Tom C wrote:
Yesterday I received my FA 31/1.8 Limited. Purchased from another list member who heard my pleas for help in finding one at a decent price. He had one in brand spanking new condition, which I was able to purchase at a reasonable price.
A beautiful lens.
I'm curious why it has a flower-shaped lens hood with only hooded sections at the top and bottoms, and is not a complete circle. Probably a foolish question to which the answer is obvious.
The 35mm negative frame is not circular, it's rectangular.
If you ray-trace the 35mm frame out thru the lens, and project it outward (the reverse of how light enters the lens, but for now, allow me that reversal...) you'll find that it projects an ever-widening rectangle as it heads outward, away from the front element.
Starting from approximately the O.D. of the lens, where a lens shade might attach, project that cylindrical shape outward until it has intersected the pyramidal wedge which is the light path.
You'll see that it cuts the light path in a double scallop shape, top and bottom, and on each side, just like the "flower-shaped" lens hoods you're asking about.
If you had a perfectly cylindrical shape to the lens hood, it would interfere with the incoming light in all the corners, and make them very dark, or occlude the light rays entering there.
If that makes no sense, I'll see what I can find to help clarify it...
keith whaley
