Extension Tubes and the Inverse Square Law

[Dan Fromm]

QG, thanks for pushing back. You were right, I was mistaken.

 

[T42]

Gentlemen,

I have read through this thread slowly, deliberately, with great interest.

I thank each of you sincerely for responding to my request for help in better understanding the at play of the inverse square law as it pertains to close-up imaging. I am confident now that the original idea I held was well-placed in the basic idea at least.

This thread has turned out to be quite educational, thanks to the many seasoned imagemakers and experts among you who have taken an interest in it.

The dialogue that has resulted has rendered a wealth of additional points of understanding with respect to the various nuances, such as exit pupil, entrance pupil, actual location of the effective center of the optic and so on. I had assumed thin, single element lenses in my question, but am pleased to read about the more complex considerations. In the final analysis, and for practical purposes, it appears that the ISL works fine enough for practical purposes.

I appreciate also the math and various associated diagrams, links, and explanations.

Thanks again.