I have heard these two lenses are the same. Are they just made by the same company (Samyang) or are the physically the exact same lens? If not, what differences are there, physically and optically? Also why is the Opteka $150 while the Rokinon is $240?
The Opteka 6.5mm, Vivitar 7mm, Samyang/Rokinon/Bower/Pro-Optic/Phoenix/Walimex 8mm fisheye lens for dSLRs are all the same lens optically, and are all made by Samyang in Korea. The external differences are going to be in the casing and branding and (obviously) the flange focal distance for the specific mount. And between different mount versions, the direction the rings rotate and aperture control from the camera (Canon versions don't allow it; I believe Nikon versions do if you lock the lens to f/22).
The difference in price, I would assume, basically come down to different retailers and distribution chains.