The AF confirm light will lie to you just like that LCD screen will. If you're having trouble telling if your images are in focus through your viewfinder you might want to adjust your diopter. If that doesn't help enough you might consider getting a better focusing screen for your view finder. (I love my split focusing screen) To me the auto focus system is a lot like your light meter. It's often right on the money, but it's still gonna miss sometimes so you need to really be able to tell in the view finder what's going on.
ok maybe I'm thick but isn't the point of buying say a more expensive camera, like my 7d and better lens is so they work together and focus properly. why should i have to buy a third party focus sensor to get better focusing on my camera.
aslo what do they look like when looking through them? the same or diff? any links for me to see an example?
thanks!
jimi