I know enough about cameras and lenses to do the job but I can't really answer questions when people ask me. Mine may sound simple but no one has given me a simple answer. 1. if you have a 16mm lens, what exactly is the 16mm measuring? Some have said that the smaller the number the more you get in frame, which theoretically is true, but doesn't truly tell you everything. 2. What is bokeh and what causes it? 3. What causes vignetting? This is what happens with my Tokina when at 12mm. Not a big deal, but curious. 4. What is aspherical glass and SLD glass and what is the difference/similarities?