Thread: F stops
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Old 17th October 2017
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Re: F stops

Originally Posted by MJ224 View Post
Following a thread here by Paul, I was always of the opinion that f stops were f5.6 was double or half of f2.8

When evaluating a lens, this is obviously very important knowledge......

Need educating........
OK - I'll adopt my teacher stance and try!

The key thing to do is not to confuse f-stop and f-number.

The f-number of a lens is the inverse ratio of aperture diameter to focal length. That is, if you have an aperture of 25mm wide on a 50mm lens, then it's f2:
- f-number = reciprocal of 25/50 = 2
So, if you double that ratio - e.g. f2.8 to f5.6 then it means that you've halved the aperture diameter.

Now, the amount of light passing through the aperture is proportional to the area of the opening. Since Area = pi * r^2, if you half the aperture width then you reduce the area of the opening by a factor of four. That means that the light intensity is reduced by a factor of four too.

Now, photographic stop, or f-stop represents a halving (or doubling if you go the other way) of light. So, going from f2.8 to f5.6 means a four fold reduction in light intensity which equates to halving twice - i.e. 2 f-stops.

The progression of aperture f-numbers from f1 downwards in f-stops is thus:

f1, f1.4, f2, f2,8, f4, f5.6, f8, f11, f16

You'll see that each doubling of f-number is actually 2 f-stops.
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