This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
Contributed by (pe).
The quality of binoculars shows up in several important areas. this is
certainly one product area that the quality can range from junk to
excellent, and you get what you pay for. The areas of prime concern are
1) Eye relief: This is the distance back from the eye piece that the image
is formed. Most binoculars have a rubber eye piece that positions your
eyes in the proper place. This rubber piece can then be folded out of the
way for people who wear glasses. A longer eye relief is more forgiving to
those who wear glasses.
2) EXIT PUPIL: Generally tied closely to eye relief, this is the diameter
of the image comming out of the eye piece. The larger this is, the less
sensitive it will be to having your eye is in the exact right spot. Generally
speaking, larger is better. But to make it larger, the overall size of the
3) Light Transmission: The percentage of light that enters the front lens
that makes it out the eye piece. For daylight use, this is not too critical.
For nightime use, a few percent improvement in the amount of light
making it through can make a hugh difference. The type of optics (glass
versus plastic), the coatings on the lens elements, and the overall quality
of teh lenses make the difference. Large, GLASS, coated optics give much
better performance than plastic, uncoated optics. Of course, large glass
elements start to get heavy.
4) Depth of Field: As a side effect of the above three items is an improved
depth of field. This is the distance that an object remains in focus. The
really good units don't even have a focus knob, as the depth of feild is so
large that it isn't necessary.
5) GAS FILLED: The better units are sealed, and purged with dry
nitrogen. This keeps moisture out, keeps the lenses from fogging, and
helps improve the overall optical qualities.
6) THE CASE: A rubber armored, rugged case will help prevent damage.
Lens caps that stay with the unit keep them from getting lost, and make
it much more likely that you will put them back on to protect the lenses.
You may want to check out the West Marine catalog. They have a chart
listing all the important characteristics of the binoculars that they sell.
Compare it against the specs of a unit you are considering. Decide if you
might ever need to read the number on a channel marker at night.
My advice is to go with the best that you can afford. Properly treated,
they will last forever and you will not be sorry.