eli5: Why there’s a difference between focusing your eyes on something distant and focusing your eyes on a screen projecting the image of something distant?
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It’s because the screen doesn’t preserve the angle of the light coming from the original image. Focusing is required because the pupil of your eye is a few mm across, so that light entering on one side of the pupil enters at a slightly different angle than from the other side. The angle difference will be tiny for distant objects but much greater for nearby objects.
Screens generate new light that fans out in all directions from up close so you have to focus close. Mirrors preserve the relative angles as they reflect light so they don’t reset the focus distance. Projection screens reflect light from a projector but they’re diffuse reflectors that scatter light in all directions so, again, you need to focus on the screen.
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