The Sigma uses gel filters in the rear of the lens and after trying an IR gel back there I threw up my hands in surrender. They are not practical, cant clean them, scratch too easily, too small, and aggravating! I knew there must be a solution to his lens using regular filters. Since I am not using full frame I tried inserting a filter into the lens from the front and checked for vignette. The D70 had none. I needed to keep the filter off the front element so what to do. I eventually ordered a 67mm (almost perfect fit) filter extension ring and inserted it into the front of the lens. I shimmed it up with some tape and if fit very nicely and snuggly and doesnt turn when I screw in a filter. This setup will not work with a full frame, vignetting will occur.