Hi guys! I just got a cheap 70-210mm f/4.5-5.6 Nikkor lens, but when i mount it and see through the viewfinder, the circle in the middle is darkened in the bottom (like when you press the DOF previewer) It also makes a little noise when I use the DOF preview lever (when I use the 50mm it doesn't make any noises) Is this normal or am I doing something wrong? This is what the viewfinder looks like: The darkness varies on that little circle, sometimes is on top, sometimes on the bottom, sometimes darker, sometimes lighter... Somebody told me the lens might be defective, but I just bought it from bhphotovideo.com Any sugesstions? Thanks!!