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Why are these numbers so important? Because not only will it tell you what magnification of scope is but also diameter of exit pupil once you know formula.
Before I get into how to calculate exit pupil, I’ll bet you’re wondering what an exit pupil is. This may get a little technical so please bear with me.
Here’s how it works. A scope gathers light over face of its front or objective lens and concentrates it out into eyepiece in an area called exit pupil.
The exit pupil of your scope should match pupil size of your eye as closely as possible (or just a little larger) to get maximum low light performance and prevent scope blackout. You calculate exit pupil diameter by taking objective lens diameter (front lens closest to your target) and dividing it by scope power.
For example, a scope with a 20mm objective lens and a power magnification of 2 will have a 10mm exit pupil. 7 to 8mm is about maximum size that human pupil will dilate in low light, so this scope will work well under low light conditions. A smaller exit pupil would not deliver as much light to eye as eye is willing to accept, however, a large exit pupil means that eye doesn't have to be as precisely centered behind scope to receive a full image.
For adjustable power scopes, 3-9x40mm for example, you get exit pupil based on power setting of scope. For example: 40 divided by 3 = 13.33mm exit pupil and 40 divided by 9 = 4.44mm exit pupil. As you can see, higher magnification, smaller exit pupil, hence lower light gathering capabilities at higher magnifications.
When talking about light gathering capabilities, you’ll want your exit pupil to be same diameter or just a little larger than conditions you're using scope in. For instance, if your eye’s pupil is dilated to 5mm, and exit pupil of scope is 7mm, then you have 2mm of leeway before your eye position behind scope becomes critical.
If exit pupil of scope is smaller than diameter of your eye’s pupil, you’ll start to encounter problems seeing full image of your target.
Just as a frame of reference, human pupil is normally dilated to about 5mm in standard light, 2-3mm in bright light, and 7-8mm in low light conditions.
Whew… I know my description was a little technical, but knowing what exit pupil is and how it works will help you determine how to best utilize your scope.
You might also be wondering how exit pupil calculation, light gathering characteristics, and scope magnification tie in with extended eye relief? Just remember, for EER handgun scope applications you’ll be holding your scope anywhere from 5-30 inches away from your eye. This can be a significant distance if your scope doesn’t illuminate your target well enough and position of your eye is not aligned with exit pupil. These have a significant affect on EER, making it difficult to get off an accurate shot if not taken into account.
And… speaking of accurate shots, there are a few handgun scopes I recommend to give you best chance of accomplishing those shots.
I think some of best handgun scopes for Extended Eye Relief and light gathering capabilities are Nikon EER Monarch series, Leupold FX and VX series, and Burris LER series. All three companies offer exceptional choices in optics as well as full lifetime guarantees. They’re rugged and durable and 100% waterproof. They have unmatched lens coating systems which excel in brightness, clarity, and contrast.
Few companies are able to stack up, however, you’ll have to make best choice for your shooting application and go with scope and company you feel most comfortable with.
They’re not cheapest scopes, but they’re best overall value and definitely worth investment.
Hopefully I’ve clarified questions you might have had about scope definitions and how to calculate critical components of a scope. Now, with this information, you’ll be armed with knowledge you need to make a wise EER scope selection.
Oh… one more thing. Usually, amount of eye relief and magnification is listed in specifications by most manufacturers, so you should be able to get a good feel for what you need after applying knowledge you’ve gained from this article.
Copyright 2005 © John Voight
John Voight is an avid hunter and sportsman and the owner of http://www.eer-scopes.com. He offers excellent discount prices on all Nikon EER Handgun Scopes with an unbeatable one-year customer satisfaction guarantee and full lifetime warranty.