Both lenses can go to an f/3.5, so not the fastest on the block. I've been using my canon lens (for my 350D) for a few months, and haven't had any problems with it. However during my routinely surfs on ebay I found a cosina 28-80mm lens for £3, and thought I'd buy it - just to see.
Well it arrived today, and surprisingly works. My question, considering they have the same focal range, should I use the canon or cosina? Or should I do some test shots myself.
Also, when my Sigma 28-200mm Aspherical IF DG Macro arrives, should I stop using the canon/cosina and only use the sigma? As the sigma covers the 28-80mm, plus the hypothetical 80-200mm. Unless the canon/cosina will be sharper at a specific focal length.