Twilight Factor of Spotting Scopes

Both magnification and aperture will have an impact on image resolution and detail depending on the amount of light available during observation and the resulting eye pupil size. In twilight hours, both magnification and aperture impact on resolution effectiveness and the term, twilight factor, gives us a means of comparison for an optical instrument under these conditions. 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 telescope when observing under dim light conditions. This partially explains why some spotting scopes of identical exit pupil size will have differences in observed image sharpness and detail in twilight. For example, compare the twilight factor of two spotting scopes: a 60mm aperture at 20X (t.f.= 34.6) and a 90mm aperture at 30X (t.f. = 52.0). Both have 3mm exit pupils but the 90mm scope has the potential for greater resolution in twilight conditions. Note that the twilight factor is not a term used to indicate observed image brightness so that we cannot assume, in this example, that the 90mm scope is brighter than the 60mm because of the higher twilight factor. Also, the twilight factor does not take into account the ability of the optical system to transmit light, or transmittance; consequently, a higher-quality spotting scope will typically outperform a cheaper scope of equal aperture and magnification, due to the higher transmittance of the more expensive scope (due to the use of special high-density glasses and/or flourite, as well as special proprietary coatings). Unfortunately, transmittance is typically difficult and/or expensive to proper measure; the best a potential scope buyer can do is compare several scopes under the same conditions.

Thanks to our friends at Eagle Optics for providing this information!

See also:
Bird Watching Binoculars—Tips for choosing the right birding binocular
Birding Tripods—Tips for choosing the right tripod for you binoculars and scopes

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