I own the EF 300/4L IS as well as the EF 300/2.8L IS, and shoot on a 5D2.


The 300/4 is very sharp, very portable, and performs best from f/5.6 - 8. The corners are a bit soft at f/4, but it is still really quite good. There's some very slight CA that is easily fixed if you shoot RAW. The main issue I have with this lens is that it has relatively old IS, and AF is not the fastest. It does not take the EF Extender 1.4x II as well as the 300/2.8, but it does reasonably well. The one big advantage it has over the f/2.8 is the close focusing ability--it has one of the highest maximum magnifications of any telephoto lens. More than the light weight, it is the one reason why I have not yet sold my copy.


The 300/2.8...well, where to begin.... This lens is ridiculously sharp but also terribly heavy at 6 pounds. Best performance is from f/2.8 - f/5.6, although it is still sharper at f/8 than the 300/4 is at f/4. Contrast is superior to the 300/4. Lateral CA is pretty much nonexistent, though you can see some axial CA if you shoot in high-contrast situations wide open. There is some vignetting at f/2.8 but it becomes unnoticeable by about f/4. It takes the EF Extender 1.4x II quite well. AF is very fast.


In summary, the 300/4L IS will give you excellent image quality on a full-frame sensor, better than any zoom, including the 100-400/4.5-5.6L IS, but if your intent is to use an extender, you'll find the 400/5.6L or the 100-400 to be a better solution.