What do the numbers on my scope mean?
All binoculars and telescopes are marked with a set of two numbers separated by a X sign; 8 X 20, 8 x 30, 8 x 40, 10 x 50, 30 x 80, and so on.
The first figure indicates the degree of lens magnification and the second, the diameter of the objective lenses, in millimeters.
Magnification
The magnification is the lens power of a telescope or binoculars.
Magnifications of between 6.5 x and 10 x are normal for hand-held scopes and binoculars. As magnification increases, the size of what you look at will appear larger, but any slight hand movements will also be exaggerated so that you may not be able to keep the binocular steady.
A magnification of factor 8x, for example, produces an image as if one were 8x times closer to the subject. A magnification of factor 30x produces an image as if one were 30x times closer to the subject.
The amount of magnification you need depends upon the subject matter you want to look at.
A larger magnification, the more powerful your telescope or binoculars will be. You will see more detail and get closer to your subject matter.
However there is a trade off...
The greater the magnification results in a smaller field of view and smaller the magnification results in a greater field of view.
The Objective Lens
The objective lens is the big lens at the front of the telescope or binocular.
The diameter of the objective lens is just as important at the magnification as its size determines how much light can be gathered to form an image.
The diameter of the objective lenses will control how much light the binocular can collect. In bright conditions the lens size will not be too important, but larger lenses will be able to maintain the brightness of the image even in poor lighting.
A small diameter objective lens gathers less light than a large diameter lens.
In general terms, the more light that is gathered through the objective lens, the brighter and sharper the image will be at your eye.
Usually the greater the number of times that the magnification can be divided into the objective lens diameter, the brighter the image will be.
Therefore a 30x80 objective lens will produce a brighter and sharper image than an 30x60, even though both enlarge the image an identical thirty times.
Field of view
The field of view of a telescope or a pair of binoculars is determined by its optical design.
It is usually notated in a linear value, such as how many feet or meters in width will be seen at 1,000 yards or 1,000 m, or in an angular value of how many degrees can be viewed.
Generally, the more powerful the scopes magnification the less field of view you will see.
Exit pupil
Telescopes and binoculars concentrate the light gathered by the objective into a beam of light - "the exit pupil". The exit pupil diameter is the objective diameter divided by the magnifying power.
For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye — about 7mm, reducing with age.
Eye relief
Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point.
It is the distance the observer must position his or her eye behind the eyepiece in order to see an unvignetted image.
The longer the focal length of the eyepiece, the greater the eye relief.