The f / number related to describing the speed of a lens. I've seen people rate this lens or that lens as a 1.8 or a 1.4 and tie that in with clarity and sharpness. I wouldn't think that a slower lens would necessarily have less clarity or sharpness. I would think that say a Canon ef mount L series lens at f4 would be clear and sharp compared to a tamron or sigma 50 or 85 mm at f1.8. What gives?