Q. Which is better, a binocular or spotting scopes?
Both eyes are used to look through a binocular while one eye is used to look through a spotting scope.
Generally, spotting scopes have a much higher magnification than binoculars, so far away images appear larger.
But a better three-dimentional image can be expected from binoculars, and they are easier to handle and have better portability.

Q. What is magnification?
Magnification describes how many times closer an image appears. Magnification is also called "power" and is abbreviated by the symbol "x". A magnification of 20 (20 power) means that an object appears twenty times closer than you would see it with the naked eye.
When looking through an optic with a magnification of 20x, an object at a distance of 100 yards /meters appears to be only 5 yards/meters away.

Q. What is the actual field of view (angle of view)?
Actual field of view is the view when looking through an optical device indicated by the angle from the center of the objective lens. An optical device with a of wide angle of view makes it easier to find an object. In general it is more difficult to find an object with an optical device with a higher magnification.

Q. What is a field of view at 1000m(1000yds)?
Field of View (FOV) is the width in yards/meters that can be seen at 1,000 yards/meters.
FOV equals the real angle of view x 52.4'. Every degree of real angle of view can see 52.4' at 1,000 yards/meters.

Q. What is objective lens diameter?
The measurement in mm of the diameter of the objective lens of an optic. As the diameter of the objective lens increases, the optic will have better light gathering and higher resolution.

Q. What is an exit pupil and a relative brightness?
Exit pupil is the bright circle that is visible when the ocular lens is viewed at a distance of about 25㎝(10inch)with the objective lens facing towards a bright background or light source.
The larger the exit pupil, the brighter the image will be under low-light conditions.
Exit pupil = Objective lens diameter ÷ Magnification
Relative brightness is a number used to compare the brightness of binoculars or spotting scopes of similar magnification. The larger the relative brightness number, the brighter the image. Relative Brightness = Exit Pupil2(or Exit Pupil size in mm, multiplied by itself).

Q. What is resolution?
Resolution, or ultimate sharpness, is the measure of the ability of a spotting scope or binocular to distinguish fine details. Optical glass is at its sharpest in the center, while resolution decreases toward the edges in relationship to the quality level of the lens. Larger objective lenses (with quality as a constant) tend to improve resolution. This is because they have a larger amount of sharp, center objective lens area, as opposed to the amount of lesser sharpness at the edge of the lens surface.

Q. What is minimum focus distance?
The closest distance to an object that the binocular/spotting scope will focus sharply.

Q. What is eye relief?
Eye relief is the distance, measured in mm, between the eyepiece lens to where the exit pupil is actually positioned in space (where the pupil of the eye must be positioned for a full view). A long eye relief makes observing for long periods of time easy and tireless.

Q. What is multi-coating?
When light enters a lens or prism, light is reflected from their surface at a rate of about 4%.
This reflection diminishes the brightness of the optic and causes flare or ghosting.
By coating the surface of the lens, the amount of reflection can be minimized, decreasing flare and ghosting.

Different Types of Coatings:
Coated optics: one or more surfaces coated.
Fully coated: all air-to-glass surfaces coated.
Multi-coated: one or more surfaces are multi-layer coated.
Fully multi-coated: all air-to-glass surfaces are multi-layer coated.

Q. What are the benefits of a gas filled optic?
Spotting scopes and binoculars are used in all types of weather, so it is important to protect the instrument from moisture. To keep the inside dry and keep moisture from penetrating from the  outside all of the air is purged form the inside of the optic and replaced with nitrogen gas.
The nitrogen gas prevents moisture from entering the scope and keeps the inside dry.

*If condensation appears on the optic, it is important to wipe the moisture from the optic immediately. Failure to wipe moisture from the optic might stain or promote the growth of mold or rust.

*In extremely high temperatures, materials that keep the optic airtight (rubber rings, oil, etc.) might degrade and the air tightness may become compromised.

Q. What is the difference between Achromat and Apochromat?

An Achromat lens system compensates the wavelength differences of two colors to diminish chromatic aberration.
For observation the two colors usually compensated are blue and red, and for photography, purple and red.
An achromat diminishes chromatic aberration so that only a little remains.

An Apochromat lens system compensates the wavelength differences of three colors or more. For example, an apochromat system is designed to make purple, green and red focus at the same point so that chromatic aberration is significantly reduced. It has become standard to use Fluorite Crystal, or a kind of XD (ED) lens for this type of lens system.

The above are general explanations and differ among optics manufacturers.
Recent refraction telescopes are called apochromat when Fluorite or XD (ED) lenses are used and achromat when other lens materials are used.