When using a rectangular fisheye like the Nikkor 10.5mm f2.8, you get heavily distorted pictures (of course). You might want to use the ability of the fisheye to shoot "wider" than your widest lens (typically a 12-24 or 10-20 zoom on a APS-C sensor).
There is an interesting option available in the Nikon Capture software to "defish" pictures. The principle is to apply a non-linear transformation to the picture to recover a "normal" perspective and geometry (straight lines). This happens at a cost because the picture is seriously streched vertically on the sides, meaning that a lot of detail is lost on the left and right side of the picture. Another issue is that such a fisheye is often prone to CA (color/chromatic aberration), meaning that those issues, if not compensated properly with a specific tool, will get worse in the de-fishing process.
Anyway the technique can be very useful, even if it is not my favorite. Sometimes the original fisheye picture looks better than the de-fished version.
BUT, next the Nikon Capture defishing function, recovering straight lines in all directions, there is a better possibility, recovering only the vertical lines, allowing to get very decent defished pictures with people in them. Have a look here, on the Imagetrends site.
Also have a look at my own minireview.
Back to the Nikon way:
The fisheye version
(shot with a D70)
The de-fished version
Here is another example: I had to take a picture of a painting in a room without enough space to get an overview and I didn't have a wide angle with me. I decided to take the picture with the fisheye and to apply several transformations to get the right result.
The first picture is the original fisheye shot:
The defished picture as generated in Nikon Capture (4.4), with the remaining black part to show the "distorted" result
The defished picture in Nikon Capture, cropped (as proposed by Capture), with a remaining distortion,
but at least straight lines are straight.
