what's the diference between -2 and -3?!

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There is a sort of rough calculation to estimate lens power needed, but it doesn't account for astigmtism. Take something with good sized writing on it preferably black on white. Without correction, see how far away you can see the letters [IN CRISP FOCUS!]! (NOT how far away you can still read them blurry!). Measure the distance from the letters to your eye in millimeters and divide it into 1000, this will give a rough approximation of the diopters you would need to correct your vision to 20/20. Note: don't use this result to order glasses on-line! it is only an approximation!

I wear glasses, in my case, doing the above test, the distance was 8.5 inches or 216 mm, dividing into 1000 gave about 4.5 diopters and when I went to get new glasses, that was the power needed. As far as "how far away can I see": It seems like you don't wear glasses, I will try to describe my experience. Despite the test I mentioned above, I can see a lot further than 8.5 inches away! It is not like if something is further away than 8.5 inches, it suddenly becomes invisible or a complete blur, it is just that, beyond 8.5 inches, I start to lose the fine detail, of course, the further away something is, the more detail is lost. In my situation, it isn't like I couldn't navigate without glasses, but things would be pretty blurry and inconvenient not being able to read signs, recognize people at a distance, etc. As someone becomes more and more nearsighted, the furthest distance they can see something in focus, without glasses, becomes closer and closer and the rate at which detail is lost with distance due to blurryness becomes more rapid. In extreme cases, it can get to the point where someone can't even see enough to walk around safely because things are too blurry. You hear those stories of somebody not being able to see past their own nose without their glasses, or if they drop their glasses they have to feel around to find them, yes, sometimes nearsightedness gets that bad. Fortunately, even if it is real bad, they can usually do a pretty good job of correcting it.