How to Evaluate a Binocular

There are some reasonable-quality inexpensive binoculars available nowadays. There are even more very poor quality inexpensive binoculars available. Here's some hints on telling the difference. These tests are done in daylight., either in the shop or at home if you bought by mail order (you did check the returns policy, didn't you??)

• Reject zoom binoculars. A decent zoom binocular has yet to be made.
• Reject any with "ruby" coated lenses. Ruby coatings are a gimmick designed to disguise poor optics.
• Reject quick-focus binoculars. They are also quick-defocus and are exceptionally difficult to focus properly.
• Give them a good shake. Reject any that have any internal noise.
• Check that the focus mechanism is smooth throughout its range, with no loose spots and no binding.
• Check that the right-eyepiece dioptre adjustment is smooth throughout its range, with no loose spots and no binding.
• Check that the hinge is smooth throughout its range, with no loose spots and no binding.
• Check that the bridge connecting the eyepieces (in porro-prism binoculars) does not rock under light or moderate pressure. This will cause the eyepieces to defocus in use.
• If they are to be used with spectacles, verify that the full field of view is available with the eyecups folded down and the binocular at "spectacle distance" (about 25mm). The edge of the field of view should be a sharp black circle.
• Check that your interpupillary distance (IPD; the distance between the pupils of your eyes) can be accomodated by the binoculars. In particular, if you have close-set eyes and/or a wide nose bridge, ensure that your nose can be physically accomodated between the eyepieces of the binoculars. This is a particular problem with some wide-angle eyepieces.
• Hold them at arm's length and look into the eyepieces. Is there a round circle of light, or is it diamond-shaped? If the latter, the prisms are under-sized (cost-cutting). If the cut-off bits are blue-grey, then it's full sized prisms, but they are BK7. This need not be a problem for binoculars used in good light.
• These three steps enably you to compare different binoculars:
  • Take them to the door of the shop and look at a distant, high-contrast, target (e.g. TV antenna against a bright sky), and focus it as best you can (Cap right side, focus left side with focus wheel, cap left side, focus right with dioptre ring). Does it snap to a good focus or is there a small range of "nearly there"?
  • How bad is the colour fringing? (there will be some).
  • How far out to the edge of the field of view is the image sharp? (it will break down towards the edge).
• Close your eyes while looking at something straight across the field of view (e.g. a roof ridge). Open them again. Were the images initially vertically displaced from each other? If so, reject the binoculars. (This is "step", aka "dipvergence", and should not be perceptible at all -- it leads to eye strain and headache.)
• Repeat the above with something vertical (e.g. antenna mast) to test for lateral displacement (convergence or divergence). A tiny amount is tolerable, but it's better to start off with none at all. (Eye strain again.)
• Move your vertical object to the edge of the field of view. It should curve slightly inwards at the middle. This is "pincushion distortion". A small amount is necessary to combat the "rolling ball" effect that you get if it is absent; "rolling ball" can make you feel nauseous (wearers of varifocal specs will probably know what I mean!). There is no "right" amount of distortion, but it shouldn't be obtrusive near the centre of the field of view.
• Back inside the shop, target something small and bright, like a halogen ceiling light, or carry a maglight or similar in your pocket for this purpose (use it in "candle" mode). As you move it out of the field of view do you get any false/ghost images? (poor or absent internal coatings) Do you get bits of light from it even when it is out of the field of view? (poor baffling)

How to evaluate a telescope