Without reference material: F over R equals D where F is the focal length (any focal length for generality), R is the aperture ratio and D is the aperture diameter. For example...

100mm/2.0=50mm, a 100mm f/2.0 lens.

The f-number is not the Focal Length as its name implies but it is the Aperture Ratio and should be called the "R-Number." f-number has got to be one of the stupidest terms used by photographers.

From grade school math "The larger the divisor the smaller the quotient."

"The larger the divisor (R value) the smaller the quotient (D value) where F (focal length) divided by R (aperture ratio) = D (aperture diameter).

The larger the f-number (cough, cough) the smaller physical size of the opening allowing light through the lens, e.g. 100mm/2=50mm v. 100mm/16=6.25mm.

I hope I'm not talking trash here.

Dave

*What we really need to know is any f/2.0 lens of any focal length will allow the same amount of light to strike our image sensors (ignoring f/stops v. t/stops). t/stops are relevant to videographers and cinematographers, not so much photographers. *