Focus at ENTRANCE PUPIL

[Ram Nanda]

According to AC Magazine April 2020 edition 

" The Pupil Marks the Focus Spot The next thing to understand about the entrance pupil is that it is at that point — not at the film plane (or sensor plane) — from which focus should be measured. It’s a long-standing practice in cinematography to take focus measurements from the film or sensor plane of the camera. "

(read the attachment)

Well, the topic seems to be fine and interesting but, the basic question is never raised nor answered - WHY FOCUS SHOULD BE MEASURED FROM ENTRANCE PUPIL?

Can some one explain these :

1) Do all of the points of light from the scene converge to a single point at the entrance pupil and then expand back out to the image plane ?

2) What role does exit pupil play in measuring focus ?

3) Can Iris be located not at aperture stop ?

4) How is DoF related to entrance & exit pupil ?

And please provide a reference website or material to understand these technical concepts clearly.

 

Thanks,

Ram Nanda.

Edited May 3, 2020 by Ram Nanda