I think it also depends on your 100-400 and body combo.
Some are sharper than others.
the 60d does not allow at home adjustment so what you have is all youre getting unless you send it into
Canon to make sure all is well.
I shoot a 7d and 100-400L and regularly crop above 50%. If you check my flickr page MOST of them are 50-80% crops. Some of them are 100% and some are hardly cropped at all.
I would say its RARE if I get a subject to fill even 1/5th (20%) of my viewfinder. I hike and shoot so I dont get the opportunity to easily set up for shots in my garden or backyard.
See if you can tell the difference when viewing on a big monitor (21" +).
Keep your shutter speed up high as that will help with stopping any subject or camera motion.
I use a monopod for most of my shots as I THINK it gives a slight bit more stability if I need to drop my shutter speed below 1/800th. It may not do a thing but I like it.
For a quick rule of thumb for me
tiny birds like hummingbirds, I dont bother shooting them over 10ft away (3m or so)
warblers/sparrows, I basically dont bother more than 20ft or so (6m)
Doves, quail, and other normal sized birds 25-30ft ( 10m)
Rapors I will take whatever I can. 75ft or so (25+meters)
Large shore birds like Egret and Heron are about the same as raptors for me.
With that said, I sometimes get away with nice shots at farther than the above distances but its not typical.
Heck sometimes they look like garbage even when I'm closer. LOL
I hope those approximate distances help.
Oh and post processing can REALLY take a decent in camera photo and turn it into a nice printed shot.
I print 11x14 mostly but sometimes if I really like a shot I have done 20x30 without issue.





