Amongst the digital SLRs based on the 35mm camera, there is a divide between full-frame cameras, with sensors the same 24×36mm size as used in 35mm film, and smaller sensors. In Nikon's case, that's FX and DX (16×24mm). The smaller DX cameras don't use all the lens image from FX lenses, so for easy comparison, the effective 35mm focal length for DX camera has a 1.5× multiplier.
That's great for telephoto lenses -- the same 200mm lens in FX acts like a 300mm lens on a DX camera. Not so great for wide angles. A 35mm short wide angle is a DX normal. My 20mm superwide angle is barely wide on a DX. And worse, Nikon doesn't even make many DX wide angles, and many are expensive zooms.
So here's the problem -- one of my bucket lens list items is a full-frame fisheye lens. It's a specialty thing which you don't want to overuse. Yet it really opens up tight spaces and emphasizes items in the central zone of the picture. But I own both an FX camera -- the Kodak DCS Pro SLR/n -- and the DX D1 series. So I would need both the 16mm f2.8D and the 10.5mm f2.8G. They look almost the same and cost about the same at $700 each. That's a lot of money for a specialty low use lens.
Here's where knowledge and research comes in. When I first got into photography in the late 60s and early 70s, there were a ton of trash lenses made by other than the big names like Nikon, Pentax and Canon. Many weren't worth buying from a quality point of view, but they could add capabilities to the camera bag.
Enter The Oddball
So while Mrs. Dr. Phil was away I discovered this on eBay.
It's a Sigma 12mm f8 Fish-eye with a Nikon T-mount adapter, circa 1968. Now, 12mm is too short to be an FX full-frame fisheye (16mm) and too long for a circular fisheye (8mm) -- so what the hell good was a rounded edge 165° fisheye-like lens? I dunno, but it becomes a 150° full-frame fisheye DX lens.
(Click on photo for larger.)
In some ways, it's not a great lens. The focus at the edges is poor. It's an f8 lens and offers only three discrete apertures (f8 f11 f16). And it has no focusing ring, though at 12mm f8 the depth of field is huge.
On the other hand, it's all metal, the coating is sufficient to keep the lens flare down, despite the sun being in the frame, and the damned thing is cheap.
Front yard in mid-May. (Click on photo for larger.)
For comparison, this is from a Nikon 12-24mm f4 DX wide angle zoom at 12mm -- 18mm equivalent and 99° angular coverage, not 150°. Remember, that extra angular coverage is compressed in the edges and corners. (Click on photo for larger.)
Amazingly, the wide angle SB-80DX electronic flash reasonably covers the whole thing, since the edges are compressed right where the light falls off.
You can read more about its strange history here.