While I packed a teleconverter in the bag also, I have to say, I'm not convinced by them. They slow focus and degrade the image; I avoid if I can. But this led me to an idea: wanting nothing to go wrong in three days of shooting at the British GP, I felt it prudent to pack a spare camera body just in case and it struck me that if I took my old D5100, not only would I have a back up to the D4, but also, if too far away from the action, I could use the small sensor crop factor to my advantage, turning my 400mm lens into a 600mm lens. Genius! And with the D5100 having 16.2 megapixels, and the D4 having, err, 16.2MP, suddenly this felt like quite an interesting experiment.
BUT, and it really is worth capitalising, when I got home and loaded the pictures from the D5100 on the computer, they looked simply superb. They achieved focus almost every time, and it needs to be remembered here that these are among the fastest cars on the planet (focus set to continuous auto) and while the effective 600mm lens length reduced the need for cropping, when you do want that close in frame, so crop aggressively, the D5100 resolved the detail beautifully. There were so many 'keepers' in the batch that selecting one or two to display was not easy, but the three captures below taken on the D5100 show just how good the results were.
However, it also proves what the experts say again and again, the lens is more important than the camera body and given the choice between spending a ton of money on a D4 body with a budget lens, or buy a budget body (D5100) and a first class lens, the latter is surely to be preferred. What's made all of these pictures possible is how good the Nikon 200-400mm lens is, whichever of the two bodies it's attached to.
I wont be leaving the D4 at home any time soon in favour of the D5100, but I am more than happy to have the D5100 as a back up camera. And it does highlight just how good even budget DSLRs are when taken out of Auto and given purpose with good glass.