The photographic effects of Star Wars were filmed in 8-perf 35 so that when the shots were composited they would look good when intercut with the live action footage shot in 4-perf vertical format. Compositing numerous ships, stars, planets, and animated effects required dozens of successive exposures copying original footage into the final shot, with each successive generation increasing grain and contrast. The larger negative not only allowed for higher quality at this stage, but made it easier to engineer highly precise cameras and optical printers that would maintain all those elements plus their mattes in registration. Most of this I know from tech articles and interviews with the ILM team not long after the film's release. Most release prints were in 4-perf anamorphic ('squeezed' widescreen) but some of the larger markets exhibited the film in 'blown up' 70mm, especially once it took off in popularity. 70 also allowed for the highest quality sound reproduction at the time via multiple magnetic stripes on the film. I used to have a few frames trimmed from a 70mm print and still have numerous 8-perf bits from ILM's dumpster -- mattes, color seps, test footage -- and a ton of 4-perf clips from release prints. The 70s were beautiful but not especially 'sharp', while the 8-perf stuff is quite sharp (ILM used Nikon SLR lenses at this point). The 4 perf clips are really pretty grainy and the FX show significant color shifts but quite a lot of them are from trailers, yet further removed from the original negative.
Originally Posted by snaggs
By contrast (if you'll pardon the expression) the effects for Star Trek: The Motion Picture were mostly done in 70mm, and in my area at the time the film premiered in 70. The shots of the Enterprise look amazingly lush and real and there's virtually no telltale special-effecty look. The opening sequence, however, was contributed by ILM in the 8-perf format and blown up for release. There's a definite 'grittiness' to the shots and the mattes are not as clean (the grit kind of works, though, because it's the Klingons).
70MM is very rare as a camera negative anymore due to expense. The economics of theatrical distribution (multiplexes with small screens) don't show it to its best advantage. Producers even economize by using 3-perf 35 (Lord of the Rings) and of course there's the 'other' origination medium we don't discuss on this forum...