PanoTools:
Re: entrance pupil - nodal point
David Sykes 2004-Nov-11 15:19:30
First of all, the entrance pupil is the image of the LIMITING
aperture as formed by the lenses in front of it, if any.
Just what constitutes limiting aperture can change with subject
distance and angle, it does not have to be the iris diaphragm.
Anyway, why don't we just measure it ?
So, I have.
I illuminated Sigma 8mm fisheye from rear via translucent film and
red filter.
Using a simple achromat, I projected an image of the entrance-pupil
onto a screen.
The rear surface of the projection lens was 42mm from the sigma front
element.
I replaced the Sigma with a needle and adjusted it's position so that
it was in-focus.
The needle was 61mm from projection lens.
So, when perpendicular to the Sigma, the entrance pupil is 19mm into
the lens.
That is about 0.5mm on the camera side from the gold band.
I will try this more accurately and at various angles.
David Sykes
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