Originally Posted by
ccphoto
So much stuff to contemplate. I feel for you as I learned all this through trial and error. There are always "other" factors to consider when going telephoto or telephoto zoom. Weight, for me was one of the factors that pushed me away from the Sigma 150-600 Sport, and softness at the edges at 600mm killed the Tamron 150-600 for my birding needs.
I ended up with the Nikon 200-500 5.6 which in solid light is a winner in either DX or FX format when coupled to either my D800 or D810. I let a friend shooting a D7200 shoot it for an afternoon and she got extraordinary results. I was actually rather surprised. Other factors are called the "Sweet Spot" of the lens. With the Sigma, I had to lose almost two stops of light to get the optimum focus at f/11 and with the Tamron, one stop at f/8. The 200-500 seems to do well at all apertures, however it is a fixed f/5.6 lens whereas the Tamron and Sigma are variable at f/5.6 @ 150 and 6.3 @ 600. Losing one or two stops more depending on focal length puts you almost into mid day sun to have enough light to shoot or, into higher ISO's to compensate. There are no free lunches in the world of birding lenses.
I can tell you from experience and lots of it, anything under 400 is only going to put you into the padded frustration room. The exception to that might be the Nikon 300mm 2.8 but you might want to weigh in the 5X cost factor. Same goes with the 400mm 2.8 or the 600 or 800 f/4 which go from $17,400 down to a mere $6,500 give or take. The lesser expensive Nikon zooms, the 28-300 or 70-300 are okay, but unless you can either fly with them or climb into their nest unnoticed, they are a waste of money.
And then, there are the tele-extenders that go from 1.4 to 2.0 and I've only had marginal results while using them. Again, 1.4 loses you one more stop of light while the 1.8 and 2.0 cost you two stops. Shooting birds is about clarity in the eye, and details in the feathers. Shooting birds is about shooting something that moves much faster than you think and thus requires a camera and lens that can acquire a very fast focus lock and which can carry that focus lock throughout the length of the shot.
In general (and I know I am being snooty) the more expensive the camera, the better the birder camera. I get far better shots with the D800 than I did with the D7200 and I get even better over the D800 with the D810. Not sure I've been a great help but my best advice is to not buy something that is only going to continue your frustration. Oh, I did have great results with the Nikon 80-400 4/5.6 but not as good as I have with the 200-500. The trade-off was losing the bottom end so I bought a 24-120 to compensate when I need less. The 80-400 is a pricey little beast but is one of the best lenses I've ever shot.
Give and take, give and take, give and take....Photography lens buying guide in a nutshell.