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Lens with f/1.2 are rather expensive and not very widespread. Lens with f/0.95, f/0.85, f/0.8 do exist but they are also very expensive and quite rare. Lens with f/0.7 are so rare that some of them are manufactured in batches as small as ten items.

What's the intended realistic use of such lens with extremely large aperture? What can they do which slower lens cannot?

  • They collect more light. 'nuff said. Tho' they also allow for more bokeh. – Carl Witthoft Jan 20 '17 at 12:23
  • "bokeh" is the quality of the out of focus blur, not the quantity of it. With more out of focus blur comes less depth of field, which makes it much harder to use well. – StephenG Jan 20 '17 at 13:17
  • The arguably most famous such lenses were made by NASA and a subset of them were sold to Stanley Kubrick. He used them in eg "Barry Linden" to allow "natural light" photography in very low light candlelit and similar situations. The results were as impressive as much in some cases for how bad they were with fancy gear as anything else. ie the result was good because the results looked like they had been obtained at the extreme edges of possibility. Note that a f/0.9 lens compared to a f/1.2 allows (1.2/0.9)^2 = 1.78 more light in for the same exposuretime (slightly < 1 stop) so the gain ... – Russell McMahon Jan 20 '17 at 14:39
  • ... is useful but not stunning. A f/0.9 is 2 stops better than a low cost f/1.8 prime. |In Barry Linden the about zero DOF made photography exceedingly hard indeed. – Russell McMahon Jan 20 '17 at 14:41
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    They weren't made by NASA, they were made by Zeiss for NASA. – Michael C Jan 20 '17 at 15:18
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There's a few things to keep in mind.

First is pushing the boundaries. It's like Formula 1 or really any high end sports car. They're special things that people buy because they have a love of them, not because they're fooling themselves into thinking they'll always be going 250mph. From the manufacturer side its PR and pays for the development which eventually trickles down to their more consumer grade products. Well its the same thing with Lenses. Most don't need faster than f/1. But its a thing of beauty. And the manufacturer's know this and put their highest grade of research into these lenses. Even if you never once shoot a Noctilux at wide open its still an incredible lens for sharpness, contrast, colors, build quality, etc.

The second issue which perhaps more precisely answers your question is there are cases when the low light capabilities are not only nice but really required. Caves, Underwater, Night are three such cases.

Famously Stanley Kubrick used a f/0.7 lens to film parts of Barry Lyndon illuminated only by candlelight:

Video discussing the Cinematography of Barry Lyndon: https://www.youtube.com/watch?v=FmSDnPvslnA

enter image description here

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One thing to remember is that many of these lenses were introduced at a time where the film we had access to made it difficult to get usable images pushing film even to ISO 1600 or so. So I'm not sure they were chasing bokeh (I don't recall coming across that term pre-Internet) as much as they were trying to get usable images without flash in dimly-lit conditions. Candle-lit portraits were the canonical use case if I remember correctly from old ads for similar lenses. Also (but not at the f/1.0 level, and not for rangefinders) in the days of manual focus a wider maximum aperture made it easier to focus (because the shallower depth of field made it easier to see when you were out of focus) -- even if you never used the maximum aperture.

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Ever bit helps and adds up. A brighter lens allows more light in and shallower bokeh. If you are looking for an edge in either case, you buy a brighter lens.

We had similar questions like Are ultra-high ISO sensitivities useful? or even Why do you need image stabilization in a wide-angle lens?. Each is about reaching more, there is always the case where what you have is not enough, so you push the ISO and the aperture or slow down shutter-speed. So, yes, a F/1.2 lens gives you more light but if it's not enough, you may want an F/1. As a side-note, I don't know why we more often see F/0.95 than F/1, seems arbitrary precision or maybe just to say it is faster.

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