As I await the arrival of the EF 300mm f/4L IS USM, which I expect will be replacing the EF 400mm f/5.6L USM in my bag, I decided to play around with my EF 70-200mm f/2.8L USM with the Extender EF 1.4x II attached. This results in a lens combination with a maximum focal length of 280mm at f/4 with a closest focusing distance of 4.9 feet. Almost identical to the lens which is in the mail.
I expect the image quality of the 300mm lens will be far better, of course. The difference between a prime lens and a zoom with an extender attached is significant. I'll do some comparisons and post them later on. I'll also compare the 300mm lens with the extender against the 400mmm lens.
In any case, here is a shot of a backlit maple leaf taken with an EOS 50D and an EF 70-200mm f/2.8L USM with the Extender EF 1.4x II at the full 280mm, ISO 200, f/8, 1/800 second. I simply spot metered on the leaf and processed in Canon's Digital Photo Professional using the Autumn Hues Picture Style while reducing highlights a touch and adding a touch of sharpness. I also cropped to a square format to remove some distracting twigs in the corners.
So, stay tuned for comparison shots once I get the new lens. Also, watch for this year's shot to be added to the annual fall comparison. You can read last years entry here.