# A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
Re: A reminder for some memebers as to what the F stop is all about.
5

mapgraphs wrote:

bobn2 wrote:

If you say 'the aperture of the lens is f/2' what you are saying is 'the aperture of the lens is f divided by two', so then you ask, what is 'f' - of course it's the focal length, so 'the aperture' is 'the focal length divided by two'

So, if someone tells you that the aperture of a 50mm f/2 lens and a 25mm f/2 lens is the same, they are wrong. in the case of the 50mm lens it is 50/2 mm, = 25mm and in the case of the 25mm lens it is 25/2 mm = 12.5mm.

Um, no Bob.

The f/ number is the focal length divided by the diameter of the entrance pupil.

The aperture diameter is the diameter of the entrance pupil. Let's call it A and the f-number N.

You tell me that N = f/A

I say that A = f/N. Can you not see these are the same thing? Lets do it one step at a time.

N = f/A

multiply both sides by A gives A*N = f

divide both sides by N gives A = f/N

Which means in your example above, if the element entrance diameter of the 50mm lens results in an f/2 maximum aperture and the element entrance diameter of the 25mm lens results in a f/2 maximum aperture, both lenses are in fact f/2.

I'm not sure what you mean by 'element entrance diameter' - let's stick to 'entrance pupil'. The entrance pupil (aperture) diameter of a f/2 50mm lens is 50/2 = 25mm. The entrance pupil (aperture) diameter of a f/2 25mm lens is 25/2 - 12.5mm. The two lenses have different entrance pupil sizes (as the must, since they have different focal lengths)

It's Elementary.

I'd have thought so, but you got it wrong nonetheless.

Sorry.

You don't have to be, we all make mistakes sometimes.

-- hide signature --

Bob

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