Originally Posted by
Bobster
should have posted this in here too!
with the lens on the camera, set it to f/8 press your DoF preview button and while still holding the DoF button, take the lens off the camera
now look at through the lens, can you see the aperture blades? if not, you def have a faulty lens and need to send it to be repaired (my 70-200 just did the same and has been sent away for repairs)
and looks like you'll have to pay to have it repaired, because unlike other lens manufacturers Canon only gives you 1 years worth of warranty with their lenses