The Opteka 6.5mm f/3.5 fisheye lens is now available on Amazon for $160. List price is $500, and it is usually available retail for around $300 IIRC.
This is a stereographic fisheye design that produces a fisheye effect covering APS-C sized sensors and produces a partial circular fisheye on full-frame sensors (or film) when the lens hood is removed. The lens is completely manual focus, and has an aperture ring. I’m assuming that the “Opteka” branding is put on Samyang optics as are sold under bunches of brand names (Bower, Rokinon, etc., etc.). If true, this would be similar to the Bower-branded 8mm f/3.5 I have in my kit. Wide-open, the Bower is pretty sloppy, but stop it down to f/8 and it sharpens up tremendously (watch out for diffraction limits, though). (I found a review that pretty much confirms the sharpness observation, see it below.) The distance markings are notorious for being displaced from where they should be on some versions of these lenses, so calibrate or adjust the setting on the focus barrel (at least, I can do that on my Bower… YMMV). If you want to try out VR (virtual reality) without breaking the bank, this is likely a good lens to start with. It is possible the price drop is a harbinger of a new product shortly to appear replacing this one. It is unlikely that if that is the case that they will offer the new one at anything substantially lower than the previous retail for this one.
Here’s a sample shot with my Bower:
Notice how it is still pretty darn sharp into the lower corners (where there is something to see). This picture benefits from downscaling, but I did not apply any other sharpening. Again, that’s with the Bower, and not exactly this lens, but I expect they are pretty similar under the branding.
Here’s a review of this lens under other branding. There do appear to be some differences in the build between it and my Bower 8mm.
If you click these links, I get an associate’s reward. If you don’t, I don’t. Check out the comments for more details.