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Autocollimation Check of Binoculars

By Rafael Chamón Cobos

Created: November 2011

Reviewed: November 2011

Reviewed: March 2012



The present article describes a method to check the alignment of binoculars with the help of a mirror. The target for the alignment test consists of the set of the two images of the binocular’s objectives, reflected by the mirror. These images are observed through the binocular using the “paralell vision” technique to check if the target images overlap or not. An aligned binocular shows perfect overlap of both objectiv’s images, whilst a misaligned binocular shows two displaced images each other.


A strong misalignment of a binocular is inmediately detected by looking through it because it produces double images. However small misalignments are hardly detected in this way because there is an involuntary mechanism in the eyes that compensates misalignments of the instrument by slightly adjusting the sight direction of the eyes, though it produces eye strain.


There is a well kown method for checking small misalignments of binoculars, which is called the “paralell vision” method. According to it, you must look at a distant target through the binocular in such way that the eyes are forced to keep their  looking direction along two paralell lines, so preventing any angular readjustment performed by the eyes.


Following procedure is used:


  1. Adjust the interpupillary distance of the binocular to match the interpupillary distance of your eyes
  2. Choose a very far away located target, say 1 kilometer or more.
  3. Look at this target through the binocular
  4. Slowly move the binocular away from your eyes while keeping seeing the target in the center of the exit pupil of the instrument
  5. When the eyepieces of the binocular are at least 20cm away from the eyes try to see a single exit pupil by slightly leaning the instrument or your head.
  6. If the target is clearly visible inside one single exit pupil, i. e, if both target images fully overlap and both exit pupils fully overlap as well, then the binoculars is aligned. Otherwise the alignment of the binocular is faulty.


Looking in this way through the binoculars from a distance from the eyepieces, the eyes do not move at all and look ahead along two paralell lines, so that the misalignment of the instrument itself is detected.
For an accurated test it is recommendable to attach the binocular to a tripod and to move your head away from the instrument rather than moving it with the hands.

The autocollimation check method with a mirror

The availability of a distant target may be an issue, and the present article describes another way to carry out the check indoors and at any time, by using as target the image of the binocular itself reflected on a mirror. More to the point, the target consists of the two objective’s images of the binocular reflected by the mirror.

The binocular is set in front of a common mirror, so that we can see the image of the binocular itself (and also of our head) when we look through it. The binocular is focused in its specular image. The distance to the mirror is not critical, it is only limited by the minimum focusing distance of the binocular. A light source -window or lamp- must illuminate the binocular frontally in order to have a clear image of both objectives (rim and lens reflexes of the objectives should be easily visible).


In this situation, if we look through the binocular in normal way, each objective receives the reflected image of the whole binocular, but if we apply the “paralell vision” procedure, i. e., if we look through the binocular from a slowly growing distance from the eyepieces, we will gradually see that two objective’s images merge into one single image inside the merged exit pupils. This single image is the superposition of the images that each objective “sees of itself”.


If the binocular is aligned the merged images correspond to a perfect single objective; rim and lens reflexes are sharp reproduced, and vision is relaxed and stable. You will easily recognize this fact; the effect is very clear. However, if the binocular presents a certain amount of misalignment, this is inmediately detected, because it is not possible to overlap both objective images onto the two overlapped exit pupils.



Refer to Figure 1.

Figure 1. Arrangement and explanation of the autocollimation check of binoculars
The target consists of the set of the two images of the binocular’s objectives, reflected by the mirror. Since both objectives have the same outward appearance and the distance between them is equal to the distance between objective’s images, the binocular always receives the same target information in both objectives, and the situation is equivalent to a binocular receiving collimated beams of rays emitted from a single target (a binocular’s objective) located at the infinite, so that the misaligments of the instrument can be detected. This situation is independent of the distance between binocular and mirror, and for this reason the accuracy of the method is independent of this distance. Choose the distance you can easily see the target images from.


If the binocular is perfectly aligned, then exit rays at both eyepieces are paralell and the image of the target lies at the same position in both exit pupils, so we see a single binocular’ objective inside. If the binocular is misaligned, then exit rays at the eyepieces are not paralell due to the misalignment; the target is imaged at different positions in the exit pupils and it is seen as two displaced images.


The use of a twin target consisting of two physically different objects, like in the present method, has the advantage over the use of a single distant target that each side of the target can be marked in order to identify the corresponding target image on the visual field of the binocular. This is especially helpful when the collimation adjustment is performed. For insteance, a small sticker on the rim of one objective could serve to discern from both target images.

The quality of the mirror is an important issue; the mirror should be as flat as possible, because a non perfectly flat mirror can distort one or both target images, so disturbing a perfect match of them. However, most common wall mirrors may be suitable for the test.

Since target images are observed inside the small field of an exit pupil seen from a distance, the method works better with binoculars having large exit pupils, where target images are better discernable. Small pupil binoculars are harder to test.

The precision of this method with mirror is not superior to a normal examination of a very distant target outdoors. According to my tests, it can be even worse due to the scarce visibility of the exit pupil that surrounds the target. However, it can help if you are working indoors at home.  It can be also particularly useful when buying a binocular in a store, if a wall mirror is available in the store.
As in the case of alignment check with a distant target, for most accuracy, the “paralell vision” procedure should be executed by attaching the binocular to a tripod placed in front of the mirror. Then you move your head away from the instrument instead of moving it with the hands.
The method only checks the conditional alignment of the binocular at the interpupillary distance set by the observer. The true collimation, i. e., the alignment at all possible interpupillary distances, is not regarded in this method. The terms "conditional alignment" and "true collimation" commonly used in the binoculars literature have been coined by Bill Cook.



The method allows the conditional alignment check of most binoculars indoors and at any time with a minimum of means.

(End of the article)