Testing Binoculars

Repair : Learn how to repair binoculars.

One of the most important features of binoculars is the hardest to assess - optical quality. Look through each barrel of the binoculars to see if the full field of the binocular is in focus and relatively free from distortion. Good alignment is probably the most important optical characteristic. The two halves of the binoculars are essentially two telescopes. If they don't present essentially identical images to your eyes, this can cause severe eye strain.  Here are a few things you can try to get a rough measure of alignment.

An especially critical point for eyeglass (frames, not contacts) wearers is the "eye relief" of the binoculars.  Eye relief is a measure of how far away from the binocular lenses your eyes can be and still see the full field of the binoculars. Most binoculars you will consider will have some form of eye cups which are extended for people who don't wear glasses and retracted for those who do. You can check the adequacy of the eye relief in a couple of ways. If you have to move the binoculars from side to side to see the full field, the eye relief is inadequate. Similarly, if you can see a larger field when looking through the binoculars without your glasses than you can with your glasses on, there is inadequate eye relief. Your eye is the real test here. However, if you have the specifications on the binoculars, look for eye relief of 15 mm or more. If you do wear glasses or contacts, wear them when you test and use binoculars. If your correction is for myopia only (no astigmatism), you can use binoculars without glasses and not develop eyestrain.

If your eyes happen to be closely spaced, or if a child will use the binoculars, be sure the distance between the eyepieces (interocular distance) can be adjusted to match.

Don't look through store windows when testing binoculars. Windows introduce a great deal of distortion. As part of your test, look at a dimly lighted area to see how bright the image is. This is comparable to looking at birds in shadows.

The ultimate test of comfort is how the binoculars work for you. If possible, buy binoculars on approval and spend several hours in the field using them.