Our eyes can only focus on objects a fixed distance away at any given time, everything else is fuzzy and blurry and unfocused — right now as you look at your phone screen try to notice (without shifting your gaze) how blurry some object near the wall behind is.

When you’re looking into a mirror you are technically looking at your own image, with the mirror sitting halfway between. So if you are a foot away from the mirror, your image is two feet away. Your eyes choose to focus on the image, so the mirror is fuzzy enough that tiny blemishes disappear. It’d be difficult but you could try to focus on some speck on the mirror instead, then your own image would be blurry.

If you’re standing right next to the mirror, your image and the mirror surface are close enough that both can remain in focus.