I've been inspired to provide this after all the discussion here regarding AFoV, and the great amount of guesswork going on. It's not

*quite*as simple as the objective aperture test, but not arduous at all.

This method also involves shining a flashlight into the bino, but in reverse. First, test things out to get a feel for how big an illuminated circle you can get before getting too faint. The bigger this circle, the more accurate the measurement.

If you can't see the full circle at a time, simply wiggle the flashlight a bit so that opposite sides of the circle can be seen alternately. In such case, you can make small pencil marks and measure their separation afterward.

Set the focus at least close to infinity focus. Some binos have field stops which are fixed with respect to the body; as the eyepiece is focused in/out AFoV actually changes somewhat! (If the field stop is fixed within the eyepiece itself, AFoV doesn't ever vary. If you know this to be the case, eyepiece position will not matter.)

Some binos suffer significant edge-of-field darkening due to a too-small rear prism aperture, which can result in a non-sharp field edge. Simply measure to the farthest visible 'edge' of the illuminated circle.

1) Set the bino up on a tripod, near a white wall and aimed so that the eyepices are facing the wall as prependicularly as you can obtain. (The wider the AFoV, the more critical is perpendicularity. Any tilt will result in an elliptical circle of light due to the geometry of projection.)

2) Shine a flashlight into an objective, from up close so that you get no spillage of light onto the wall.

3) Measure the diameter of the illuminated circle of light on the wall.

4) Measure the distance between the wall and the

*location of the eyepiece's eyepoint (exit pupil)*. If the eye relief is stated as, e.g., 18mm, measure right to the eye lens itself and then subtract the 18mm.

5) AFoV = 2 * ARCTAN((circle diam. / 2) / distance to eye point)

An example:

- The illuminated circle of light measures 300mm in diameter.

- The distance from wall to eye lens = 280mm.

- The eye relief is 15mm; the wall-to-eye point distance is therefore 280 - 15 = 265mm.

AFoV = 2 * ARCTAN((circle diam. / 2) / distance to eye point)

AFoV = 2 * ARCTAN((300 / 2) / 265)

AFoV = 2 * ARCTAN(150 / 265)

AFOV = 2 * ARCTAN(0.566)

AFoV = 2 * 29.5

AFoV = 59.0 degrees

With care you can surely derive the AFoV to within 1/2 degree via this method. And as you can see, a larger projection distance lessens individual measurement error.

Lastly, this value will conform to the real AFoV, which is the apparent angle subtended on the retina by the illuminated field. For larger AFoVs particularly, there will usually be some discrepancy when compared to the 'fictitious AFoV' simply calculated from TFoV and magnification. This is normal.

Let's see some results!