Binocular Vignetting-Indoor tests

This article is devoted to tests of vignetting which can be done indoors. For less technical instructions on doing tests of vignetting outdoors please visit Outdoor tests. For those with more optical knowledge please visit Ray diagrams  

When you look into the objective end and see a bright region with sharp borders as in the figure below then this is a good sign. What you are seeing is the eyepiece field of view and the border of the bright region is the edge of the eyepiece field stop.

If the border of this region is fuzzy and ill defined and and the size of the bright region is rather small as in the figure below then you have severe vignetting which makes the binoculars almost useless for astronomy because resolution and visibility of faint objects is reduced. Such a view is shown in the figure below.

To further test this, move your eye sideways as you look into the objective. If the border of the bright region now becomes visible then this confirms the severe vignetting. An example of this view is below where the gray region is the entire eyepiece field stop but only the portion in white inside it is visible.

If your binocular passes this first major test then continue below.

What Is An Internal Aperture Stop?

An internal aperture stop is anything that restricts the light cone from the objective reaching the eyepiece. This is done to reduce the portion of the light cone which contains the greatest aberrations. When you look into the objective end of the binoculars and move your eye from side to side you can see there is something blocking the view of the eyepiece field of view. This is the internal aperture stop shown in the figure below. 

Vignetting is defined as the blockage of light anywhere between the eyepiece and objective lenses. The suggestion is that if you look from the objective end into the binocular along the edge of the barrel and if you see a "cat's eye" shaped region of light then this necessarily means you have internal aperture vignetting. The purpose of this article is to be able to identify vignetting due to the internal aperture stop and the eyepiece itself. 

Determining The Portion of the Eyepiece Field of View Which Has No Aperture Vignette and is Therefore Fully Illuminated by the Objective Lens 

The eye is located at about one foot in front of the objective lens. Look into the left side of the objective end of the binocular so that the edge of the internal aperture stop appears at the very left edge of the objective lens. The center of the eyepiece field stop is marked by an X in the figure below. You will be able to see that portion of the eyepiece field stop between the two short vertical lines  at the focal plane, one left and one right of the position X. This is the "cat's eye" region that is visible to the eye. The portion of the eyepiece field stop which is 100% illuminated is a circle with center at X and radius equal to the distance from X to the left vertical line in the figure below.  It is the white region inside the eyepiece field stop which is shown in gray. In this example, about one-half of the diameter of the eyepiece field of view is 100% illuminated. 

 On the other hand, if you can see the entire eyepiece field stop from the edge of the objective and the edge of the aperture stop as in the figure below  then the entire eyepiece field of view is 100% illuminated. This is an ideal but unlikely case however.

If the X appears at the very left edge as in the figure below then only the point at the exact center of the eyepiece field stop is 100% illuminated. ( In this case the radius of the circle is zero) This indicates a moderate amount of vignetting and caution is advised.

 If the location of X is even further left as in the figure below then no part of eyepiece field is 100% illuminated. There is such severe vignetting then it would be unwise to purchase such a binocular.

Determining Whether Vignetting Occurs In The Eyepiece Itself

Once again position the eye about one foot in front of the objective lens. Move your eye even further to the left side so that you see less and less of the eyepiece field stop. Note that eyepiece vignetting occurs on the right side of the eyepiece field stop which is opposite that of the vignetting from an aperture stop. Also the boundary of the eyepiece vignetting region in the figure below is much less curved than for aperture vignetting. This would indicate severe eyepiece vignetting.

If you move your eye still further left and can still see the thin crescent of the eyepiece field stop without the typical shading of eyepiece vignette then there is no eyepiece vignette. This is terrific.

As you move your eye even further to the left, the crescent shaped portion shown in black above will grow larger in size until it covers the entire eyepiece field of view.

ESTIMATING THE PERCENT ILLUMINATION AT THE EDGE OF THE 

EYEPIECE FIELD OF VIEW DUE TO APERTURE VIGNETTE

When the eyepiece field of view is not fully illuminated by the objective lens then the edge of the eyepiece field of view is only partially illuminated by the objective lens. This section describes how to estimate this amount keeping in mind that this is a very rough estimate. In the diagram below we look into the objective so that the bright eyepiece field of view is tangent with the internal aperture stop. The portion of the objective that is wasted is the portion in grey to the left of the eyepiece field of view. The actual shape of this region depends on the size and location of the internal aperture stop so this is an estimated shape. The percent of the objective in grey is estimated to be about 15% so we can say the edge of the eyepiece field of view is at most 85% illuminated. This may even be less because of eyepiece vignette described in the section after this.

ESTIMATING THE PERCENT ILLUMINATION AT THE EDGE OF THE 

EYEPIECE FIELD OF VIEW DUE TO EYEPIECE  VIGNETTE

Look into the objective on the right side now and move your eye to the left until you just start to see the shading, shown in green of the eyepiece field of view which arises from eyepiece vignette. If you moved your eye further to the left the green region would cover more and more of the eyepiece field stop. The portion of the objective still illuminating the edge of the eyepiece field is the portion shown in blue (and also the white region of the eyepiece field of view). In this case the portion in grey is about 25% of the total objective area so the estimated illumination at the edge of the eyepiece field of view is about 75%. 

When you have both types of vignetting at the edge of the eyepiece field of view the percent illumination is estimated to be 100-15-25=60%. The portion of the objective which illuminates the edge of the eyepiece field of view would be similar to the blue region in the picture below. The grey region of the objective is unused

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