Eyepieces of Spotting Scopes
One of the most important considerations in purchasing a spotting scope is selecting the right eyepiece or eyepiece configuration. The eyepiece magnifies the primary image formed of some distant object by the objective lens or mirror and delivers this image at a certain distance behind the rear most ocular (eyepiece) lens. The diameter of this image is called the exit pupil and the distance at which it is located behind the eyepiece is called the eye relief. The amount that the image is magnified is determined by the ratio of the focal length of the objective lens (or mirror) to that of the eyepiece (the longer the eyepiece focal length, the lower the magnification for a given aperture). The eyepiece design is therefore a factor in magnification, field of view, exit pupil size, and eye relief. The optical design and glass quality of the eyepiece can also affect the amount of color or linear distortion of the image.
Eyepieces differ in many ways. Some are fixed in focal length; some are variable (zooms). Others are designed to give either wide fields of view (wide angle) or long eye relief (for eyeglass wearer comfort). They can attach to the scope by different means: screw threads, bayonet mounts or by fastening with a set screw. Some spotting scopes have eyepieces which are non-interchangeable (usually these are either zooms or waterproof scopes).The eyepiece placement may be configured for straight-through, forty-five degree, or ninety degree viewing. Also, some eyepiece designs are available in different diameters, varying from .96" to 1.25" or even as large as 2".
Eyepiece placement is usually a personal preference with the straight-through design being preferred by most nature observers. This design makes it easy to sight an object and follow as it moves and is also convenient to use with a car window mount. A spotting scope with an offset forty-five or ninety degree placement is easier to use for viewing above the horizon (close-in observing at tree-top level or for celestial observation) and does not require a tall tripod (stability can be a factor here). Also, this configuration is convenient for sharing your observations with others.
Thanks to our friends at Eagle Optics for providing this information!
Back to Bird Spotting Scopes