Get yourself a filefolder (or some relatively stiff opaque paper file folders are made of--cardstock--but make it thin).
Cut a long, thin, trapezoid out of it. By long, I mean maybe eleven or twelve inches--or whatever the width of the file folder. By thin, I mean eight or nine mm wide at one end, down to one mm or so at the other end.
Label the thing according to width. Mark it where it is 2 mm wide, 3 mm wide, all the way to about 8 mm.
Go out at night, close one eye, pick a bright star, and center it in the open eye.
Hold the trapezoid in front of your pupil, with the narrowest point in front of the pupil. You will be able to see the star because, although the center of the pupil is blocked, the edges of your pupil can still see the star. Wiggle the trapezoid back and forth a bit. If you can see the star constantly, your entrance pupil is bigger than the trapezoid at that point.
Move the trapezoid down a little bit, so that the wider part of the trapezoid is in front of your pupil. You can still see the star. Give it the wiggle test.
Move it again, and--poof, the star goes out. The width of the trapezoid at the point where the star disappears is the point at which the trapezoid is just wide enough to cover your pupil. Measure that point, or interpolate from your width markings. Note that the wiggle test at this point has the star blinking. At some point, when directly centered, the trapezoid is big enough to block your whole entrance pupil.
Move it further down (past the point where the star first disappears), and the wiggle test hides it even when you wiggle. You want the point where it is JUST wide enough to hide the star, but no wider.
Now, beware, if you put the trapezoid too far from your pupil, you will get a false reading.
And, of course, if you are not steady, you might not be putting the trapezoid right in front of the pupil, and you can see the star from light getting in on one side or the other of the trapezoid. The wiggle test checks for this.
But, you get the idea.
Try it with dark adapted, and light adapted eyeballs.
Edited by Alex McConahay, 16 December 2016 - 08:36 PM.