difference between 8mm fisheye and 16mm fisheye?

what's the difference between an 8mm versus a 16mm fisheye? Apparently both are available for the A100 (sigma makes both, I think, and sony makes the 16). Will the 8mm just capture a bigger field of view?

Sample images would be nice, if anyone has them.

Also, opinions about the usefulness of one versus the other would be helpful.

AFAIK the 8 mm is circular while the 16 mm is diagonal which means from what I've understood that the 16 mm will give you a 180 degree viewing angle diagonally on a full-frame imager, be it film or digital. The 8 mm however is circular and will put the whole image circle inside the edges of a full-frame imager, meaning you'll have a circle centered in the frame with completely black parts around it. This means you'll have a 180 degree viewing angle in all directions, not just diagonally.

As far as APS-C is concerned, an 8 mm will still be a lot more pure fish-eye than a 16. The 8 mm will still give you black corners on an aps sensor while the 16 mm will only be a bendy wide angle lens. It'll give you a wider view than a 16 mm rectilinear lens but it won't really be a fish-eye anymore as you're not getting the 180 degree view.

groovyone wrote: The 8mm is a 12mm with the crop factor and will give more of a fisheye effect on the crop sensor.

Actually, that FOV multiplier doesn't work the same way for fisheye lenses. As Blind Boy said, the 8mm is what's called a "circular fisheye" while the 15/16/17mm are "diagonal fisheye".

Someone did the math once and figured out that an image from a diagonal fisheye, when projected rectilinearly, is equivalent to a 10.6mm rectilinear lens.

http://www.fredmiranda.com/A9_rico/

The same math is a bit... more difficult on a circular fisheye. On a crop sensor, you're cutting off more from the top and bottom than you are from the sides so you no longer have this. I don't think anyone has done the math as to what the FOV is once you project the image rectilinearly but it's a good deal greater than what you'll get from a diagonal fisheye.

Edited by MostlyHarmless - 21 March 2007 at 01:42