So my unwieldy question is neither stupid nor irrelevant, projection methods and the like, while I understand the explanation qualitatively, quantitatively I have too little understanding of optical physics to completely make sense of it without digging into mathematics of some college text books I would rather not. Another unwieldy sentence... I do apologize.
I have a Tokina DX 10-17mm fisheye zoom, which is a diagonal fish eye lens, that I used both in DX and FX (from 15mm onward, effectively making it a 16mm f/4.5 FE).
And Akira-san (I assume) said that Samyang does not make a circular fish eye lens, and I am fundamentally not interested in a diagonal fish eye lens, the simpler question might be what are my options for FX? For a lens that will not see too much action, I rather not pay the Sigma's asking price. Opteka 6.5mm?
And I suppose the other option is to use the Samyang 8mm and PP the circle to it...
I am not partial to using a DX lens and push it to FX service, as I have done with the Tokina 10-17 DX FE with a loaner D700 and my D800 and Df.