I've discovered 2 bald eagles nesting in a park not very far from my house, and while the 55-250mm did admirably grabbing a few good shots of the male, I almost immediately began wanting even more reach, to really get up close and isolate him without having to do much cropping. The other issue I had was focusing: I lost a couple of shots because autofocus focused on part of the branch just in front of him. I tried switching to manual focus, but when I got home and opened the full-size image, I saw that my focus was still a bit soft. I just couldn't get enough detail of him in the viewfinder to really hone in my focus.
That said: my contemplations are now what my next lens (or lenses) will be. However, I've reached a bit of an impasse, mostly in terms of cost to quality.
I've seen some brilliant photos taken with the 100-400mm L series lens, which seems like it will have that extra reach I'm hoping to get. Downsides I can see are that there's not a lot of versatility; 100mm isn't very wide on a 1.6 crop. There's also the higher f4-5.6, which means as soon as my light begins to go, so does my ability to shoot.
Now for roughly the same-ish price, I could go with the Sigma 70-200mm 2.8 OS and a 2x TC to make up that extra reach. I know I'm losing stops when using a TC, but when light started to go, I could take out the TC and keep shooting -I'd just have to do some cropping etc.
My only hangups with that are that I've heard enough tales of woe with the Sigma being soft and needing to be sent in to be focused, etc. I know the Canon 70-200 2.8L IS mk II is a much better lens, but it's unfortunately far beyond my budget for some time to come.
Any thoughts/comments as to what the better setup might be? Is there something else I haven't yet considered?