To get a pair of binoculars that is worth the money, you should perform a few simple tests. Since the variations in quality even with identical models can be very large, you should not perform these tests on a demonstration model, but on the device that you really want to buy. Mainly because of the quality fluctuations, price alone is not a performance criterion – very good units of a less expensive series may have better value for the money than poorer units of a good manufacturer.
Quality of prisms: If you hold the binoculars at arm‘s length and look into the eyepieces, you should see two circular light spots that are not distorted to diamond shapes or are surrounded by reflections.
Interior: Dust and scratches on the inside of the binoculars can be seen if you hold the binoculars up to a light (not the Sun!) and look into to the tubes through the front lenses.
Image quality on the edge of the field: Look at a straight edge (for example, an antenna or a building edge). Is the image severely distorted at the edge of the field or is there a blue shimmer (color error)? Only very expensive equipment can provide a near perfect image, but also less expensive binoculars should be corrected to the point that the error is only visible close to the edge and is not to obvious going too far into the field.
Mechanics: Can eye relief and sharpness be adjusted and are neither too stiff or too loose? Is there a thread for a ball head available?
Alignment: When the two tubes do not point in the same direction, you will see in extreme cases two images. The eye can compensate a small maladjustment though, but after a short time you will get a headache. Cover one lens with one hand and remove it again while you look through the eyepieces – if you can see a double image for a short time, the binocular is defective.
Picture Quality: Stars should be depicted as sharp points and appear circular when out of focus, instead of being distorted into ovals. You can build an artificial star by puncturing a hole with a needle into a piece of aluminum foil and lighting it from behind with a lamp or a candle. From about 10 yards (meters) away, the view is similar to that of a star.
Handling: The focus should not be a problem – on some devices, the focus range is very narrow. Some binoculars cannot focus both eyepieces with a common central focuser, but are focused directly at the eyepieces, which takes a little more time. The eye caps must be adjustable to a comfortable level.
For used equipment: Are dust covers and case still available?