Using Mirrors in eye Tests

My niece’s husband recently asked a question about the use of a mirror during an eye exam in his ophthalmologist’s office. The exam room was very short, only about ten feet in length, and when the ophthalmologist asked my niece’s husband to read the various lines of letters to determine his eyesight, he had him look at a mirror positioned out in front of his face in order to see the letters that were being projected on the wall behind him. My niece’s husband surmised that the purpose of the mirror was to simulate conditions in a much longer room, but this then generated another question. Why couldn’t the letters be projected on an LCD screen on the wall that he was facing, and the size of the letters simply increased or decreased in order to simulate their size in a much longer room?
- Tim (age 58)
Montgomery, AL, US

The procedure you suggest would indeed give the same effective angular sizes. However, the effective distance from the eyes to the screen would be shorter. It would test for a different ability to focus. To pick a more extreme case, testing eyes by reading a book a foot from your eyes gives different results than testing your eyes with an equivalent-sized projection across the room. One is easy to read for near-sighted people and the other is easy to read for far-sighted people.

Mike W.

(published on 04/24/2013)