[Leica] Ideal hood shape

[George Huczek <ghuczek@sk.sympatico.ca>]

   For those of you who don't get Photo Techniques magazine, the current
issue's Reader's Questions column has an item regarding lens hoods.  The
writer states that rectangular lens hoods are more efficient than circular
ones because the rectangular shape matches the same rectangular shape as
the film. There are other points made in the article, regarding bellows
hoods, filters, screw-in hoods, etc., but I have trouble with the idea that
rectangular hoods are more efficient. 
   If anyone wants to check the reference, it is the Feb. 1998 issue, Vol.
19. No. 1., page 20, by Ron Jegerings. He said, "An ideal hood has the same
square or rectangular shape as the film format and masks light that isn't
used for image formation from the image circle."
   The cylindrical hoods can be built-in to the lens.  The light entering
the lens forms the shape of a cone, determined by the angle of view of the
lens.  As long as the edges of the hood lie just outside this incident cone
of light there will be no vignetting and the hood should operate most
efficiently, no?  The long dimension of rectangular hoods leave gaps
between this entering cone of light, offering more space for stray light,
and leading to a greater likelihood of flaring the lens. This is the way I
understand it, anyway. Off-axis light rays that can get into the lens can
contribute to a loss of contrast.  So what's this business about the ideal
shape of a lens hood being the same as the format of the film?  The only
case where I can see that this might apply is for panoramic cameras. Can
anyone fill in the gaps here for me?

- -GH