Upside. It works very well optically, stop it down a stop or so and it produces very good images. The shot below was taken over 500ft through heat waves, since it was 34 C. here today in Toronto. Downside: 2 seconds to lock focus, then, press shutter, wait 1.5 seconds and then it fires! NOT an action lens! But for $85.00 plus a free adapter (Olympus deal) it's fine. P.S.: The old 4/3rds lenses do NOT focus on my Panasonic G1!