Has anyone found a solution to get sharp edges and corners with the 16mm fisheye on a Nikon full-frame camera? After spending the last several years in the Canon digital camp, I have recently started using my first digital Nikon – a full-frame D700. After reading the praises of the u.w. performance of the Nikkor 16mm fisheye on these pages, I was expecting stellar performance. However, after my first use of this combination in the field, I have come to the conclusion that these rave reviews must have been written by people using this lens on DX cameras.
Am I alone in finding that edge sharpness leaves a great deal to be desired when using this lens behind a dome port? So far, I have only tested it in the configuration recommended by Subal – behind their DP-FE2 (8-inch) dome, with no extension. The lens front sits even with the back of the port, so it looks like a good placement. Yet, at apertures wider than f10, more than 30% of the frame may be uselessly out of focus. The nature of my photography does not always allow me to stop down, so I may be forced to use apertures of f5.6 or wider, with results that make me wonder why I am shooting FX, instead of DX (when shooting the Tokina 10-17 at 10mm on DX you get the same field of view, with a center that is not quite as sharp, but edges that are much more acceptable, although plagued with blue fringing on highlights).
When shooting horizontal, as expected, the left and right edges are much worse than the top and bottom. It appears that the virtual image curves away from the sensor plane in such a way that near objects suffer the worst. For example, if the center of the image is perfectly sharp at a distance of 3 feet, at the edge the most acceptable focus might be achieved on something at a distance of 6 feet, although nothing on the edges will be really sharp. I find this terribly disappointing because the center is phenomenally sharp!
It is not possible to place a close-up diopter lens on the front of the 16mm to bend the plane of focus, as is often suggested for zoom lenses. The other usual suggestion for edge problems is to use a larger dome. However, for some of us it is not practical to travel with the giant heavy glass domes that are offered, even if one can afford the purchase price. The lens may only be looking through the center portion of a large dome anyway, so all that glass around the edges is wasted. Logically, if the edges are focusing differently from the center, it should be because the dome is either too curved, or not curved enough. If a bigger dome gives better results, that implies that the problem is “too curved” (as long as the lens focuses close enough that the distance to the virtual image is not an issue). In theory, a housing manufacturer should be able to take the mount from a 6-inch or 8-inch dome and put in a piece of glass or acrylic that is a section from a much larger dome. This would have less curvature than a dome which is actually half of a 6 or 8 inch sphere. But would it really work better? I would love hear from anyone with either a practical or theoretical background to shed some light on this or offer any suggestions.
Is the Sigma 15mm any better around the edges at wide apertures on full frame? Has anyone done a critical comparison between the 16mm and Tokina 10-17mm + 1.4X tele (at the wide end) for center and edge sharpness? Thanks.
Doug
One is focusing on a curved virtual image. 
