What Does Interpupillary Distance Mean On Binoculars?

Interpupillary Distance (IPD) is the distance between the centres of both your pupils. Everyone’s IPD is unique, which is why the lens tubes on most binoculars are adjustable. The IPD is an important thing to be aware of especially when looking through binoculars. This is because when looking through the lens of the binoculars, if the ocular lens (the lens closest to your eyes) doesn’t line up perfectly with your eyes, you’ll see a dark halo around the image you’re trying to view.

As this dark halo will be blocking part of the image, you won’t be able to see the full image unless your eyes and the ocular lens line up correctly. Practically all binoculars have a way to account for the differences in IPDs. The majority have some sort of hinge system between the lens tubes, which allows you to adjust the ocular lenses along with the lens tubes to fit your personal IPD.

What Are The Differences Between Human IPDs?

The variations in the Interpupillary distance of human beings is influenced by many factors including;

• Gender
  The differences between male and female IPD can be as much as 4mm.
• Race
  In one study ^[1] the differences between Turkish and Azerbaijani Turks was noted to be significant.
• Age
  In a study by the US government ^[2] the aging process can cause IPD to decrease by almost 1mm in a 15 year period.

Interestingly enough, a survey conducted in the US army ^[3] found an IPD variation of between 53 to 77mm for men and 51 to 74.5mm for women. 

Anyone at the lower end of those figures could find it difficult to see a clear image through binoculars with a minimum IPD of say 56mm, because there will not be a complete overlap of the right and left ocular image. Some binoculars that fold in the centre, might offer enough overlap to be usable. If for example your IPD was 56mm and you use a pair of 7×50 binoculars with a minimum IPD of 58mm. 7×50 binoculars have an exit pupil of 7mm which means it’s very probable that you wouldn’t even notice the 2mm loss of IPD.

However this wouldn’t work out so well with binoculars of say, 10×32, as they have an exit pupil value of 3.2mm, which leaves no room for era. It is quite important therefore, that you are familiar with your own IPD so you can be sure to get the correct binoculars for the perfect viewing experience.

How To Measure Your Own Interpupillary Distance (IPD)

As we’ve just established, everyone has a different IPD, and it’s important to get binoculars with the correct IPD to get the full image as clearly as possible. To measure your own IPD just follow the steps below.

1. Stand In front Of A Mirror
   A distance of 8 inches (20cms) is the perfect distance.
2. Place A Ruler Against Your Forehead
   As close to your eyebrows as possible.
3. Close Your Right Eye
   With the right eye closed, align the 0 of the ruler to the centre of the left pupil.
4. Close Your Left Eye
   Open your right eye, and look at where the ruler measure is at the centre of the right pupil.
5. Take Note Of The Figure On The Ruler that Lines Up With The Centre Of The Right Pupil
   The number you just noted is your interpupillary Distance.

How Do You Adjust The Binoculars To Fit Your IPD?

Now you have your IPD you’ll need to adjust your binoculars to get the best possible image through the lenses. On many binoculars all you need to do is adjust the width of the lens tubes by opening or closing the hinge until you see just one image through the binoculars. At their widest setting, you’ll see two circles through the lenses, as you slowly adjust the tubes inwards, you’ll see those images merge. 

Once the image has completely merged into just one image, you have successfully adjusted the binoculars to the correct setting for your eyes. There is often a dial with a set of numbers on the hinge, you can note down the setting at its perfect point for future usage. If they don’t have any numbers don’t worry too much, because once you’ve worked out how to set the binoculars to your IPD, it will become second nature.

How Will Incorrect IPD Settings Affect The Binocular User?

If the binoculars haven’t been set up to perfectly fit your IPD, you will not see a full, clear, crisp image. But that’s just part of the problem, The physical effects on you will be far more uncomfortable than just a dodgy image through the lenses. The first problem you’ll probably encounter will be eye strain.

Eye strain can lead to headaches as our brains try to make sense of the poor image. This can in some cases lead to migraines, caused in some part by fatigue. So to prevent eye strain, headaches and possible migraine, be sure to set the correct IPD on your binoculars.

Frequently Asked Questions