The shot is also grainy because the lightsaber scenes were shot in regular 4-perf 'scope, and 35mm gets noticeably grainier with each extra generation (exactly the reason why ILM chose to use VistaVision for the effects).

I think the only live footage shot in VistaVision were the bluescreen cockpit scenes, and the splinter unit footage in Guatemala; so all other live scenes with effects added were standard 35mm, which is why every shot with lightsaber glows or laser bolts looks grainy as shit, and why (as much as we hate it) Lucas chose to recomposite everything for the SEs.

I wonder why the animation comps in Empire and Jedi look better - behind the scenes footage of the lightsaber scenes shows that the film magazines are standard 35mm and not VistaVision...