why is it harder to focus close-up with myopic glasses on then it is without?

Answer 1

Optics 101

Answer 2

Myopic lenses are designed for helping patients see at a distance. However, you should be able to see up close as well. If you are having difficulty focusing on close-up items, you may need a bifocal. I don't know how old you are, but as we age the muscles that help us shift our focus begin to weaken. That is why you may notice that some individuals that are 40+ begin to hold books further away in an effort to focus on the small print or they may simply remove their glasses if they currently wear a pair. Sometime this same problem happens in younger people, but it is not as common in younger individuals. If you are having difficulty focusing on close-up items then I would recommend a new eye exam and be sure to discuss this problem with your doctor.

Answer 3

If your glasses correct you for distance vision, your ciliary muscles will have to refocus the eye from far distance down to the distance you're interested in. (Like everyone who doesn't need glasses.)
If you're not wearing your glasses, your furthest point of focus is already considerably closer, so the amount of "shift" required is less.
This shows up markedly as people approach 40-45, as the refocussing reserves, "accommodation", become less.

There are odd exceptions. In a few myopic individuals the binocular coordination is such that they are more comfortable reading with their glasses on. For them, the muscles inside the eyes may be working harder, but the muscles aligning the eyes are less stressed, and that can be more important.

Answer 4

Because glasses with lenses for people with myopia (near-sightedness) are designed to help said people with distance.

Answer 5

I agree with squachi, 100%