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Old 23rd July 2010
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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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Old 23rd July 2010
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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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Old 23rd July 2010
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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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Old 23rd July 2010
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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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Old 23rd July 2010
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Re: Definition of what the 'f' number means... & how calculated...?

Have a look at

http://en.wikipedia.org/wiki/F-number
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