Well, not so fast. The inverse square law accounts for only one of
the cosines in the Cosine**4 law. I think you are describing the
Slyusarev effect which lens designers utilize to increases the apparent
size of one or both pupils for off-axis points and greatly mitigates
the cosine**4 law.
A second factor that causes light loss off axis is that the lens pupil
is no longer round but elliptical when viewed from the periphery. It
also strikes at an angle off axis. There is additional attenuation due
to the Lambert effect. I think that makes four, but I am tired after a
long day and a couple hours left to go and can no longer count.
Mike (protégé of Dr. Focus)
see
http://toothwalker.org/optics/vignetting.html
have a neat trick where the aperture
> is optically magnified (increasing the apparent size)
> as you move off-axis, it's quite neat to see when looking into the
> front of the lens.
However, this still can't account for the fact that the light has to
> travel more than 2x as far to the corners of
> the film plane, than what it has to the centre. This, and only this,
> causes the light falloff, and it's this that the
> centre filter tries to correct.
--
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