Telescope Reviews: Testing Binoculars in the Store Before Buying

We got this question not long ago from an interested forum participant.

“I was wondering if someone could point me to a web page that describes how to adequately test binoculars in a store before buying?”

Lots of people responded. This is a collection of the advice that was given. There was other discussion.. You can see the original thread here.
Binocular Testing

There is no doubt that some things cannot be checked in a short period of time. Personally, I'm of the opinion that to check out a binocular takes weeks of use. Some things just don't come out at you until you've used it for a while. It takes quite a while to really evaluate a pair of binoculars.

If you are looking at binoculars for astronomy, then I think a respectable business would provide a means for you to somehow have a look at night.

I think the best you can do in stores is to judge the brightness and resolution and to see if the ergonomics are to your liking. A $1 bill taped to a wall 15 ft or more distant can be a way of checking resolution. Larger binoculars like 12x50 need about 25 feet, 15x70 need 60 feet and 20x80 might need 100 feet.

Look at some reflections of lights in the coatings. In the objective lens you should see several reflections and ALL of them should be color reflections, none white. Generally a white reflection indicates some uncoated surface.

Better coatings reflect less light back at you. Look for coatings that reflect the least amount of light back.

Look down inside the binocular through the objective lens. In the circular metal ring that surrounds the prism face, you should not see any prism edges exposed in better binoculars. Also you should not see any sides of the other prisms protruding into the light path. A very small clipping of the exit pupil is often seen in mid-priced binoculars.

Hold the binoculars 6 inches to a foot in front of you and look at the exit pupil, the little beam of light that comes out of the eyepieces. It should be perfectly round. If it is noticably misshapen (oval) in any way, reject it.

Look very closely at the exit pupil for any straight edges cutting into the round exit pupil. Extrememly small edges cut off of the round exit pupil won't do much harm, but in a premium binocular reject any with big edge cuts.

Once more look at the exit pupils. If you see a diamond shape around the exit pupil where the light is dimmed in the diamond shape, these are Bk7 prisms. these will probably show the dimmed view in use.

Check to see if the binoculars can come together narrow enough for the width of your eyes. Inter-pupilary distance, IPD, can range from 52mm to 78mm on some binoculars, but on others only 60mm to 72mm. If you have eyes that are set apart by only 56-58mm, you must spend the time to find a binocular that adjusts narrow enough to fit. If you are endowed with a large nose, you may need a binocular with a very wide setting.

If the binocular is to be used by children as young as 7 or 8, the IPD will need to accommodate very narrow set eyes. Of the hundreds of young kids that I've had out to view, quite a few had a problem at 59mm IPD (Orion Ultraview, Minolta Activa), most were served well with a 57mm IPD. Only very few need a binocular with IPD narrower than 55mm. Two young 4th grade girls out of 13 kids in my recent class needed less than 55mm.

Press against the eyepieces while holding them up to your eyes. If the eyepieces are loose and move in allowing you to change the focus, reject that binocular.

Make sure the right diopter has enough range to accommodate your prescription. Some binoculars do not have enough right diopter adjustment.

Look AT the edges of the body outside of the front objective lens. Look to see that both objectives are recessed from the front of the binocular by the same distance. If they are substantially different it will throw off the diopter settings when you focus and you may not get the use of all the diopter adjustment that you need for your eyes.

Focus at some distant sign. Check the image across the field of view, not only at the very center of the field, but also at the edges and midway between center and edge. How does the image look, clear and sharp or blurry? A binocular should have at least 60% to 70% of the central view clear and sharp. Any less and I would reject it.

Edge sharpness of binos is quite easy to tell when you get a pair under the stars but difficult to evaluate when viewing cityscape in the daytime. Binoculars that seem perfectly sharp in daylight can have miserable off-axis performance on the night sky, even costly premium instruments. Pinpoint star images are a much more critical test for sharpness than anything that can be done in daylight.

Shake the binocular. If you hear anything rattle, reject it.

Must you wear glasses while observing? Check to see if you can observe the entire field of view with your glasses on. If not, there is not enough eye relief.

Even non-glasses wearers should check the eye relief. If you can't see the entire field of view without glasses, pick another binocular.

Observe some very high contrast object, like a black pole against a brightly lit background, or a white post against a dark background. Do you see a lot of color fringing around the edges of the object? Do you find it objectionable?

If you don't like the way a binocular feels in your hands, it probably isn't a good choice for you no matter what the specs indicate. One of the primary advantages of binoculars over telescopes is comfort and ease of use. No matter how outstanding a binocular is optically, if it isn't comfortable for YOU, it's probably not a good choice.

Lighter is not always easier to hold for longer. After you've tried a few different binocs, you will find some that feel good and some that feel not-so-good. Not-so-good will usually get worse the more you use them.

An outstanding binocular has to be excellent optically AND ergonomically.

I've never seen ANY daytime test that gives as precise a collimation check as does checking on a star. Checking collimation at 100-200 ft. is not the same as collimation at infinity.

Certainly you can give this a try. With you eyes about an inch or two behind the binoculars, allow your eyes to let the images in each side go loose and watch to see if they come back together or merge. If collimation is really way off, you can tell in an instant.

Move every hinge and turn every dial, feel for sticking spots. Any bad humps or real sticky spots, try out another pair.

Turn the focus dial so the eyepieces are half way out. Now grab the whole eyepiece bar assembly with your fingers and try to rock it. Does it require some force to make it rock, or does it seem flimsy and rock in and out real easy? Reject flimsy.

If you don't know the differences between what the fields of view (fov) are like, then you need to try about three different sized pair with different fov. Put one edge of your view on the exact same spot for each binocular and make a note of how far across the wall it can see. Compare with the others. Every 1° field of view is 1.75 feet across at a distance of 100 feet.

You can test for pincushion distortion on a distant high-contrast sharp line, distant TV antenna or power lines or a vertical post and moving it to the edge of the field. Look for a curve in the vertical or horizontal edge. This is not necessarily bad for astronomy, but might be a reason to reject if terrestrial use is your priority.

While looking through the binocular, see what other items are for sale in the store. If the shop is full of items such as thermos flasks, thermal socks, walking boots and cheap compasses, you are probably in the wrong store.

For waterproof binoculars, it can be very useful to take along a bucket full of salty water. Ask the assistant to dunk the binocular in it for 3 minutes. Then remove it and see if there are any signs of water ingress.

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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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