You cannot set that lens at F2.8 when you are zoomed at 80mm. If you look at the lens, above the aperture ring there are three markers, one red, one white and one green. The red marks the F-stop when using 40mm, the white when at 60mm and the green when at 80mm. If you look at the focal numbers they are color-coded also.
If you turn the ring all the way to fully open, the markers are (red = F2.8, white= ~F3.5 and green= ~F4.5) which should be the real F-stops.
The iris of the lens has a fixed size and does not change as you zoom in and out. It changes only with the ring. Since the definition of the F-Stop is Focal Length / Iris diameter you get:
At 40mm and F2.8, the iris is 14.3mm wide open and will stay at that while zooming.
When at 60mm the effective F stop will be 60/14.3 = F4.2
At 80mm it will be 80/14.3 = F5.6.
Mathematically it looks that the F4 and at 80mm is rather optimistic.
Last edited by demp10; 04-07-2012 at 09:03 AM.
|