So, now that I have added the Canon EF600mm f/4L IS to the lens lineup, I just couldn’t resist trying it out with my Canon 2x TC. Of course, I lost autofocus on my 7D due to the 2-stop light loss, but I figured “Hey, I used to focus manually way back in the olden days, so why can’t I do it now.”
All I have to say is “WOW”! Canon really knew what they were doing when they built this stuff. I was shooting at ISO 800 and 400, and considering I was manually focusing at 1200mm (with my almost 50 year old eyes), I’m pretty impressed with the outcome.
These are just simple birds from my back yard, but they really show off the power of this lens. Thank you Canon! These shots, while having been resized for the Web, have not been cropped in any way. The EXIF data is intact on these images if you are at all curious of the settings.
To begin, I focused on my feeder and waited. Eventually, along came one of my local House Finches. I have lots of these guys around the house and they are always willing to model for me.
Wow, not bad. Not quite as clear as at the native 600mm, but not bad, especially when it comes to posting on the Web!
Listening, I could hear chirping coming my neighbor’s yard, so I turn to find another one of the little fellows up in the top of one of their trees! A distance shot! Great test!
Not too shabby. Looks like I might take this 2x with me to Yellowstone next month! Might help out with those long shots of the wolves in the Lamar Valley!
Next up was a local Mockingbird that landed on my bird bath. This was the first time I have seen him/her there so I fired away. (One of these days I need to figure out how to tell the guys from the gals for this species.) You can certainly tell that the depth of focus is really narrow shooting at this range. Still, very clear! 
As I’m sitting there enjoying the early afternoon, I hear my buddies the Bluejays approaching. They always announce their approach and all the other birds scatter. Funny thing is though, these guys really are bullies! They make a lot of noise and scare other birds, but they are about the most skittish birds that visit my back yard. I have the hardest time luring them out for a photograph. They will hide in the trees, swoop down to be a peanut or two, and then right back to the safety of the tree before the camera can even focus on them. But not tonight! With the big gun, I can get him even when he is hiding in the tree! I’m really liking this lens! (I’m starting to get redundant.)
And last but not least, there are always a few doves around sitting on the fence, all made up with their blue eyeshadow. They seem to be able to sit there for hours, just watching me and the other birds. Then sooner or later, a whole flock of them will descend to the ground and start picking up the seeds that the finches have knocked out of the feeders. These guys are good models because they don’t move much, so even as the sun is setting, you can still use a pretty slow shutter-speed with them.
So that was it for the evening. I didn’t see my Northern Cardinals around, but they can’t be far away. Maybe by Saturday morning I’ll be able to catch the “early birds” as they are rooting around looking for those fabled worms.