Use of lenses under water

if you want to use lenses designed for the use “in air” in a housing under water, please do NOT use a plane window! The reasons get clear from the interactive graphics below.

If there is no other chance than to use a plane window, then place it close to the lens.

instead you should use a spherical window that shares it’s center with the entrance pupil (center of the appearant hole when looked from the front: