What are "IPD" and "eye relief", and why are they important in selecting a binocular?
It's not uncommon for the novice optics buyer to be daunted and confused by a list of binocular specifications. "Don't you just pick them up and look through them?", you think. Well, yes, you do just exactly that - after you've set them up for yourself by making up to three different adjustments. Those adjustments include moving the bridge or hinge to match your IPD, turning eyecups up or down, and adjusting the diopter for one eye.
While most people can use just about any binocular on the market, for a not-insignificant minority, that's simply not true - there are binoculars that will be a bit of a struggle to use, or even just won't work for them, usually because of the IPD range or eye relief. So let's delve into those two factors because they're important to know before you choose a binocular.
Eye relief is simpler, so let's tackle that first. All multi-lens magnifying optics, whether compound loupe, microscope, telescope, rangefinder, or binocular "project" the final image above the surface of the ocular lens or eyepiece. The distance this image "floats" above the glass is the eye relief. With binoculars, it's usually as least 10 mm, though a few cheap compacts may be even less. If you don't wear glasses, or wear contacts, eye relief usually isn't critical. But if you DO wear eyeglasses, don't buy a binocular without at least 15 mm of eye relief! This is because your eyeglasses are a few mm thick, and they're a few mm from your eye. More eye relief such as 17 to 20 mm is even better. You also want adjustable twist-up/down type eyecups, ideally with intermediate positions, meaning at least one firm click-stop or detents between "up" and "down".
Now, on to "IPD", which stands for interpupillary distance. That's the distance between the centers of your pupils (in other words, how far your eyes are apart). As adults, our IPD is essentially fixed, though it can vary slightly depending on whether you are focusing on a near or far object. When focusing very close, your IPD effectively becomes narrower by up to two or three millimeters, which is why with some extremely close-focusing binoculars, we might experience parallax, a situation where the left and right images appear to separate or diverge in opposite directions and you see double.
IPD is among the few measurements Americans are accustomed to seeing in millimeters. The range, from children of about five years old to the widest-faced adults, is 40 mm to 80 mm. Some double-hinge folding compacts go as close as 35 mm, and we carry several inexpensive Porro prism binoculars which fold down to 50 mm; either of these may be suitable for elementary school-aged children and adults towards the lower end of the IPD range.
The mean IPD for all adults is about 63 mm. Over 90% fall between 58 and 73 mm, so it's no surprise many companies have IPD ranges close to this. Still, even a 2012 sample in the US Army found a range of 51 - 74.5 mm for women and 53 to 77 mm for men. For anyone at the lower end of those ranges, a binocular with a minimum IPD of 58 or even 56 mm will not allow complete overlap of the left and right images.
Even if a binocular won't fold quite close enough for you, there may be enough overlap to make them usable. Say, for example, your IPD is 56 mm, and you try a 7x50 with a minimum IPD of 58 mm. The exit pupil of that 7x50 is a very forgiving 7 mm, with an area of nearly 40 sq mm, so you might not notice any shortcomings, since it covers most of your pupil. But the very much smaller 3.2 mm exit pupil (8 sq. mm) of a 10x32 won't work in the same scenario.
Since many binoculars have a minimum IPD of 58 mm, if your IPD is less, you should know what it is! And you shouldn't settle for "usable" when "just right" is out there.
References on IPD Range: https://www.researchgate.net/publication/229084829_Variation_and_extrema_of_human_interpupillary_distance