You would just use the lens wide open. That's a lot better than the lever-operated aperture on older Nikon lenses that was stuck at the minimum aperture when not operated by a camera / special adapter. 
With the 300/4 PF I imagine you'd want to spend a lot of time wide open. 
As for the Sigmas... for what it's worth, the future is here.
https://www.dpreview.com/news/1942581285...art-lenses
With the 300/4 PF I imagine you'd want to spend a lot of time wide open. As for the Sigmas... for what it's worth, the future is here.
https://www.dpreview.com/news/1942581285...art-lenses