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??)
If you are satisfied with the binocular you have evaluated, buy that one, not an untested one in an unopened box!
- Reject zoom binoculars. A decent zoom binocular for astronomy 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.
- If they are of Zeiss (aka "European") construction (objective barrels screw into the prism housing), check that they are not cross-threaded. The tell-tale sign is an uneven gap between the objective tube and the prism housing.
- With a bright light behind you, look into the objective end of the binocular for signs of dust or other foreign matter.
- 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.
- Fold-down rubber eyecups should be checked for splitting. You should also check under them (by sight and/or palpation through the rubber) to make sure that they are not concealing any damage to the eyepiece tubes.
- Twist-up eyecups should be checked for smoothness of operation and that they lock in place when extended or in steps (as appropriate). You should also check under the rubber cups (by sight and/or palpation through the rubber) to make sure that they are not concealing any damage to the eyepiece tubes.
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Check that your interpupillary distance (IPD) 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 or with narrow fields of view.
- Some budget binoculars effectively stop down the aperture internally; this often makes a 10x50 effectively a 10x42, or a 15x70 effectively a 15x63. You can check for this with a piece of graph paper ruled in millimetres; Hod it behind the eyepiece with the binocular pointing towards an open window or distant light source (not the Sun!), and move the paper until you have the smallest possible circle of light. The size of this circle should be the aperture divided by the magnification in millimetres (e.g. a 10x50 should give you a circle of 50mm/10 = 5mm diameter). If it is smaller, there is probably internal stopping that reduces the effective aperture (a reduction of less than 5% should not alarm you, since this could be due to an increased magnification that falls within industry-standard tolerances). For a more accurate method of measuring aperture, see here.
These next three steps enable 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 except in very high quality binoculars).
- How far out to the edge of the field of view is the image sharp?
(it will break down towards the edge except in very high quality binoculars).
The next two steps are easier if you can "fool" your vision into thinking the objects in each side of the binocular
are different. The easiest way to do this is to use different colour filters on each side, such as cellophane sweet wrappers or the red-blue 3-D spectacles.
- 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.)
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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 desirable 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, the LED flashlight on a mobile phone, 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)
Additional tests for Used Binoculars
- Give the binocular a thorough visual inspection for evidence of repair or tampering. If there is any, try to find out what has been done.
- Wherever possible, check for damage under eyepiece rubbers (by palpation if they cannot easily be lifted).
- Inspect external optical surfaces for scratches. This is best done viewing the surface at a glancing angle, which may reveal fine sleeks that result from improper cleaning. Eyepieces are particularly prone to this, since they tend to accumulate more gunge, which often wiped off with a tee-shirt hem or similar.
- Look for evidence of failing optical adhesive between lens elements. This may have a milky appearance in patches, and often starts at the edge of the lens. It is usually prohibitively expensive to correct.
- Look for signs of fungal growth. This is most likely to be seen on the edges of lenses, where it can penetrate between the lens elements. If it has penetrated, it is expensive to correct.
- Compare the view through both sides. If there is internal optical damage, it is usually not symmetrical, so one side of the binocular may have a different apparent colour, a different clarity, or a different amount of light scatter.
Acknowledgement: I am indebted to the following members of Stargazers Lounge forums, all of whom have made suggestions for inclusions or improvements to this page: cantab, mdstuart, pete_gamby. (If there's anyone I've missed, I apologise, and please tell me!)
