I have a little question regarding the usage of extenders. Namely, I have a 20D with a 70-200 F/4 L as my "long" lens. In the majority of cases, the 200 mm gives enough zoom, but sometimes a few mm more wouldn't hurt.
A separate, good-quality lens in the 300-400 mm range is way too expensive for the amount of shooting I, for now, expect to do with it, so I'm looking at an extender. I'm leaning toward the 1.4x, effectively giving me a 98-280 mm lens if I did the math okay, which isn't at all bad.
In this post, the 70-200 F/2.8 with the 1.4x came out quite well, but what about the F/4?