Q: I am in the market for a spotting scope to use for varmint and big-game hunting. I want the highest magnification I can get, but there’s a big difference in the price of different scopes of the same power.
Looking at different model specifications, I see the terms “exit pupil” and “relative brightness” a lot. What do these terms mean, how do they relate to performance, and how much importance should I attach to them?
Earl Osbourne
A: A high-quality image doesn’t necessarily mean high magnification. Image clarity and brightness are more important than magnification. Exit pupil (EP) and relative brightness (RB) give a good indication of how much brightness and clarity the scope has.
The exit pupil is merely the diameter of the image that exits the eyepiece lens. It is calculated by dividing the scope’s aperture (ie, the diameter of the objective lens in millimetres) by its magnification. If the EP is as large or larger than the diameter of the pupil of your eye, the image will be essentially as bright as what you could see without the scope (human pupillary diameter varies from about 2.5mm in bright sunlight to about 7mm in total darkness).
Relative brightness is simply the square of the EP, which means brightness increases exponentially as the EP gets bigger.
For general-purpose daylight spotting scope use, an EP of 3.0 is about right, and an EP of 1.0 is still adequate for close looks at game (a 20mm scope at 20x, or 60mm at 60x). But if the EP drops below 1.0 you are getting too dim. The same holds true for RB below 1.0, as your scope blanks out very quickly (since RB is EP squared, your RB is 1.0 if the EP is 1.0).
Doing the maths is not really complicated. Say you pick up a spotting scope with a 60mm objective and 20x eyepiece. The scope’s EP is 3.0 (60 divided by 20), and its RB is 9.0 (3.0 times 3.0).
Then look at another scope which has an objective lens diameter of 20mm. It looks just as good and promises 20-60x zoom magnification, and costs only half as much. Performing the same calculations, you can see that at 60x this scope’s EP is only 0.33 and its RB a tiny 0.11. Even at its low end of magnification of 20x, the EP and RB are both just 1.0.
So even at its lowest setting, this scope is just barely usable in full daylight and its higher powers are worthless. By comparison, the 60mm scope has excellent brightness at 20x, and if you want to attach a different eyepiece and increase the magnification, you would still have usable 1.0 EP and RB up to 60x.
Optical quality must be at the top of the list when choosing a spotting scope. This is particularly true due to the fact that the varmint hunter spends a lot more time looking through his scope than does the typical big-game hunter. And the longer one looks through the scope the more important optical quality becomes.
From a practical point of view, a maximum magnification of 30x to 60x is all we actually need for field use. Choose a top-of-the-line scope available from established companies such as Nikon, Zeiss, Swarovski, Burris and Leupold. When it comes down to optical quality, you get exactly what you pay for.
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