On 10/07/2011 4:30 PM, charles wrote:
On Sun, 10 Jul 2011 22:10:20 +0200, wrote:


F number is focal length over diameter.

So, for /f 2.0 with 300 mm lens, the diameter would be 150 mm, or
about 6 inches.

That is pure math, a 300mm f:2. Nikon did make a special order 300/2
that used a 160mm filter, so about 6% bigger in diameter than math to
compensate for light loss.

Mike