Ebay a week ago and it showed up in the mail so I thought i'd do a little write-up on it. I bought this unit because I hated pressing my nose up against the camera and because it allows for a greater Diopter range so I no longer need to wear my glasses when i'm using my camera. They're easy enough to fit, just unclip your OEM eyepiece and fit it to the Extender... Then slide the extender onto the camera body where your OEM eyepiece used to be. What does it do? Basically it gives the user a larger diopter range and prevents the users nose from pressing against the camera back. As for Viewfinder difference; the extender effectively 'zooms out' the viewfinder and things may appear smaller but really they are not, the focal points are still visible enough and I can still read the shutter/appeture information with ease. well worth the money if you wear glasses or if you're after some comfort for those long shoots in the field. The unit will fit most Canon EOS SLRs.