How Do You Know If Your Binoculars Are Out of Collimation?

Collimation is the process of aligning the prisms inside your binoculars so that the images reflected into each eye can be processed as one image.

When your binoculars aren’t collimated, the images your eyes receive are too different for your brain to align.

Sometimes, the binoculars are very narrowly out of collimation. This means that it can be difficult to identify whether there is a collimation issue. Before you start adjusting the collimation, it is important to make sure there is a need. Otherwise, you could be messing with a perfectly good set of binoculars.

In this article, we’re going to cover a few different tips and tricks that you can use to test the collimation of your binoculars.

What is Collimation?

Before we dive into diagnostic methods, it is important to understand what collimation is and how it relates to binoculars. This will help you understand what it is you’re looking for when diagnosing collimation issues.

A collimated beam of light is one that has parallel rays of light. This means that no matter how far away you are from the light source, the image will never distort naturally because the rays of light don’t cross.

In the case of binoculars, the beam of light is the image collected by the objective lenses at the end of the barrels. The light travels down the barrels and into the prisms.

The prisms are supposed to be aligned perfectly so that the image can be rotated through them but not distorted. This means that when the image comes out of the prisms and into your eyes, the light rays should still be exactly parallel, and you should see a sharp image.

Binoculars that are out of collimation do not have perfectly aligned prisms. The prisms could be knocked out of place thanks to a drop or a knock. It doesn’t take much movement to mess with the collimation. A fraction of an inch will ruin collimation.

Fixing collimation is an involved, intricate, process that needs to be carried out by a professional. However, it is possible to perform ‘conditional alignment.’ This means that you adjust the prisms so that they produce a collimated image when adjusted to your interpupillary distance.

Conditional alignment does not fix the issue completely. If you make the binoculars wider or narrower to suit someone else’s eyes, the collimation will be off.

You can learn how to conditionally align your binoculars here.

Testing Collimation

There are a few different ways to identify whether or not your binoculars are out of collimation. Some of these methods are more involved and fiddlier than others, but they also make it easier to see the collimation.

You should choose the method you are comfortable with and have the tools or resources to carry out.

Physical Symptoms

You may notice that you are getting headaches and eyestrain when looking through your binoculars for any period of time. This is an early indication that your binoculars are out of collimation.

The headaches and eyestrain are a result of your brain working overtime to try and merge the two images into one. It is much harder for your brain to do this when there is a collimation error. This is because the images look different thanks to the distortion.

Getting headaches or eyestrain aren’t foolproof methods of identifying collimation issues, but they are a good early indicator. Even the slightest collimation error can cause these symptoms before your eyes even notice the error.

Naturally, if the headaches and eyestrain aren’t relieved by collimating your binoculars, do seek medical advice from your physician or optician.

Double Image

One of the major signs of a collimation issue is the presence of a double image. This happens because your brain simply cannot merge the two images due to distortion.

If you’re seeing double when you look through your binoculars, consider collimating your binoculars to fix the issue.

If you’re seeing double without your binoculars, seek medical attention!

Use the Night Sky

Some collimation issues are difficult to spot with the naked eye, but they are easier to spot at night. This is because the night sky is more intricate and demanding on the brain.

When it comes to everyday things viewed in daylight, our eyes and brain can sort of make up the differences because we are familiar with these images.

The night sky is less familiar to us, and it has a lot of small details that can’t be guessed by our brains.

So, if you think your binoculars are out of collimation, train them on the stars and see if that shows any double images.


The most accurate way to check for collimation issues at home is to trick your brain into thinking it is looking at two images. This way, the brain won’t try to merge the images, and you can see the differences.

The easiest way to do this is to defocus one eyepiece using the diopter adjuster on the eyepiece. When you look through the binoculars with one defocused eyepiece, you should see one blurry blob and one focused star.

If the focused star does not sit centered in the blurry blob, then your binoculars are out of collimation.


You can also trick your brain into seeing two images by coloring the lenses. You can use colored cellophane from sweet wrappers or decorations, or you can use red/blue 3D glasses.

Again, you’re looking to see whether the different colored stars line up with each other. If they are off center, you have a collimation issue.

Bahtinov Masks

This is another accurate method of checking for collimation. A Bahtinov mask has a specific pattern that, when placed over the objective lenses of your binoculars, causes the star to spike.

Each lens creates an ‘X’ through the star and another spike running through the middle of the X. When using Bahtinov masks on binoculars, you need to rotate one mask 90 degrees. This means that one eye sees a vertical spike through the X and the other eye sees a horizontal spike.

Uncollimated binoculars will display these images separately while collimated binoculars will show a single star with lots of spikes.

Final Thoughts

Identifying collimation issues can be tricky, especially if the prisms are only off by a fraction of an inch.

The key to identifying collimation errors is to trick your brain into believing It is looking at separate images. This way it will not try to merge the two images. Instead, it will show the differences between the images your eyes are receiving.