About Spotting Scopes

Objective Lens
This is the lens in front, nearest the object you are viewing. Like Binoculars, the light gathering capacity of a spotting scope is in direct relation to the size of the objective lens. Lens diameters generally range from about 50mm to 100mm. The larger objective lenses are recommended whenever practical. You'll find that the larger objectives provide the best and brightest images, although weight may be a consideration.

Eyepieces
The eyepiece you select for your scope will make a big difference in your viewing enjoyment. It's important to spend some time learning about eyepieces in advance so you will make informed choices. It's possible to pair a mid-range scope with a low-end eyepiece and receive very disappointing results. Likewise, a high-end eyepiece can be paired with a low-end scope and perform much better than expected. The point is, do your research.

Eyepieces come in two configurations; zoom and fixed focal length. Zoom eyepieces generally comes in magnifications ranging from 20x to 60x. The 20x setting is the most popular setting used by most birders. Due to the higher costs of zoom eyepieces many birders choose to use 27x or 32x non-zoom eyepieces.

Another consideration when selecting a spotting scope is eyepiece placement. Placement is obviously a personal preference, with the straight-through design being preferred by most nature observers. Having the eyepiece angled at forty-five degrees makes viewing above the horizon easier and makes sharing observations with others more convenient.

Magnification
Magnification is determined by dividing the focal length of the spotting scope to the focal length of the eyepiece. You can change the magnifying power of a scope by putting in a different eyepiece. A zoom eyepiece lets you dial the power you want. Increasing the magnification usually results in a narrower field of view, a decrease in observed image brightness and a decrease in the depth of field. Scope magnifications go from a low of 15x to a high of 75x. It's a good idea to use a tripod at higher magnifications to eliminate hand tremors.

Aperture
The aperture is the diameter of the front lens (objective lens) of the spotting scope, usually expressed in millimeters. The size of the aperture has a direct relationship to the weight of the scope. When purchasing a spotting scope pay close attention to the aperture spec. The larger the aperture, the sharper and brighter your images will be.

Eye Relief
Eye relief is one of the most important considerations to eyeglass wearers for their spotting scope. Eye relief refers to the point where the complete field of view becomes clearly visible to the observer. Generally, 12mm to 15mm of eye relief is adequate for most eyeglass wearers to be comfortable.

Focal Length
Focal length is the distance between the objective lens and the point where the light rays from the image comes sharply into focus (the focal point).

Exit Pupil
The exit pupil is the diameter of the circular beam of light from the light rays that have entered through the aperture and then exit the eyepiece of the spotting scope. Larger exit pupils produce brighter images. Dimensions are measured in millimeters.

Exit pupil size can be calculated by dividing the objective lens size (in millimeters) by the eyepiece magnification. For example, if you have a 60mm scope at 20x you'd have a 3mm exit pupil.

Field of View
Field of view describes the widest area of land or sky that the optics presents to your eye. Manufacturers typically express this either in linear feet at 1000 yards or in angular degrees. Spotting scopes generally have a field of view ranging from 1 degree (52.5 feet) to 3 degrees (157 feet).

Be aware that field of view for a fixed focal length lens is generally larger than for a zoom lens of equivalent size. A wide angle lens provides a wider field of view and a lens with long eye relief generally has a more restricted field of view. Higher magnification will also reduce field of view in most cases.

Focusing
There are three different types of mechanisms used on telescopes, although only two are primarily used on spotting scopes: helical focusers and knob focusers. Helical focusers are knurled or rubberized collars that encircle the scope barrel. These offer convenient, rapid focusing and are great for tracking objects that are changing distances rapidly. Knob focusers are much slower but allow for very precise focusing and are probably the most popular among birders. It's a good idea to try each style to find which you prefer before making a purchase.

Near Focus
This is the nearest distance you can focus on an object and still receive a clear sharp image.

Glass
The best spotting scopes have objective lenses made with special glass. The HD (high density), ED (extra-low dispersion) or Fluorite (FL) glass produces a noticeably brighter and clearer image to lower quality glass lenses. This is especially true in low-light viewing conditions and when viewing at higher powers. The ED/FL/HD scopes come at a higher price but are worth the extra expense if you can fit them into your budget.