Quote Originally Posted by neuroanatomist


Not exactly. An f-number is also relative - in this case, to focal length. The f-number is the ratio of focal length to aperture(iris diaphragm) diameter. So, a 100mm lens at f/4 has an aperture diameter of 25mm, whereas a 30mm lens at f/4 has an aperture diameter of 5mm. But, those two lenses would let in the same absolute amount of light. The aperture is 1/5 the diameter because the focal length is 1/5 as long.


Very Interesting discussion!


Neuro..., I think you may have a typo or you lost me, doesn't a 30mm lens at f/4 have an aperture diameter of 7.5mm, and then shouldn't it say;The aperture is 1/3 the diameter because the focal length is 1/3 as long.


In other words, that these two lenses will let in the same amount of light because their respective focal lengths are approx. 1/3 each other (100mm to 30mm) and their respective aperture diameters (25mm to 7.5mm) are also approx. 1/3 of each other, therefore the relative ratio of light they let in will be the same?


Stop me If I'm off track, Pun intended.


Rich


PS. I love the golden waterfall.