Very soon after I purchased my Nikon D3100 starter kit, I purchased a Sigma 70-300mm f/4-5.6 APO DG MACRO lens. I've started experimenting with that lens so I can start to understand how to use it and can become comfortable with it. Unfortunately, when I've looked at photos I've taken with that particular lens, I've noticed a small, dark smudge in the upper left quadrant of my photos. I've tried gently cleaning the lens, but it is not removing the smudge in photos. I have only used a clean cloth (the type you use to clean glasses or pc/laptop screens) in an attempt to remove any surface dirt or oils that might be causing the smudge, so maybe I need some type of liquid solution that's specifically for cleaning lenses before I can really determine if the smudge is caused by something inside the lens? Thanks, in advance, for any advice you can offer.