Focusing on Rokinon 8mm + Nikon D40

[the.ronin]

First time poster here ... hi!

I'd like to ask for some advice on focusing with a Rokinon 8mm fisheye lens on a Nikon D40. This is my first fisheye lens but not my first MF lens - my 50mm prime. But I can get absolutely crisp photos on the prime.

Here is an example of a shot (no post production) taken with the Rokinon in manual mode shutter 1/2000s aperture f/3.5 ...

(Note that the little girl's face was purposefully blurred for privacy.)

I know Rokinon has some zoom lenses that aren't exactly quality but I've read very good reviews of this lens (also branded Samyang, Bower, etc). I've also seen photos taken with this lens that look very good and crisp (amount of post production unknown however).

I also know some have manually adjusted the focus ring on this lens but that's something I would prefer to avoid right now.

Any other advice would be greatly appreciated.