Last weekend I attended a meeting of a German optics forum in the old observatory in Bonn.
One of the attendees, Walter E. Schön, a regular contributor to the forum and a technical author gave a talk on the effects on perspective when using binoculars and also on the so-called rolling ball or globe effect.
He also demonstrated his method of measuring the apparent field of view, which was so interesting, I thought I would share it here.
The binoculars (or scope) are focussed to infinity and are set up with their optical axes perpendicular to a flat wall and with the eyepieces facing towards the wall. For the sake of accuracy, this distance should be maximized, ideally over 5m.
A laser pointer is then shone through the centre of the objective (Walter Schön uses a green laser to minimize any effects of lateral CA). The magnification causes the emerging angle from the eyepiece to be correspondingly higher than the angle of incidence.
The maximum horizontal divergence of the laser projection on the wall before it becomes obscured is marked left and right by an assistant. "Post-it" sheets would be ideal for this purpose.
The distance between the two "Post-its," d1 and the distance from the exit pupil to the wall, d2 are measured.
The AFOV is then: 2.tan^-1 (d1/2.d2).
Now why didn't someone at the ISO think of that? ;-)
quote:
The AFOV is then: 2.tan^-1 (d1/2.d2).