Ever since I started using the D800, I've been excited by the prospect of usable autofocus at f/8. However, as most of you know I've used the 400mm f/2.8 VR with a 2x TC as my long lens combo for a while. While this combination delivered outstanding sharpness and accuracy, there was simply no way to go longer than 800mm even if I could afford to use an aperture smaller than f/5.6.
      I recently bought the Nikon 600mm f/4 VR II lens, mainly with the intent of shooting it with the 1.7X and 2X teleconverters to gauge how usable it is. I did some field testing to assess performance (albeit VERY unscientific!) and thought I'd share the results with you. Hopefully this will help those trying to choose between these two lenses and trying to figure out how effective the teleconverters are with the 600.
      I shot all three combinations (bare lens, TC-17EII, TC-20EIII) from a tripod with a shutter speed fast enough to eliminate any camera movement, and the light was good enough to keep ISO's down too. All shots were taken on the D800, at a fixed exposure and within minutes of each other.The files were processed identically in LightRoom 4 - the only change was a sharpening value of +62 and then an output to JPEG. The 100% crops were taken from the center of the image.

Full image with the bare 600mm lens @ f/5.6 (click on the image for full size)


100% crop with bare 600mm lens @ f/5.6

The bare 600mm is pretty darn sharp - the detail on the fence post is captured pretty well. No surprises here then...


Full image with the TC-17EII Teleconverter mounted on the 600mm lens@ f/8 (click on the image for full size)


100% crop with the TC-17EII Teleconverter mounted on the 600mm lens@ f/8

In the past, I've seen the TC-17 deliver a bigger image without necessarily resolving more detail than the bare lens at the same distance, especially on the 70-200mm f/2.8 VR (version 1). No such complaints here - the quality of the glass on the 600 shines through and really delivers a sharp, usable image. The compromise in aperture hasn't been a big deal for me YET - the D800 seems to take it in stride and focuses swiftly and surely. However, busy backgrounds can still throw it off so you need to give focus a manual tweak every now and then. 


Full image with the TC-20EIII Teleconverter mounted on the 600mm lens @ f/10 (click on the image for full size)

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100% with the TC-20EIII Teleconverter mounted on the 600mm lens @ f/10

With the TC-20, the image in the viewfinder and on the sensor is certainly bigger, but it looks to be like the detailed resolved is similar to the TC-17. Contrast also seems to take a slight hit. Overall, a decent performance but it loses you more light than the TC-17, slows down AF a little and doesn't really resolve any more detail.

The Verdict
I'm pretty happy with the results the 600mm and TC-17 provide, and can safely say that I'll use this combo whenever I need the reach and don't need the AF speed. I wouldn't say that the AF is speed is bad, but I'm not sure it'll be up to handling birds in flight.
      I've always sworn by the performance of the TC-20EIII teleconverter on the 400/2.8, but it looks like the 600 doesn't work as well with this teleconverter. The results are certainly usable, but doesn't provide enough of an advantage over the TC-17 to recommend it. The allure of 1200mm is still too much to resist though, so I'll continue doing more field testing before I discount this combo.
What do you think? Any thoughts and comments are welcome!