How To Collimate Binoculars

When you look through the lens of a pair of binoculars, you should see a clear image with no blurry or double vision. If you do experience a blurred image or you start to get eye strain or headaches, then your binoculars are probably misaligned. Or they are collimated incorrectly (which means the same thing).

This article is all about how to collimate your binoculars, but we feel we should give you a warning. It is possible to make things far worse if you don’t know what you’re doing. You’ve probably spent a small fortune on a decent pair of binoculars and at some point you’ve knocked them or dropped them. And now they only show a blurred image and every time you use them, you get a headache.

The problem is, binoculars are precision optics and some of the parts are fragile and easy to dislodge from their correct position. This is especially true of the prisms in Porro prism binoculars. Once you start messing about with the internal prisms you could cause even more problems.

Binocular Warranty

Most decent binocular manufacturers have a lifetime warranty and even some of the cheaper ones have at least a 24 month warranty. If your binoculars are still under warranty, you should contact the manufacturer. Because if you start fiddling with the inner workings of the binoculars you will probably void your warranty.

If your binoculars were expensive and they’re out of warranty, you’ll be wise to take them to a professional to get them repaired. If they were only a cheap pair to begin with, then it’s worth having a go at fixing them yourself. But only if the cost of any repair would be greater than replacing them with a new pair.

Learning The Lingo (Binocular Terminology)

Before we start with the how to, let’s be sure of what the various parts of the binoculars are called. There are quite a number of parts that make up a pair of binoculars, and you wouldn’t want to mess with the wrong bits.

The Barrel

Binoculars have 2 barrels, they are the long tubes where the lenses are at each end.

The Eyepiece

The eyepieces are positioned at the end of the barrels, the ones closest to where you look through with your eyes. The lenses at the eyepiece are called the ocular lens, and the lens at the other end, the bigger lenses are called the objective lens.

The Diopter

On some binoculars the diopter is found on just one eyepiece, others have a diopter for each eyepiece. It is used to focus the binocular eyepiece while compensating for any differences in your eyes.

Binocular Hinge

The hinge is located between the barrels and allows for interpupillary adjustment (you open or close the hinge slightly to make the eyepieces fit your eyes and you can see a clear image.

The Barrel Bridge

The barrel bridge houses the hinge and the focus wheel.

The Focus Wheel

The focus wheel is located on the barrel bridge and is used to focus the binoculars. The focus wheel usually adjusts both barrels at once unless you have a military pair of binoculars that focus independently.

The Objective Lens

The objective lens is the lens furthest away from the eyes and closest to the object you are looking at. The objective lens is the only way any light can enter the binoculars to illuminate the image you see through the lens.

Porro Prism Binoculars

Porro prism binoculars have two offset prisms for each lens, they are found inside the barrels of Porro prism binoculars. They are the reason for the bulky midsection of the binoculars. It’s usually Porro prism binoculars that get misaligned and need collimation.

Roof Prism Binoculars

These are smaller and far more compact than Porro prisms, as they have less moving parts, and roof prisms are far less likely to need collimation.

How To Tell If Binoculars Need Collimation

When using your binoculars for any length of time, if you find you have lots of headaches or eye strain, it’s possible that the binoculars need collimation. However, this is not a fool proof way of telling if your binoculars are misaligned. If after having your binoculars fixed, you still get headaches and/or eye strain, you should consult a doctor.

Binoculars only need to be knocked slightly from alignment to cause eye strain and headaches so although not 100% proof, it is pretty accurate.

Double Vision

One of the major indicators of a collimation issue is seeing a double image through the binocular lenses. This occurs because your brain finds it difficult to merge the 2 images because of distortion.

The Night Sky

It is often difficult to spot collimation issues with the naked eye in daylight. But at night, when looking at the night sky, it’s easier to spot. This is because our brains compensate for some image discrepancies during the day with objects we’re familiar with. It’s a different story at night when looking at the stars. The images seen in the night sky are far less familiar to us so our brains can’t make up the difference. 

If you suspect your binoculars are misaligned, look at the night sky with them and see if you get any double images.

Defocus The Eyepiece

One of the most accurate ways of checking the binoculars for misalignment is to defocus one eyepiece. Looking through binoculars at night with one defocussed eyepiece, you should see a clear view through one lens and a blur through the other one. If the clear image of a star is not in the centre of the blur the binoculars are out of collimation.

The Use Of Colour

You can also trick your brain into seeing 2 images by colouring the lenses. Use some cellophane or sweet wrappers or even a pair of 3D glasses. The different coloured stars should line up exactly. If they don’t line up you need to collimate your binoculars.

Using Bahtinov Masks

A Bahtinov mask has a certain pattern that will cause the star to spike when the Bahtinov mask is placed over the binoculars objective lens. Each lens should show an x through the star and a spike running through the centre of the star. If you rotate one of the masks by 90 degrees this will show one star with a vertical spike through the x and the other one sees a horizontal spike.

If your binoculars are collimated they will show a single star with lots of spikes and if they’re not, they’ll show two separate images.

How To Collimate The Binoculars

Now we’ve identified that the binoculars are misaligned and need collimation, we can easily collimate them by following this step by step guide.

1. Locate The Collimation Screws
   To find the collimation screws check under the armour at the eye end of the barrels (the prism housing). They are usually small flat head screws located under the rubber or leatherette coating. Some binoculars have a small flap that covers the screws, but cheaper models often just cover the screws with the casing coating. The screws you’re looking for have flat heads not phillips, the philips screws are part of the binoculars construction.
2. Support Your Binoculars
   Once you’ve found the collimation screws, you need to support your binoculars on a tripod or some other support.
3. Focus On A Bright Object In The Night Sky
   Once the binoculars are supported, focus on the North star.
4. Defocus One Eyepiece
   Once you’ve located the North star use the diopter to defocus one eyepiece. This tricks your brain into looking at 2 different images. The image should look like a blurry blob.
5. Look Through The Binoculars
   Adjust the binoculars by using the hinge to get the interpupillary distance correct for your eyes. Now use the focus wheel to get a clear, sharp image. You should see the image of the star is off centre from the blurred blob.
6. Adjust The Collimation
   While looking through the binoculars adjust the collimation by turning the flat head screws. Just turn the screw no more than an 8th of a turn and only turn one screw at a time. As you turn the screws, the sharp image should begin to line up with the blurred blob. Once the defocused image sits directly below the focussed image you have completed collimation.
7. Replace The Cover
   Now collimation is complete, all you need to do is replace the cover on the adjusting screws and your binoculars are good to go.

Could The Misalignment Be Caused By Something Else?

After doing our research on this, we realised that sometimes, only sometimes, the objective lens tubes can become dislodged. To remedy this, simply unscrew the objective lens and retighten it taking care not to cross thread it. If this doesn’t work then sadly you will need to go through the above process to repair your binoculars.

Frequently Asked Questions