Here’s something I’ll bet you haven’t thought about all that much, if at all: every Nikon digital camera is capable of autofocusing when there’s an AF lens mounted that has an aperture of f/5.6 or wider. Since all AF NIKKORs have f/5.6 or wider apertures, why should this be of interest? Why would I even bring it up? Because it’s not always true; sometimes an AF NIKKOR won’t focus automatically.
Say you’re using a telephoto NIKKOR that has a maximum aperture of f/4—like the 500mm f/4D, or the 600mm f/4D, or the 200-400mm f/4G—and you happen to be using it with the 2x teleconverter TC-20E III or the 1.7x TC-17E II. Do the multiplication, and the math will tell you that you’re effectively shooting with an f/6.3 or f/8 lens, and that means only manual focus is available.
Except when you’re shooting with Nikon's newest HD-SLR cameras, the D4, D800 and D600.
The reason for the f/5.6 AF limit has to do with the transmission of light through the lens and the contrast of the image reaching the AF sensor. But now, in the three newest Nikon HD-SLRs, advanced sensor technology has resolved the issue, providing greater light detection and critical contrast.
This is big news and a big advantage for professional and advanced amateur shooters who use big glass and a teleconverter for whatever reason and subject. Could be a nature photographer in need of extra reach from a safe distance, or a fashion shooter using a 500mm lens and a 2x teleconverter to compress the scene to create a stylish effect. Imagine a wildlife photographer trying to capture an eagle in flight and having to manually focus his lens, or a sports photographer trying to capture a Philadelphia Eagle running down the sideline. With the D4, D800 and D600, AF with any lens and any teleconverter is available to them.
Granted, these examples will be very special circumstances for most of us—that is, until we find ourselves using one of the long lens and teleconverter combinations I mentioned, and we absolutely need the speed and certainty of autofocus.
The development of the technology that makes this autofocusing possible comes from Nikon's ongoing relationships with pro photographers; from asking about what they need in the real world and then going to work in the R&D lab to figure out how to make it happen.
I talk with the R&D folks, and I know that what drives them is expanding the capabilities, going beyond what might have been considered necessary a few years ago and making it possible for all photographers to get the best images under any circumstances.