Fisheye lenses must be allowed to break the rules when it comes to focal length measurement. I don't know exactly why. If you look at field of view in degrees, the Pentax DA 10-17mm fisheye zoom has the same field of view at 17mm as the Pentax DA 12-24mm f4 zoom does at 12mm.
To make up for not knowing that explanation, I offer these photos. The first is with the Pentax 16-45mm zoom at 16mm. The second is with a Sigma 16mm fisheye lens meant for film use. Because I'm using it on an APSC-sized sensor, the sensor only sees the center of the lens and misses out somewhat on the fisheye effect.
The same lens used on film, with a subject that shows the distortion:
Last edited by Just1MoreDave; 01-23-2011 at 05:49 PM.