Look at the number 0, like the zero on a speed sign that says “60 km/h”, what do you see? Likely two sharp ovals, with a pigment between them, making the shape of a zero. If you have trouble seeing it, it’s likely because your eyesight is poor and it’s blurry.
Well, I see sharp ovals. I just don’t see only two sharp ovals. I see dozens of fragments of those two ovals, both on the interior as well as the exterior. Those fragments are razor-clear and razor-sharp with a good prescription, but the smaller the zero is, the closer these fragments intrude on each other, and the harder it is to make out that zero as a zero.
Plus, when something like a speed sign gets small enough (enough distance, in terms of speed signs), I even get a distortion of the overall number - a zero starts looking like an egg balanced on its pointy end. It is fatter at the top than it is at the bottom.
This gets immeasurably worse with more complicated glyphs, like a 6 or an R. All those extra sharp-clear edges make them look like other things, like an 8 or a B.
Now, these “fragments” are kind of like looking through an insect eye at the same time as looking through a human eye. I see a part of the glyph that is clear and sharp, which then fades out around a roughly-circular distance from the place of maximum sharpness like it’s a mirage. And these cluster together such that they overlap, and also hover over the “master image” that my eye sees. So I am seeing the same part of that edge multiple times.
For example, if I look at the letter T, I can see the top corner of the left arm multiple times, both as a part of the full image of the T as well as multiple fragment overlays. If I choose a T that has the right contrast, the right thickness, great clarity and the right sizing, I can easily count how many replications of any one point are visible. Even for a precise point like the top left corner of a sans-serif uppercase T, it can be anywhere from 3-5 replicated corners. Teeny-tiny replicated fragments, and clustered tightly around that area, but multiple copies that can dramatically confuse the image and make the letter look like something else.
And this problem is in both eyes, pretty much equally. I take the glasses off, and so long as the text is close enough (I’m nearsighted), it happens without glasses equally severely.