Optical Quality
An optical quality test
Even binoculars with large objective lenses can have poor
light-gathering ability if the optics are poor. Examine the edges of
the exit pupil to see if it forms a complete, bright circle (above,
left) or if it is partially shaded in gray (above, right). If only the
center of the exit pupil is bright, then inferior optics are blocking
some of the light, counteracting the advantages of the large objective
lenses.
Coated optics
Light entering the objective lens passes through up to eight pieces of optical glass in each barrel. At each glass surface some light is reflected backward rather than passing through the prisms and lenses. The optics of well-made binoculars are coated with non-reflective film that helps deliver more than 90% of the light gathered by the objective lenses. Without non-reflective coating, binoculars may lose as much as 60% of that light.
Although you should never look at the sun directly through binoculars,
coated optics also help when you're looking at backlit subjects. Within
uncoated binoculars, reflected light causes annoying glare.
Always choose binoculars with "fully coated" optics. Beware that,
although most manufacturers coat exterior lenses, the interior optics
of some inexpensive binoculars may be uncoated, causing a significant
loss of light. Also note that the quality of coating varies; price
makes a difference, with more expensive binoculars having better
quality optical coating.
Resolution
The quality of the optical glass used in binocular manufacture affects
resolution (the ability to separate objects visually) and image
sharpness. High quality optical glass is extremely expensive, and each
lens and prism must be professionally ground and mounted with expert
precision. Top-of-the-line binoculars usually have excellent optics,
producing images that are crisp across the entire field of view.
Manufacturers of lesser products cut corners, often by using cheaper
glass and looser quality control.
To check the center-to-edge resolution of a pair of binoculars, focus
them on a map or newspaper tacked to a wall. Stand back about 25 feet
and see if you can read the print at both the center and edge of the field of view.
