Jwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 20 Reply 1, posted (11 years 6 months 1 week 11 hours ago) and read 776 times:
According to my old university optics course the f/number is equal to f/D' where D is the diameter of the entrance pupil of the lens and f is the focal length.
It goes on with an example, a 50mm lens with a 25mm aperture has an f/number of 2.0.
Then it goes into photographic lenses, stating that one f-stop in those is equal to a step equal to the square root of 2 (rounded), yielding the set we know of 1.0, 1.4, 1.8, 2, 2.8, 4, 5.6, 8, and so on.
Thus closing down 1 f-stop decreases the lens opening by 1 divided by the square root of 2 and the total light reaching the focal point of the lens by a factor of 2 (thus halving it). (it assumes you know that the area of a circle is defined as pi * the square of half the diameter of that circle, therefore the square root in the formula).
Hope this explains the mathematics behind it somewhat.
Jwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 20 Reply 4, posted (11 years 6 months 5 days 2 hours ago) and read 663 times:
Not too bad, front lens in the area of 18cm diameter or more. Add the tube and you're up to a diameter of maybe 20cm.
Length of 50cm or more.
Weight a trivial 20kg approximately (not counting the dual tripod mounts it would need).
Price? €1 per gram like most lenses in that class for an inconsequential €20000,-- estimated