# Focal length and FStop

I have been reading about Focal length and Fast lenses when I came across this lens which has 85mm focal length and F1.4 as FStop.

https://www.sony.co.in/electronics/camera-lenses/sel85f14gm

Generally lenses with 1.4mm Fstop have focal length below 35mm but here its 85mm!!
As we calculate FStop as `FocalLength/diameter` this implies that this lens must be having huge diameter to achieve 1.4 Fstop.

Looking at cost this lens is very expensive, so must be a premium lens.

Can someone please explain how having a large diameter makes it a premium lens.