Originally posted by baro-nite I've been on something of a longer focal length (FL) quest myself lately. As they say, it's a never-ending quest. Getting closer to the birds is always best, when possible. Of course with eagles it often isn't possible. Once you go past 300mm you are definitely getting into tripod-only territory, and to get anything reasonably fast you are also talking large, heavy, and expensive. I settled on the Pentax-A 1:5.6 400mm for now. Light enough that it doesn't require an expensive specialized mounting system for good results; still very portable and even hand-holdable in bright light. Can be had for $500 or less. Manual focus.
You could also look into "digiscoping", i.e., attaching a spotting scope to your camera.
My wife got a photo of two eagles in a tree. She had the DA L 55-300 and she pulled it in as close as possible, and switched to a 6MB picture. When I cropped the photo, it started to get grainy. I thought if we made it a larger picture that we would be able to crop it and would still have good quality. Any comments?