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#1




Definition of what the 'f' number means... & how calculated...?
Hi All
I was asked a question the other evening that I was not able to answer, and the question was; "what exactly does fn.n mean"? Now of course, I was able to explain what aperture is, it's relationship with exposure, DoF and shutter speed etc, but I have no idea how the actual value for a given lens is actually calculated. What is the mathematical formulae for calculating the aperture of a given lens? Is is something like the ratio of physical aperture size to distance from sensor or something like that? Can some tekno person help me here please...? Thanks.
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John 
#2




Re: Definition of what the 'f' number means... & how calculated...?
Isn't it = f/D where f is the focal length and D is the diameter of the aperture... So a small hole = big "f" number, and 200mm lens with a bay window on the front is still only f4
That only works for really simple lenses mind you. Quite how they calculate it with the ZD8mm who knows ... Pete
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Look, I'm an old man. I shouldn't be expected to put up with this. Pete's photoblog Misleading the public since 2010. 
#3




Re: Definition of what the 'f' number means... & how calculated...?
Yes, just found out from Ian that the f number is basically the focal length divided by the aperture diameter.
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John 
#4




Re: Definition of what the 'f' number means... & how calculated...?
And before anyone asks  I believe the f stands for focal ratio.
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Paul 
#5




Re: Definition of what the 'f' number means... & how calculated...?

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