[dcstep]

I've got a full frame Canon 5D MkII that I've used mainly for landscape, kids, flowers, macro and travel so far. Since I live within a couple of miles of a state park with eagles, owls, geese, falcons, etc., etc. I'm thinking of buying a Canon EF 400mm f/5.6 L lens for bird photography.

I see lots of images taken with this lens, but most are with crop sensors, putting the equivalent focal length out over 600 mm. My sensor has exceptionally good high ISO performance, so I plan to try shooting in the 400-800 ISO ranges in the usually good Colorado light. That'll give me low noise, but I'm concerned about cropping. Since I start with a 21MP image (usually 24 to 27 MB, depending on the subject) even a 100% crop gives me a detailed, low noise image. With birds in the wild, I'm thinking that I'll need crops more like 400%.

Is anyone here using a FF body for bird images and, if so, what lens are you using and what degree of cropping? Are you pleased with the results? 600 and 800mm lenses are huge and cost an arm and a leg. Before I make that kind of investment I want to be certain that I'll make good use of the lens.

It seems to me that pushing the sensor can make up for the lack of reach from the lens, so long as the image is well focused and sharp and you don't skimp on the file size. I just don't see other bird photographers doing this much.

Dave