How to Understand Scope Magnification

by David Chandler
Good binoculars are indispensible for spotting wildlife.

Good binoculars are indispensible for spotting wildlife.

Looking through a rifle scope or binoculars makes distant objects appear closer than they really are. This magnification is useful in surveying distant landmarks or targeting a shot. These scopes are really refracting telescopes using two lenses to magnify objects and operate by many of the same principles as the refracting telescopes which employ mirrors commonly used by astronomers. The lens near the eye is referred to as the ocular lens, while the lens at the far end of the scope is termed the objective lens. The magnification of the scope depends on the focal lengths of these two lenses.

Identify the numbers describing the scope. Two important numbers are given with scopes. The first number indicates the magnification power of the scope and the second number gives the diameter of the objective lens (the lens at the far end of the scope). For example, a 4 by 40 mm scope has a magnification power of 4 and a 40 mm diameter objective lens.

Identify the number indicating the magnification power of the scope. This number describes how much larger an object will appear when looking through the scope. Some scopes give a range for magnification and can be adjusted. For example, a 6-24 by 72 mm scope has a range of magnification between 6 power and 24 power.

Identify the diameter of the objective lens. A large objective lens allows more light to pass into the scope making an image seem brighter than a smaller objective lens. A large objective lens also increases the field of view of the scope. A scope with a small objective lens can produce high magnification but with a very limited field of view.

Calculate the exit pupil of the scope by dividing the objective lens by the magnification power. The exit pupil is the size of the column of light exiting the eyepiece. Ideally, the exit pupil approximates the size of the pupil of the observer's eye at a comfortable distance from the scope.

Items you will need

  • Rifle scope or binoculars

About the Author

David Chandler has been a freelance writer since 2006 whose work has appeared in various print and online publications. A former reconnaissance Marine, he is an active hiker, diver, kayaker, sailor and angler. He has traveled extensively and holds a bachelor's degree from the University of South Florida where he was educated in international studies and microbiology.

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