Just as people are right- or left-handed, everyone generally prefers the input of one particular eye, something called “ocular dominance” or “eyedness.” Most people rely on their dominant eye for things like aiming, and a person’s dominant eye actually has more neural connections to the brain than the other eye.

So here’s a question: do you look through your camera’s viewfinder using your dominant eye? You may assume that you do, but that might not be the case.

If you’re not sure which of your eyes is dominant, there’s a simple way to find out that’s called the “Miles test”:

The observer extends both arms, brings both hands together to create a small opening, then with both eyes open views a distant object through the opening. The observer then alternates closing the eyes or slowly draws opening back to the head to determine which eye is viewing the object (i.e. the dominant eye).

Here’s a 30-second video animation that shows how this test is done: