# Is there theoretical limit on the minimal focal length for a given sensor size?

There seems to be a limit, around 0.33, on the fastest possible aperture. Is there a theoretical limit on how wide a non-fisheye lens can be? That is, what is the minimal possible focal lens for a given sensor size?

• The theoretical limit with typical glass used for camera lenses for maximum aperture is f/0.5. If one had a diamond large enough and pure enough, one could cut a lens with a theoretical maximum aperture of f/0.235 because diamond has such a high index of refraction. That f/0.33 lens was a marketing gimmick that didn't actually work. – Michael C Apr 18 '19 at 18:27
• To clarify the question: You're looking for math to support what the shortest focal length of a lens you can get, for a given sensor/film size, before noticeable 'fisheye distortions' creep in? - You may need to define what level of acceptable distortions would be for a helpful and valid answer. – TheLuckless Apr 18 '19 at 19:30
• @TheLuckless, Yes, I'm asking about the shortest focal length. A ballpark info would be sufficient; forget about the non-fisheye clause. – Michael Apr 18 '19 at 19:41
• Well, 0mm is pretty much a hard limit for focal length. Probably not very useful, though. But, it does pretty much affirmatively answer "is there a theoretical limit ...". – twalberg Apr 19 '19 at 2:39
• @twalberg, :-) What about a positive limit? – Michael Apr 19 '19 at 3:35