I'm having a problem with a very trivial thing- magnification makes things bigger. But if it doubles or triples the size AND distance away, why does it even help? I think I'm just ignoring something about how we see, but I don't understand why making a small object twice as tall but twice as far away would help at all.
closed as unclear what you're asking by Dilaton, Manishearth♦ Jul 12 '13 at 13:11
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I'm assuming we are talking about the magnification of a virtual image.
When you are looking at a small object, the light from different parts of it comes to your eye at very close angles. This results in the very small distance between the images of these parts on your retina. Since retina has limited resolution, if the object is too small, you cannot see the difference between its parts; you just see a small dot without any details.
A lens does not enlarge the whole world behind it. Instead, it increases the difference in angles between these incoming rays, or, as it is usually put, it increases the object's perceived angular size. Consequently, the image of the object on the retina becomes larger, and you can see more details; that's what lenses are useful for.
Now depending on the context observer's brain may interpret this as the object getting larger (when you look in the magnifying glass), or getting closer (when you look in the spyglass). But the only thing that actually changes is the angular size.