I’ve had my zoom Nikkor 200-400mm f/4 AFS G VR lens for about five years, and it’s my absolute go-to wildlife lens unless I’m after very small birds (at which point the 600mm is more useful). A common criticism of this lens is that it is sharp on near (less than 100ft) subjects, but soft with distant subjects. Unfortunately, telephoto lens performance on distant subjects can be marred by more variables than one can easily isolate. I recently had a chance to test my lens in the field to see if I could make any definitive conclusions about its performance.When you focus on distant objects with a super-tele lens, here’s all the things that can degrade image sharpness:

• Focus accuracy: Your camera’s phase-detection AF systems are less precise than you might think. And if you use AF fine-tune, a small error in tuning will be very noticeable on distant subjects.
• Camera shake: the farther the subject, the more noticeable small lens movements will be likely to cause image softness. Nikon’s VR (or Canon’s IS) system can help here, but it can’t eliminate ALL lens movement. Also, VR can degrade image quality if you’re using your lens completely locked-down on a tripod. Some recent Nikon designs have a “tripod mode” VR setting to help deal with this potential problem.
• Atmospheric disturbances: Thermal (heat) waves have long been the nemesis of super-tele users on warm days. They can create distorted images or disrupt AF performance (or both).
• Teleconverters: Tele-extenders (converters) can degrade image quality especially if you are using 1.7x or 2x versions. In my experience, the Nikon TC-14E does not drastically degrade image quality on my 200-400 VR, especially if I’m shooting at f/8 (one stop down from wide-open) or more.

I have the original version (Mk I) of this magnificent Nikkor lens; Nikon recently updated it to a VR II version with improved vibration reduction and better (nano crystal coat) coatings. Optically, both the VR I and VR II versions are essentially identical. I was in the Florida Everglades leading a photo workshop when I noticed a red-shouldered hawk about 100 meters from my position. Actually, I had no idea what species it was; I figured I’d use my lens as a spotting scope. I had the 200-400mm lens with TC-14E mounted to my Nikon D300s body, giving me a 35mm format angle of view equivalent to 825mm.

Just for grins, I stabilized my setup and checked my exposure. 1/200s at f/8. I figured using VR with this setup would be ok given the (relatively) slow shutter speed. I focused with single-point AF and captured an image. I checked it. Soft. This result was not entirely unexpected; I knew this image wasn’t going to be award-worthy but instead just a documentary shot.

Still, it seemed as though I could have done better. So I locked my rig down on my Gitzo tripod with Wimberley Sidekick, and this time, I used Live View in the Nikon D300s to focus. Live-view focusing uses contrast-detection instead of phase-detection. It’s slow, but it is usually more accurate. I fired again, same shutter speed and aperture. I checked the image. Wow! Noticeably better! I still wasn’t going to win any awards, but the difference in image sharpness confirmed my suspicion: at great distances for relatively small, low-contrast subjects, the phase-detection AF system just isn’t as accurate as it needs to be. Mind you, I’m not knocking my AF-system. I’m merely testing its limits.

I passed this tidbit of wisdom on to my students for situations where you have enough time to use live view to focus on a relatively stationary subject. Sure enough, we got much better results the next day when we photographed this great-horned owl chick in its nest at Shark Valley.