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Message: <blockquote data-quote="CCRII" data-source="post: 1325926" data-attributes="member: 58093">First I have heard that the Sigma zoom 120-300mm f/2.8 is actually sharper then the prime so something to consider in your decision.300mm is a short focal length for birds. That being said the canon 300mm f/2.8 IS is a magnificent BIF lens.You will probably need to use teleconverters in order to get close to small passerines or the like. Canon takes teleconverters well (not ideal though), the sigma I am not sure on. Anyway I can tell you that the image quality at 1.4x is good, but not as sharp as say the 400 f/5.6 however they are near identical at f/8 with the 400 f/5.6 getting the slight edge from what I have seen (although there is more to it then just sharpness, however it is a good place to start when considering glass imho). The 2x teleconverter you get good reach, but images will appear fuzzy or slightly blurred no matter how steady your shot is. Could be the teleconverter I had, but I was never able to attain a crisp shot. Your mileage may vary! I will say this, that the Canon 300 f/2.8 IS is extremely fast focusing even with the 1.4x tc and may be argued that it is a touch faster then the 400 f/5.6.I think the canon 300mm f/2.8 is way overpriced imho considering how old the technology is. It is very heavy and even with IS you may have trouble hand holding it. I was able to hand hold it,  but it was far from easy and I think the use of a good monopod would come in handy.</blockquote>

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