Spotting Scope Terms

Spotting Scope

Spotting Scope – Before you can determine what type of spotting scope you really need, you should understand the following spotting scope terms.

Eyepieces:  The spotting scope eyepieces differ in many ways.  Some are fixed in focal length and some are variable (zooms).  Others spotting scope eyepieces are designed to give either wide fields of view (wide angle) or long eye relief (for eyeglass wearer comfort). They can attach to the spotting scope by different means: screw threads, bayonet mounts or by fastening with a set screw.  A spotting scope can have eyepieces which are non-interchangeable (usually these are either for zoom or a waterproof spotting scope).  The eyepiece placement may be configured for straight-through, forty-five degree, or ninety degree viewing through the spotting scope.  Also, some spotting scope eyepiece designs are available in different diameters, varying from .96″ to 1.25″ or even as large as 2″.

Eye Relief:  The eye relief is the distance that your eye must be from the eyepiece on the spotting scope to get a full, clear picture.  Eye cups provide the eyeglass wearer easy head positioning for the appropriate eye relief on the spotting scope.

Exit Pupil:  The exit pupil is the small circle of light that appears in the spotting scope eyepiece.  The formula for determining the exit pupil size is the spotting scope objective lens diameter size in millimeters divided by the magnification.  The larger the exit pupil on the spotting scope, the less critical the position of your head is in relation to the spotting scope.  Also keep in mind, that an older person’s eyes may only dilate to about four millimeters in low light situations while the younger person’s eyes may open up to seven millimeters or more.  Therefore, a larger exit pupil size on a spotting scope becomes less of a deciding factor for the older eye.

Field of View (FOV):  The FOV is measured in linear feet at 100 yards.  This is the amount of view you see through the spotting scope from right to left at that distance.  FOV is inversely effected by magnification – as magnification increases, the smaller the FOV, and conversely, as the magnification decreases, the greater the FOV.  For example, since the field of view will usually be smaller for a spotting scope than for a binocular, with a typical range of from 1 degree (52.5 feet) to 3 degrees (157 feet), the field of view is much more critical at close distances.  So this range is quite adequate for the medium to long distance observation done with most spotting scopes.

Focus Systems:  There are basically three different types of focusing mechanisms used on a spotting scope: Single Knob, Double Knob, and Helical.  The characteristics of each type can make a difference in how well adapted it is to your specific application and even how much you will enjoy using your scope:

  • Single knob focus is the most common type of spotting scope focusing system.  Single knob focus is slower in action but allow for precise, accurate focusing and are probably the most preferred for general purpose birding or nature observation with a spotting scope.
  • Double knob focus, as the name describes, has 2 drive knobs.  One knob give fast coarse focus, the other give precision fine focus.
  • Helical focus has knurled or rubberized collars around the spotting scope barrel where the focus is changed fairly rapidly when turning the focusing ring.  Helical focus works well for observing objects that are changing distances, near to far, quite rapidly with a spotting scope.
  • Rack & pinion designs are commonly found on astronomical telescopes and usually offer fairly fast, smooth focusing, but most components are external and subject to potential deterioration from dust and moisture over time.
  • The typical close focus distance for most terrestrial spotting scopes is about 20 to 30 feet, however, certain designs (catadioptric) will allow a near focus down to 5 to 15 feet.

Magnification:  The magnification is actually a relationship of two independent optical systems– the objective lens and the ocular (eyepiece) on the spotting scope.  It is calculated by dividing the focal length of the objective lens by the focal length of the eyepiece.  Magnification, in simple terms, is how many times larger an object will appear than with your naked eye.  Beware of the notion that “the more magnification – the better.”  In reality, the more magnification you have with the spotting scope, the less light that gets to your eyepiece thus reducing the image brightness, contrast, and clarity while also reducing field of view.  For most nature observing, 20 to 40X is quite adequate.  In situations where there is a need for fine detail, (long distance birding or reading leg bands or tags) higher magnifications of up to 100X or more may be required.

Objective Lens:  The objective lens diameter on a binocular is measured in millimeters (mm).  The larger the objective lens, the brighter the target image and the higher the contrast and clarity of the spotting scope.  However, the negative side to a larger objective lens is that the bigger glass elements are heavier in weight.

Twilight Factor:  The twilight factor is calculated by taking the square root of the product of the magnification and the aperture.  The higher the twilight factor, the better the resolution of the spotting scope when observing under dim light conditions.  This partially explains why some of identical spotting scope exit pupil sizes will have differences in observed image sharpness and detail in twilight.

Here is some additional information to consider in selecting Target Spotting Scope:

  • For distances up to 100 yards, a larger objective lens is not required on the spotting scope. Good optics or magnifications around 18-36X should be sufficient to see bullet holes at 100 yards.  A 60mm objective lens should be sufficient in size and while still maintaining portability.  If you are using a spotting scope for scoring targets in air rifle or rim fire competitions, be sure to check for “close focus” or “minimum focus” under the scope’s specifications since not every target spotting scope will focus as close as 33 feet (10m).
  • For 200 yards, a spotting scope with 60mm to 80mm objective lens have enough optical quality to handle 200 yard scoring, assuming reasonable observing conditions.  This is an optimal distance that most shooters need for sighting in and/or checking load performance.
  • For 300 yards, the requirements are much higher for a spotting scope.  The minimum objective lens size is at least 80mm to ensure greater clarity at higher magnifications.  The glass making up the spotting scope lens should be of exceptional quality and be preferably ED glass.  You will be wasting your money if you do not invest in an eyepiece that matches the exceptional optical quality of the objective lens.
  • For 400 yards, you will have to battle with the ever changing observing conditions which severely challenges the spotting scope optical system.  There are no shortcuts since the target remains in focus for only a short time and only better quality optics can ensure a clear view.
  • For 500 yards, you will need the best optical system quality available.  The premium spotting scope is more typically used at this distance to judge atmospheric quality and air currents rather than for scoring.  A spotter or tagger at the target will typically be needed to assist in scoring.

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