Help! What is f/1.2 really?

Started Sep 3, 2008 | Discussions thread
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 Help! What is f/1.2 really? Sep 3, 2008

We know the f numbers increase by a factor of root 2 and so we have f/1, f/1.4142..., f/2, f/2.8284..., f/4, etc. but we round everything to a single decimal point.

Now, what is f/1.2 really? There are two things we know/hope:

Theory A
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f/1.2 is half a stop slower than f/1 (or half a stop faster than f/1.4). According to the aperture area/light gathering formula in wikipedia [1], I estimated this "half stop faster than f/1" to be f/1.154700538... yes, lotsa decimal places and if we round this number to 1 decimal place, we conveniently/confusingly get f/1.2.

Theory B
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f/1.2 is exactly that... which would mean it is 61.16596...% of a stop slower than a f/1 lens, and not by only a half stop.

So when we say f/1.2... what do we really mean? If "f/1.2" lenses have been either one of the above, how can we know for sure about our lenses?

I don't think it's correct to perform practical tests here to measure the light (e.g. using T-stop tests or camera's own meter) because lens design would affect vignetting amounts etc. and wouldn't give the theoretical/design answer we need.

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