TelescopeBinoculars.org

Exit pupil is easy to calculate. It’s the size of the aperture divided by the power. So 7×35 binoculars, it’s 35 mm / 7 = 5 mm.

How big is the pupil of your eyes? Well, during the day, they’re quite small. Perhaps 2 mm. At night, when your eyes are adapted to the dark, they might be anywhere from 5 mm to 8 mm, depending on how dark it really is, and how old you are. So how do you find out? One way is to get a metric set of hex wrenches. Go out on a clear moonless night as much as possible away from light pollution. Find a star, and hold a small hex wrench in front of and horizontally across one eye. I’d try 4 mm first. Move it up and down a bit. If your pupil has dilated to larger than 4 mm, you should see two images – one above and one below the hex wrench. If you can see both, then move up to your next size. When you can no longer see two images of the star, you’re done. That’s your pupil size.

What does it mean? Well, if the exit pupil of your binoculars (or scope) is larger than your pupil, some of the light falls outside the pupil, and is not detected by your retina. The image will be dimmer than it might otherwise be. It’s OK if the exit pupil of your binoculars are smaller.

Let’s say that you’re on a boat at night. You’ve measured your pupils, and they’re 5 mm. And you have 7×35 binoculars. The exit pupil is 5 mm, just like your eyes. Everything is great, right? However, on a bouncing boat, it’s tough to keep the full circle of the exit pupil registered exactly on your pupils. It can be easier to keep 7×21 binoculars pointing at your eyes because the 3 mm exit pupil can wander around a bit and still be within your 5 mm pupils.

So why should some people have 5 mm and others have 8 mm dark adapted pupils? Mostly, it’s age. Really young people have smaller eyes – they’re still growing. By your late teens, they’re adult sized. But, you’re still reasonably young, and they can dilate to their maximum, probably closer to 8 mm. As you get older, they may only dilate out to 5 mm. There’s no formula, you have to measure them.

But you still might choose an eyepiece for a telescope for low magnification that has an exit pupil bigger than your own pupil. I can think of two reasons. First, you may be an old guy with 5 mm pupils, and you want to show younger folk with larger pupils. Second, you often go with low power for wide fields of view, rather than for maximum brightness. And, the view may not be noticeably dimmer with a slightly too large exit pupil – and the extra field of view may be desirable.

Astronomical binoculars often have higher power as well as larger aperture. For example, Celestron 25×100 binoculars have a 4 mm exit pupil. Vixen makes binoculars with replaceable eyepieces, meaning the exit pupil can be anything.

http://www.telescope.com/control/product/~category_id=binocular_astronomy/~pcategory=binoculars/~product_id=09119

Not to simply be contrary. My 10 mm telescope eyepiece has 6.3 mm eye relief. I like it. In particular, it means my eye is close enough to the eyepiece to limit stray light from my surroundings. And, my eyelashes can be used to gauge the distance easily so that i can hold my head steady. I wear glasses, but not while observing. While eye relief can be important from a comfort level, it’s more at matter of personal preference than most people let on.

Exit pupil is less important than eye relief. All binoculars have an exit pupil that is well within reasonable range (see more below). A 10×50 binoculars have a 5 mm exit pupil which is just perfect. A 7×50 will have a 7 mm exit pupil which is also very good. You can calculate the exit pupil by dividing the aperture by the magnification.

Now, about eye relief. Around 20 mm is a comfortable eye relief. Short eye relief (less then 13-15 mm) makes it hard for people with glasses to see the full field of view. Less then 10 mm is uncomfortable for anybody (with or without glasses). Very long eye relief (more than 25 mm) makes it difficult for some people to center the image and can introduce other artifacts such as "kidney bean" distortion. You don’t say which model you are planning to buy, but I am sure that their eye relief will be well within the comfortable range (most likely between 15 and 23 mm).

Going back to exit pupil. Too big exit pupil (more that 7 mm) means that some of the light will be "wasted" because it won’t fit within your eye’s pupil (which typically is 7 mm for young people, and decreases to about 5 mm with age). Too small exit pupil (less then 1 mm) amplifies the effect of the shadows that dead cells floating inside the eye project on the retina. Young people don’t have this problem, but it can be quite noticeable for people over 50. But this is only relevant for telescope eyepieces – binocular exit pupils don’t get even close to this lower limit.