Archived posting to the Leica Users Group, 1998/01/15

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Subject: Re: [Leica] Ideal hood shape
Date: Thu, 15 Jan 1998 23:50:12 EST

In a message dated 98-01-15 21:44:45 EST, GH write:

<< 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?  >>

No, that's not right; it's a matter of geometry. Place yourself at the film
plane and consider all of the light that is coming toward the camera from the
scene in front of the camera. Ideally, you don't want any light striking the
front lens element that is not part of the scene that is to be imaged on the
film. If you trace the outer-most edge of the film from the film, through the
lens and out the front element, you will see that just in front of the front
lens element the envelop of light rays that falls just outside the film at the
film plane makes the outline of a square or rectangle that is the exact shape
of the film format. A round lens shade, to allow light to cover the corners of
the film, must necessarily allow light from a sharper angle than is necessary
to enter the lens barrel on the sides of the image. That is extraneous light
that could cause scattering and, therefore, loss of contast, or even flare (if
Leica lenses were susceptible to flare).

The only way a round lens shade could be more efficient that a square or
rectangular one is if you are using a round film format -- like in the very
first Kodaks.