I don't like my new lens I also bought another new lens.

There's three issues. The first is that, unlike a zoom lens, it's always at 400mm - which, shouldn't really be a problem since I usually use the long-ends of zooms, but with, say, a 100-400 I could start out at 100 and zoom in while tracking the subject. With this it's in full telescope mode the whole time and it's really hard to match up what you see with the naked eye with what you see in the viewfinder. Will I get better with practice? Sure. Should I have bought the 100-400? Yes.

The second is the lack of in-lens image stabilization. I wanted to use it for video, but the in-body image stabilization isn't as effective as I thought it would be, and it still results in too shaky of an image. This is using a tripod with a fluid head. Should I have bought the 100-400? Yes, that one has in-lens image stabilization.

The smart thing to do would be to return/sell the 400mm prime, along with my 70-200/4LIS and buy the 100-400IS, but I am not a smart man. I also want to keep my 70-200/4LIS because it's the largest lens I can balance on my gimbal and that's really useful for shooting telephoto handheld.

So the second smartest thing to do would be to sell/return the 400mm prime, add another $600, and buy the 100-400IS. However, I still am not a smart man and I impulse bought a 24-70/4LIS instead last night.

That said, I do need a midrange-zoom walkaround lens for the documentary I'm working on, and the 24-70/4LIS fits that bill.

So at this point my plan is to hold onto the 400mm prime and see if I get used to it. I have a trip down to Kentucky next month and plan on shooting some horses and other wildlife with it while I'm in some relative warmth. Oh, which is the third issue. There's no wildlife out where I live right now because it's too darn cold. I could go to the airport and shoot planes taking off but that's boring and the chances of a cool plane crash are so slim it's not worth it.

I've also decided to hold onto the 16-35/4L IS because now I understand the limits of in-body stabilization and like the idea of having a well-rounded set of stabilized zooms.

Always, at this point I have the following lenses:

Primes:

EF 50mm f1.4

EF 85mm f1.8

EF 400mm f5.6L

Tilt-Shift:

TS-E 17mm f4L

TS-E 24mm f3.5L II

TS-E 45mm f2.8

TS-E 90mm f2.8

Regular Zooms:

EF 17-40 f4L

Stabilized Zooms:

EF 16-35 f4L IS

EF 24-70 f4L IS

EF 70-200 f4L IS

I'll probably sell the 17-40L, except that zoom is so light and smooth on it, I might want to use it for some documentary shots.