I've been looking into long and somewhat fast lenses for various reasons. I cam upon this lens 100-300mm F4 EX DG APO HSM made by Sigma. From what i have read it is pretty good and for the price tag of only 1k I think it fits the bill. Anyone have any idea if this is a good lens? Also do extension tubes degrade quality and slow the f stop? Thanks.. Also what are good extension tubes as I dont really see much of a difference.. Thanks all.