> After "tinker[ing] with different combinations of lenses and film stock," the production obtained three super-fast 50mm lenses (Carl Zeiss Planar 50mm f/0.7) developed by Zeiss for use by NASA in the Apollo Moon landings, which Kubrick had discovered. These super-fast lenses "with their huge aperture (the film actually features the lowest f-stop in film history) and fixed focal length" were problematic to mount, and were extensively modified into three versions by Cinema Products Corp. for Kubrick to gain a wider angle of view, with input from optics expert Richard Vetter of Todd-AO. The rear element of the lens had to be 2.5 mm away from the film plane, requiring special modification to the rotating camera shutter.
https://en.wikipedia.org/wiki/Barry_Lyndon
> The lens was designed and made specifically for the NASA Apollo lunar program to capture the far side of the Moon in 1966.
> In total there were only 10 lenses made. One was kept by Carl Zeiss, six were sold to NASA, and three were sold to Kubrick.
So yes extremely fast even with today's technologies.