Thursday, September 17, 2009


Some people report they're unable to bring the "two circles of light" together when using binoculars that have a short close focus (3 to 5 feet). Actually, this happens to everyone but some people are more sensitive to it than others. The optical phenomenon they're observing is called parallax and is perfectly normal with stereoscopic vision (as opposed to monoscopic vision when looking through a telescope or spotting scope).

You can observe the same effect without a binocular. Hold out your index finger about a foot away from your face and focus on something in the background; it will appear as though there are two fingers instead of one. This effect makes perfect sense when considering the binocular as two fixed parallel barrels. Our eyes have the ability to change their focal point distance. This is achieved as your eye's orbital muscles slightly shift the angle of each eyeball inward (near focusing) or back outward (distance focusing). This changes the width of the angle between your two lines of of sight. When you're focusing through your binoculars on something close to you (A), the two optical circles will appear split. However, when you're focusing at something distant (B), a single optical circle will be seen.

Because you're using the same eye muscles when close focusing through binoculars as you would when crossing your vision, you may experience slight discomfort or eye fatigue. If this happens, try keeping just your dominant eye open while closing the other.

Mike McDowell
Eagle Optics Staff

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