Why does a mirror image look fuzzy to a nearsighted person, even if the mirror is close?

Dear Cecil: I was looking in the mirror the other day without wearing my glasses, which I occasionally use because I’m nearsighted. I noticed that things that were far away, even when reflected in the mirror, were blurry. When I put my glasses on and looked in the mirror again, everything was in focus. I found this strange. I thought everything should have been in focus without my glasses, because the mirror was close to my eyes and so (I thought) were the reflections. I guess that’s why people don’t use mirrors for vision correction, huh? Kirsten Munson, Santa Barbara, California

Cecil replies:

You got it, babe. The reflection is out of focus, even though you’re close to the mirror, because you’re not looking at the mirror. You’re looking at the image in the mirror, a different matter entirely.

You can prove this by a simple experiment. Look at a mirror from a distance of 6-12 inches. With your glasses off, focus as best you can on some distant object reflected in the mirror — say, a bathroom towel on the wall behind you. No doubt the image of the towel is pretty fuzzy, and not just because you haven’t cleaned the lint screen on the dryer. Now look at something on the surface of the mirror, such as a dust speck. You’ll observe that (1) it requires a noticeable effort to adjust your eyes — in other words, you’re refocusing — but that when all is said and done (2) the speck, unlike the towel, is in reasonably sharp focus. This clearly demonstrates (to me, anyway) that when you look at a reflection in the mirror, you’re not looking at the mirror’s surface.

So what are you looking at? For purposes of focusing, at the object itself (in this example, the towel). Without going into the the technical details, the image of the towel in the mirror is out of focus for the same reason that the towel is out of focus when you look at it directly. In both cases the light travels more or less the same distance from the object to your eyes; the fact that in one instance it bounces off the mirror en route is irrelevant. Unless you want me to get out my giant model of the exposed human eye — and it IS looking a little bloodshot — I say we leave it at that.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.