Yeah. I don't know. I may want an f/5.6 aperture lens. The Nikon 80-400 is sharp wide open, and it could be stopped down to give a bit deeper depth of field. Maybe even an f/8. Might give you more in-focus shots. And Sigma's AF system can be a bit inaccurate (at least with the lenses I have tried).
Well, f/8 and be there was the order of the day- despite the overcast skies. Even had to stop down more to get a shutter speed slow enough for proper propeller shots.
As for the Sigma's AF accuracy on his D7100, I'll have to let SS comment; however, with the advent of the 'sport' variety of the 120-300/2.8 OS, accuracy may have improved, and effectiveness of the AF can be very highly tweaked using the Sigma dock. That lens, in particular, should honestly ship with the dock, as it has a raft of configuration options that can switch the lens from being more of an 'art' lens for more sedentary subjects to being a pure 'sports' lens for a wide range of tracking scenarios. At least, that's what I've read from those that have used it in depth.
So far, I would say that autofocus was excellent. Perhaps not quite as fast as the Nikon 70-200 but a lot of that I would attribute to shear physics of the heavier glass. I didn't have any issues with front or back focusing, which can be corrected with the doc.
Light certainly wasn't an issue. It really hasn't been an issue for the four years we've done this event. Morning shots are extremely hard because you are shooting directly into the sun and everything is backlit. For the curious, I made a histogram of the f-stop used for my collection of shots from Friday.
Yes, I really did keep so shots taken at f16 and above. The thing to remember is thay everthing is moving. The subject (an airplane) is moving, perhaps on the order of 75mph. The camera is moving as you track the airplane across in front of you and because the subject is only 100 feet away, or so, the angular velocity is very high when the subject is in the best position for pictures. Parts of the subject are moving as well (the propeller). The props turn anywhere from around 5000-10000 rpm. Finally, you have the background. In shots when the background isn't just the sky, the background will have motion relative the the subject because of the movement of the camera.
All those factors compete. To get a good blurred prop arc, say 1/8th of a revolution, max shutter speed is 1/640-1/1200 or so, assuming the motor is a full throttle. At idle when landing, it may only be turning 1500-2000 rpm. In bright outdoors these are the shots that are so stopped down. I high shutter speed will help with the inherent blur of the subject due to my swinging the camera around, sometimes almost as fast as I can physically turn. Unfortunately, the higher shutter speeds obviously kill much of the prop arc, but even more, they significantly reduce the nice blurred background that gives you a sense of motion in a tracking shot.
Extremely challenging, both technically and physically, but a heck of a lot of fun!