How to test binoculars? (1 Viewer)

What kinds of tests can one perform on binoculars to make sure they are not defective/damaged/etc? I know about the "continuous horizontal line in both eyepieces" vertical collimation test:
http://www.birdwatching.com/optics/education/alignment.html

What else is there? Anything involving stars worth doing? Any good way to test horizontal alignment?

You will get some responses. But I think the best test is to spend a few hours in the field. If they are out of alignement, you will get eye strain. I am sensitive to looking at things too close with porros, so I avoid that test, they all look bad. But at a reasonable distance they should all be, porros and roofs, comfortable.

Tero is right, provided you have some observing experience. But newer bino users often want a simply interpretable test. I'll just speak to the stars question. I spend a lot of time looking at them myself, and I'm not the only stargazer here, but since it's not too common on this forum, their optics testing benefits may not be widely appreciated.

First, they are very sensitive indicators of collimation. Look at a bird, and if the bino is not well collimated, the eyes will go to great lengths to line the two birds up, which could fool you, until you feel the eyestrain, into thinking the bino is okay. But look at a crowded starfield, and the all the stars look pretty much alike. The eyes don't know which two points to line up, and collimation errors become quite obvious. If crowded starfields are instantly seen with aligned images, that is good news.

Second, bright point sources like stars are very severe tests of optical quality. Almost too severe, some people might say. And, stars test your eyesight along with the bino optics, and it can take some thought to figure out whether a flaw is in the bino or in yourself. But if your eyes are not astigmatic, and you use your binocular to look at a bright star, you will ideally see a compact and mostly circular blob of light. If the image is big and misshapen and blobby, or there are big spikes sticking out, that is bad. Viewing one eyed, and rotating the suspect barrel, will tell you if it's the bino at fault, or your vision.
Ron

Ron says it better than I could. One of the aberrations he mentions above, when a star looks more like a tear-drop, with a short tail, is called coma. This is more something that occurs in telescopes with curved mirrors, but could be something else for you to watch out for with binoculars. You are more likely though, when looking at a star with cheap binoculars, to see a sort of hazy halo around the star. But I'm talking more about optical quality, you were specifically asking more about optical damage or defects.

spitfiretriple said:

Ron says it better than I could. One of the aberrations he mentions above, when a star looks more like a tear-drop, with a short tail, is called coma. This is more something that occurs in telescopes with curved mirrors, but could be something else for you to watch out for with binoculars.

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If a "comatic" star image is seen near the center of the field it may be a sign of astigmatism or some other aberration. Telescopes with curved mirrors (reflecting telescopes) that are made and aligned properly will show coma only at the edges when using a wide-field eyepiece.

With binoculars, if you can't get a bright star that is centered in the field to focus to a "reasonably" round and tiny dot, then there is trouble. Like my recently-repaired Audubon 804R's, I had that problem and it required a lot of internal adjustment to get things right.

Howard

Resolution Chart

Good advice here on BF, actually best I have ever found on the web.

I look a the resolution chart to set my Diopter and this usually can identify bins that are not up to snuff. Not sure who said it, think it was Kimo, but the bins should 'snap' into focus, w/o having to keep focusing. I think this tests depth of field or depth of focus but it is a great, quick, 'in-store' test.

Pick a star and "move it" around the field to see the effects described above. The bin should be symmetrical in all of it's distortions/aberations and consistent between barrels. If not that could indicate the elements in a single barrel are not collimated (i.e. coaxial and not tilted).

The star test also lets differentiation between field curvature (the blurriness you can "focus out") and astimatism (the blurriness you can't "focus out").

If you books the bin then a real star test can tell you a fair amount about the quality of the optics in the bin and how well aligned they are. Henry Link has written about this here many times.

Use one eye (your better eye!) on both barrels to minimize differences between your eyes. It's not how you use the bin in real life but it's better for testing.

I clearly have one "good" eye and one "bad" eye when it comes to star tests (even with vision correction ... I think my bad eye has some higher order aberrations).

You can also do this sort of "star" test in daylight with a convex reflective object (like a christmas decoration or ball bearing at birding distance. Even a curved car body can be used in a pinch. Just make sure it's not planar ... you have to be careful about looking at reflections of the sun.

A small mini maglight in "candle" mode (in a dark room, if possible) is also useful for checking ghost reflections between the optical elements and seeing spikes from diffraction from the roof edge in roof prism bins.

These latter too tests are useful for "in store screening" of a bin before purchase. Or a quick test.

Resolution tests are not much use unless you use a booster behind the bin's ocular. Without the booster you are just measuring your eye's acuity at that particular time as the bin should have "more resolving power" than you can see. A bin that has lower resolving power than your eye should be clearly apparent as being a "poor" bin without the need to look at a resolution chart.

statestat said:

I look a the resolution chart to set my Diopter and this usually can identify bins that are not up to snuff. Not sure who said it, think it was Kimo, but the bins should 'snap' into focus, w/o having to keep focusing. I think this tests depth of field or depth of focus but it is a great, quick, 'in-store' test.

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Any distant but just readable text is good enough for diopter setting.

The actual DOF should be constant for a given magnification (we've had this argument here before).

The perceived DOF seems to be a complicated interaction of field curvature, focus rate, equalization of diopter setting. I would consider that part of an evaluation rather than a "test".

Kevin's mention of the maglite also reminded me that using a flashlight to look inside the bins can sometimes reveal issues with particles or dust. Might be another way to do quick in-store test.

Kevin Purcell said:

A small mini maglight in "candle" mode (in a dark room, if possible) is also useful for checking ghost reflections between the optical elements and seeing spikes from diffraction from the roof edge in roof prism bins.

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Kevin, what an excellent tip, tried this method last night with the mini-mag and i am going to try the ball-bearing today. B