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

Started Jan 26, 2014 | Discussions thread
mapgraphs
mapgraphs Senior Member • Posts: 1,376
Re: A reminder for some memebers as to what the F stop is all about.
1

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.

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. It's Elementary.

Sorry.

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