PRISM SYSTEM/TYPE
The prism system of a spotting scope reduces the size of a long optical path and correct an inverted image.

Roof Prism System
The prisms overlap and are aligned in a straight line with the lenses allowing to have a slim construction.

Porro Prism System
The objective is offset from the eyepiece offering greater depth perception and a wider field of view.

PRISM GLASS
The common standards of optical prisms are barium-crown (BAK-4) glass or borosilicate (BK-7) glass. The BAK- 4  is a higher quality glass that reduces light scattering, resulting in sharper and brighter images.

OBJECTIVE LENS DIAMETER
The second number in the reference (i.e. 20-60×60) indicates the diameter of the objective lens or the front lens. The larger the objective lens, the better the light transmission, thus the brighter
the image.

ED (Extra Low Dispersion) Glass
ED Glass is designed to provide precise optical color correction. It is multi-coated to improve contrast and transmission across the visual spectrum and provides reduced chromatic aberration.

MAGNIFICATION (POWER)
Magnification is the number of times the object being viewed is enlarged. The spotting scope is often identified by two numbers, for example: 20-60×60.  The first number indicates the magnification or the power of the scope, expressed by the letter “x”, for times. With this variable zoom power, that means that the object being viewed appears to be 20 to 60 times closer than would be seen with the naked eye.

OPTICAL COATINGS
Various coating processes on the lenses and prisms will determine the brightness and the light gathering of a spotting scope, providing higher contrast and brighter images. The types of coating are:

Coated -  A single layer on at least one lens surface.
Fully Coated -  A single layer on all air-to-glass lens surfaces
Multi-Coated -  Multiple layers on at least one lens surface.
Fully Multi-Coated -  Multiple layers on all air-to-glass surfaces.

FOCUS SYSTEM
The mechanism, usually with a turn-adjustable knob, that is used to focus the spotting scope on the object being viewed.

FIELD OF VIEW (F.O.V.)
This is the side-to-side linear measurement of the circular field seen through a spotting scope. It is defined by the width in feet of the area visible at 1000 yards, or in meters at 1000m. The higher the magnification, the narrower the field of view.

CLOSE FOCUS DISTANCE
The closest distance to the observed object that the spotting scope can be used while retaining a sharp focus.

RESOLUTION
The measurement of the scope’s ability to distinguish fine detail and sharpness.

EXIT PUPIL
This refers to the size of the small disc of light that exits a spotting scope. To determine the size, divide the objective lens diameter by the power.  For example a 20×60 spotting scope will have an exit pupil of 3mm.  (60/20=3)

EYE RELIEF
This is the distance a scope can be held away from the human eye and can still observe the entire field of view. Long Eye Relief (LER) reduces eyestrain and is more comfortable for eyeglass wearers.

WATERPROOF/FOGPROOF
Spotting scopes can be O-ring sealed for complete waterproof protection, and fogproof which means that they are nitrogen-purged to prevent anti-fogging inside the optical surfaces. These models are able to keep completely dry inside after immersion in water.