A Simple Outdoor Night-time Test For Assessing Vignetting in Binoculars

Reviews of binocular performance rarely, if ever, include a comment on vignetting. Ideally all light passing through the objective lens at the front of the binocular and directed toward the eyepiece field stop, and then through the eyepiece and exit pupil, will pass through unimpeded before it enters your eye behind the eyepiece. (When you hold your binoculars a foot or two in front of your eye, with the eyepieces closest to you, the bright disk in the center of the eyepiece lens is the exit pupil. It has a specific location but behaves like a small window allowing parallel bundles of light to pass through it. There is no image there though.) Vignetting is the blockage of part of this light before it can pass into your eye by either an aperture stop which is too small, or by a prism stop or by the eyepiece itself. In effect, it reduces the useful, or effective, area of the objective lens, which this test will display.

When observing with binoculars, the pupil of your eye should be placed so that it is coincident with the exit pupil. This will reduce the possibility of even further vignetting by your own eye!

The adverse effect of this blockage is to reduce the brightness of the image. For daytime use of binoculars, this dimming may hardly be noticed. When binoculars are used for astronomy however, this dimming is more important since the image is usually dim to begin with. It is possible to lose up to ¾ of a magnitude of brightness near the edge of the field of view when 50% of the light is blocked. There is however a positive benefit to vigneting. It can increase image sharpness in binoculars, especially cheaper quality ones, by eliminating those peripheral rays that are more difficult to bring to a common focus.

For this test we require a bright, and isolated distant point source of light at night so that the light rays from a given direction, and only that direction, arrive at the objective lens at the front of the binocular in a parallel bundle. This will make it possible to assess the amount of vignetting from different directions in the field of view. If we were to do this test in daytime, then light from all directions would simultaneously enter the objective lens and the exit pupil would appear circular and equally bright even when there is vignetting. We suggest the full Moon would fulfill this requirement even though it is not a point source but is very bright and makes a bright exit pupil. (Note that the round Moon is not the reason why the exit pupil is round.) The Moon should also be low in the sky for convenient observation. A bright isolated street light located two hundred feet or more away is another possibility although I have not tried this. This is convenient because the light source does not move.

The basis for this test is the fact that the exit pupil is the image of the entrance pupil of the optical system. For a telescope the entrance pupil is the objective lens of the telescope. If a portion of the light entering the objective lens from a distant point is blocked internally, then the same portion of the light will also be missing from the exit pupil. For example, if you cover the top half of the objective lens with a piece of cardboard while pointed at the Moon at night, then the top half of the exit pupil will also be missing. The light passing through the exit pupil is a miniature version of the light entering the objective lens. Also remember that every point of light leaving the object produces its own beam through the exit pupil so the actual exit pupil is really a superposition of all of them. The binoculars should be mounted on a tripod and carefully focused at infinity on the Moon. This will ensure the light rays leaving the exit pupil will also be in parallel bundles. Then follow these simple steps.

1) With the binoculars pointing at the Moon in the centre of the field of view, use a ruler, laid against the eyepiece, and a magnifying glass if necessary, to measure the diameter of the exit pupil or bright disk. With no vignetting the diameter of the bright disk should equal the diameter of the objective lens divided by the magnification of the binocular. Should this be the case then at the very least the center of the field of view in the eyepiece would be fully illuminated by the objective lens. If this is not the case, then your 10x50 binoculars are behaving more like a 10x40 or so and there is no part of the eyepiece field of view which is fully illuminated.

2) Cover one eyepiece of the binocular with a small piece of waxed paper held in place with an elastic band as shown in the figure below. This paper should be large enough to completely cover the exit pupil and be flat. This waxed paper is translucent and one can see the light from the exit pupil on it forming a bright disk. Ideally the waxed paper should lie in the plane of the exit pupil. To test whether it is, position the Moon near the center of the field of view. Gently rotate the binoculars from side to side and watch the exit pupil. The exit pupil disk will also move from side to side. Move the waxed paper either closer or farther from the eyepiece until the exit pupil remains in the same place when the binoculars are rotated from side to side. The paper is now located at the exit pupil position. This is especially important when using the Moon as a source since it is not a point and will cause the bright disk to appear slightly larger than it should be if the waxed paper is not located at the exit pupil.

3) With the Moon half way to the edge of the eyepiece field of view, examine again the bright disk on the waxed paper with a magnifying glass. This illuminated disk may or may not be a perfect circle. The amount of the missing part of the exit pupil is in the same proportion and position as the amount of the light missing from the objective lens caused by internal vignetting. Now place the moon at the extreme edge of the eyepiece field of view. Observe the shape of the exit pupil again. It is not likely to be a perfect circle any longer. The fraction of the missing exit pupil will tell you the amount of light which is blocked by vignetting. For example if half the disk is missing then only half the light from the edge of the objective is reaching your eye.

It is useful to also know the approximate size of the eyepiece field of view that is fully illuminated by the objective lens. That is, there is no vignetting in that area. To do this, position the Moon at ever increasing distances from the centre of the field of view toward the edge while checking if the exit pupil on the paper is still a perfect circle. When it stops being a perfect circle you have reached the edge of this fully illuminated region.

This test may also be done on refracting telescopes but the exit pupil is much smaller so it will not be as easy to determine the size and shape of the exit pupil.