Quote Originally Posted by Richard Lane
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.

Good catch - it was a typo, should have read 20mm, so f/4 would be 5mm diameter.


But the bottom line is that because f-number is a ratio, any lens of any focal length, when set to the a particular f-number, will let the same amount of light through the lens as any other lens of any other focal length set to that same f-number.


Thanks for the compliment on the waterfall - that's at Yerba Buena Gardens in San Francisco.