Hi Everyone I bought a pretty nifty MD mount sigma zoom k-II 70-210 f/4.5 off ebay some time back. It's a pretty awesome lens with great bokeh and decently sharp images. Now comes the problem, the glass element at the very front (facing the subject) seems to have a circular spot (oil mark type thing) in the very center. It appears purplish in color and sort of looks like the mark the lens gets if you touch the glass with oily fingers. My hunch is that it's due to this stain that I've been getting low contrast images, that lose some of their sharpness, infact the mark shows up in on the bokeh like shown below Reigniting that fire by Amlan Mathur, on Flickr So the question is how can I get rid of this? Can I open up the lens without ruining it (the screws are on the side the cam mounts), what would be the easiest way to do so. Is there any alternate, like some people suggested that I keep it in sunlight or with some silica gel packs.