An F stop is a ratio of what part of the lens?

Sounds like a simple question, but the part I don't understand is what part of the lens. So if I have a 50mm f/1.0 lens then it has a 50mm diameter lens somewhere in it, but which part of the lens is used in this formula, is is the actual aperture diaphragm at maximum opening?

Not really an important queation, just curious. If it were the front element then my 70-200 should be f/1.0 at 70mm!

f-stop = Focal Length / Aperture

http://en.wikipedia.or​g/wiki/F-stop

I learned something too - although most of it hurt my brain a bit.

symby wrote in post #5576625
f-stop = Focal Length / Aperture

http://en.wikipedia.or​g/wiki/F-stop

I learned something too - although most of it hurt my brain a bit.

Thanks for that they nail it, it is the maximum aperture opening, not the actual pieces of glass.

Obviously they need to be big enough too though!

Now if anyone can explain how contact aperture works on a zoom...

You divide the "exit pupil" into the focal length. So a 50mm lens with an exit pupil of 25mm would be f2. (50/25=2). That's why when you put a 2x teleconverter on a 50 mm lens you lose two stops. Take the 50mm lens again only this time you put the 2x TC on it. The exit pupil would remain 25 but the focal length is now 100mm. 100/24 = 4 or f4.

With a constant aperture zoom the lens MUST be a lot faster at it's shortest focal length than at its longest, but you never see it. The variation has to be in the exit pupil and this must be closing when the focal length gets shorter. So my 70-200 f2.8 has to be faster than f2.8 when it's at 70mm.