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I've quite a few questions in this regard:

  1. Is there a theoretical limit?
  2. What has been achieved till date?
  3. What is commercially available today?
  4. If there is a sudden exponential increase in price after a certain limit, what limit is that?
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    The two answers so far have interpreted your question differently. Can you clarify? By "smallest f number" do you mean a numerically small, but physically large, aperture (like f/1.0), or do you mean a numerically large, but physically small aperture (like f/32)? – coneslayer May 10 '14 at 0:37
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    Saying f number should not b confusing. I meant the former, like f/1.0 – kBisla May 10 '14 at 6:00
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  1. Yes, there is a theoretical maximum of f/0.5 beyond which spherical aberrations are unavoidable. (according to the book "Applied Photographic Optics: lenses and optical systems for photography" by Sidney F. Ray)
  2. The most extreme would be a f/0.33 "Super-Q-Gigantar" by Zeiss, but that was purely a marketing gimmick and not practically usable. The f/0.7 (also Zeiss) was actually used for a movie and is widely considered the fastest "real" lens.
  3. Wikipedia lists a number of f/0.95 lenses as currently available. I've seen those made by SLR Magic for reasonable prices.
  4. I don't think you can make a definitive statement there, it also depends a lot on the focal length and how much you're willing to compromise on optical quality.
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    Sidney F. Ray... how aptly named. – AJ Henderson May 10 '14 at 0:52
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    It is worth pointing out that the f/.7 lens was actually developed for use in space for Nasa, but Stanley Kubrick got them for a movie as well. – AJ Henderson May 10 '14 at 0:54

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