Can I make my Nikon 18-200 lens work with my a7iii ?

[thunderseed]

I’m a beginner and just got a cheap adapter that doesn’t have automatic functions, to work with my dads Nikon lenses. for my Sony a7iii 

But the video is so dark, i have to turn the iso up so much it gets grainy. 
i think this might be because i can’t adjust aperture due to the adapter and incompatible lens

Is there another way i can adjust aperture? Or maybe is the darkness caused by something else ? 

 

[XKAES]

If your Nikon lens has an aperture ring on the rear end (by the adapter) you should be able to adjust the f-stop, but only Nikon's early AF lenses had this.  The current crop does not.

[tadwil]

As far as 'dumb' adapters go, you will need a Nikon G to Sony E adapter instead of an F to E adapter, in order to be able to control the aperture.  Nikon F and G mount are identical but the G mount denotes Nikon lenses without aperture rings.

[Pieter]

Alternatively, you could mount the lens on a Nikon body, open up the aperture and then remove the lens before putting it on the a7iii.

Do bear in mind that the 18-200 is an APS-C (DX) lens so it won't be able to cover your entire fullframe sensor. Since you're using a dumb adapter, your camera won't be warned about this limitation and you'll see heavy vignetting in the resulting image.