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What size and power are best for me?
Home » Astronomy » Binoculars » What size and power are best for me?Unfortunately, there are no hard and fast rules about what size binoculars will work best for you. Everyone’s eyes are different and binoculars are much like fitting a pair of glasses to someone’s needs. One of the first things to consider is eye relief, the distance you must hold the binoculars from your eyes to achieve proper focus. Anything less that 9mm will be hugely uncomfortable, while eyeglass wearers will need about 15mm of eye relief.
The next is to understand exit pupil. This is nothing more than the amount of light our eyes can receive. Divide the larger number by the smaller number on the binoculars to get the exit pupil size. For example: 5 X 35. This works out to a perfect 7mm exit pupil—the maximum any human eye can achieve. Any larger number is a waste—reducing the amount of light collected by the aperture.
Since chances are that many of us will never experience absolute dark and maximum dilation, some of the very best to consider are those that have around a 5mm to 6mm exit pupil. For example, 5 X 30, 7 X 42 or 10 X 50. Binoculars in this range are still small and light enough to be handheld, while larger models would probably require a tripod, or a method of steadying them.
Choose what’s comfortable for you. If you are interested in mounting them on a tripod, there is no reason not to go with a larger model—just remember your exit pupil equation. For all purpose hand-held astronomy, 10 X 50 is the best bet.
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