The lense does exact what it should. I have -6 eyesight, so taking photos with a Canon T1i and my glasses simply does not work. Without the -4 correction, I still could not get the T1i adjustment to be in focus (to -3). I have added the viewfinder extension and the diopter lense without problem. I can see through the lensefinder without problem and in focus.
Bottom Line Yes, I would recommend this to a friend