I recently sold my new 2011 300mm f4 IS lens after about six months of ownership. It was the perfect size and weight for a 300mm prime, and the short minimum focus distance was ideal for a variety of subjects. It had an excellent sliding lens hood and a nice soft case was supplied at no additional cost. I really wanted to like the 300mm f4, but at the end of the day image quality is all that really matters, and in this area the lens was a disappointment for me. I use a 40D, and if I was careful to shoot stopped down to at least 5.6 this lens could produce pretty good images, but not significantly better than the 70-300 consumer (non-L) zoom stopped down to f8 that I used to own. My 70-200 f4 IS with a 1.4x teleconverter (280mm) yielded slightly sharper images at f5.6 than the bare 300mm f4 wide open and equivalent images when both were stopped down. I bought the 300mm f4 planning to use it with a 1.4x teleconverter, but this degraded the image noticeably, even stopped down to f8. In addition, the IS on the 300mm f4 was only good for one or two stops max with no tripod-sensing feature, and the AF was not consistently accurate and was too slow to follow birds in flight when using a 1.4x teleconverter. Downloading images at the end of a day shooting with the 300mm f4 was often frustrating – too many missed shots that were either slightly out of focus or simply too soft for my expectations.
I traded my 300mm f4 for a used 2009 300mm f2.8 IS. The difference between these two 300mm lenses is like night and day. The image quality from the f2.8 lens is amazingly sharp and brilliant. The outrageously fast and accurate AF and tripod-sensing IS combined with the very bright viewfinder image make the lens a joy to use. It can be shot wide open with confidence and accepts a 1.4x teleconverter with virtually no image quality loss. With the converter the 300mm f2.8 still focuses faster and more accurately than the bare 300mm f4 lens. Yes, the 300mm f2.8 is bigger and heavier, but it is still quite portable and I routinely hand-hold it with no problems. Many people are happy with the 300mm f4, but if, like me, you expect a little more from an L-series prime lens, rent the f2.8 version and try it for yourself. Is a used 300mm f2.8 worth three times the price of a new 300mm f4? For me, the answer is absolutely yes.