ABBERATION (or image distortion)
Distortion is the enemy of resolution. Distortion (or aberration) is most often detectable at the very edges of the lens, because as the lens curves it becomes more and more difficult to maintain image quality. Some scope makers try to overcome this distortion by employing a retaining ring that effectively blots out the edge of the lens. Of course, this makes the scope appear to be sharper at the edges, but it also greatly reduces the actual amount of lens being used to transmit light. The result is a dimmer image at dawn and dusk or in any poor lighting condition. Nikon riflescopes provide tremendous edge to edge sharpness because of the quality of the lens glass, the care taken in its preparation, and the expert use of anti-reflective multicoatings.
This means that the objective lens can be adjusted to remove parallax at a variety of distances, to provide a sharp, clear image at high magnification.
The adjustment graduation varies by scope, but is either 1/2, 1/4, or 1/8 MOA, or minute of angle, which is equal to 1/2", 1/4" or 1/8" at 100 yards.
A coating applied to lenses to improve the color and contrast of the image and maximize brightness. Quality, type and method of coating applied is of critical importance to brightness, clarity and contrast. Nikon uses only the finest sulfide materials, which give the lenses a distinctive magenta or green/magenta appearance. Nikon also coats all lens surfaces within the binocular/scope for maximum light transmission and glare reduction.
APPARENT ANGLE OF VIEW
The segment of a 360§ circle that appears to be in view, factoring in the magnification of the binocular or spotting scope. It is calculated by multiplying the magnification by the real angle of view. Binoculars with apparent angles of view greater than 60§ are termed wide-angle instruments.
ASPHERICAL EYEPIECE LENS
A non-spherical shaped lens used to minimize aberration and maximize the flatness of field in less expensive binocular models.
ATB ALL TERRAIN BINOCULAR
A term coined by Nikon for its group of rugged, waterproof/fogproof binoculars.
The lenses in the two barrels of the binocular should be closely aligned. Severely misaligned barrels are immediately apparent because of the perceived "double image" they produce. Slightly misaligned barrels can cause eyestrain and headaches as your eyes struggle to compensate for the misalignment.
On Nikon Buckmasters scope lenses, the lens coating is called Brightvue and it allows up to 92 percent light transmission.
CENTRAL FOCUS CONTROL
A knob that controls the lens alignment in both barrels of the binocular. A separate diopter control allows you to compensate for the difference in eyesight between your right eye and left eye by offsetting lenses in the two barrels so the image is sharp for each eye. After diopter is set, using central focus knob will give you a quick, sharp, strain-free view.
CLOSE FOCUS DISTANCE
The closest distance to an object that the binocular/spotting scope will focus sharply.
Compact binoculars are small, lightweight and convenient, and they can be taken anywhere binoculars are needed.
DEPT OF FIELD
The degree to which the prisms flatten the viewed image. Porro prism binoculars, with their off-set design, have greater depth of field than roof prisms, producing a more lifelike, three-dimensional image.
A separate diopter control allows you to compensate for the difference in eyesight between your right eye and left eye by offsetting the lenses in the two barrels so the image is sharp for each eye. To set the binocular specifically for your eyes, first move the barrels up and down until the two images merge into one. Set the diopter at zero. Then, cover the objective lens on the barrel with the diopter control and adjust the central focusing knob until the image is sharp. Now cover the objective lens on the barrel without the diopter (do not change your distance from the subject). If your eyes were visually equal, the subject should be sharp with this other eye since the diopter is set at zero. If not, turn the diopter control, which only moves the lenses in its barrel, to change the lens positions until they match the sharp focus attained by the central focus knob for the other eye. Once done, as long as the diopter control is in this established position, and your eye prescription does not change, the central focus knob will always give a quick, sharp, strain-free view.