Here's a question I've been pondering for a while. Would it be possible to create an image that, when far away, is clear to a nearsighted person but blurry to a person with good vision?
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I'm pretty sure that's impossible because any object the nearsighted person sees is made out of focus by the lenses in his eye.
Out of focus means the place where the rays of light intersect perfectly is somewhere other than on the retna at the back of the eye. The only way to change that is to change the location of the lens, or the location of the object you want to be in focuse.
Like Ron said, focus doesn't really work that way. You can't take something that is blurry in "real life" and make a sharp image by mis-focusing light from it.
(And by "sharp" I mean the optics/photography sense, "contrasty and with well-defined detail", not in the "knife bade" sense).
You can take the light from something sharp and project a sharp image of it. You can take the light from something sharp and project a blurry image of it. You can even take a blurry image made from the light from something sharp and, using a lens, re-focus that image so that it becomes sharp. (That's what glasses do: they take the flow of light coming from something sharp that is being projected as a blurry image, and they bend that light so that it ends up being focused as a sharp image). But if it's blurry to begin with, there is simply not as much information in that image as there would be if the image were sharp. (There are algorithms that try to "sharpen" digital images that are slightly out of focus, trying to deduce the lost detail by a variety of mathematical operations involving patterns and groups of neighboring pixels, but they don't work super well, and usually exaggerate the noise from the imaging sensor more than the details of the object that was photographed).
An image "cover" that works like a pair of glasses is probably as close as you can come: You can have an image behind a lens (you can imagine a painting set some distance behind a huge window-like lens) that normal-sighted people just can't focus on, but near-sighted people would. It would basically force everyone who looks at it to see it though "glasses", and you know how hard it is to try to focus while looking through someone else's strong glasses.
Two other interesting things that are related to your question, though:
It is possible to make an image where people will see something in it when looking at it from far away, and will see something else when looking at it from up close. That is done by superimposing patterns of different levels of contrast and of diffent focus distribution sizes, so that the less-in-focus pattern is lost to you when you're close, but the less contrasty pattern is lost to you when you're far away:
http://boreddreamer.wordpress.com/2006/05/09/a-crazy-optical-illusion-dr-angry-and-mr-smile">http://boreddreamer.wordpress.com/2006/05/09/a-crazy-optical-illusion-dr-angry-and-mr-smile">http://boreddreamer.wordpress.com/2006/05/09/a-crazy-optical-illusion-dr-angry-and-mr-smile/">http://boreddreamer.wordpress.com/2006/05/09/a-crazy-optical-illusion-dr-angry-and-mr-smile
The other thing is that color-blid people see red, green, and blue in proportionally different intensities than do normal people. Just as some color differences are more easily detected by normal people than by color-blind people, some color differences are actully detected more easily by color-blind people than by normal people. (So maybe the PC term should be "color-different"... Except that those differences are always caused by one of the types of receptors being LESS sensitive to a range of wavelengths than on most people, so it really is a kind of slight blindness that works differently on different parts of the spectrum). The result: It is possible to have a bunch of colored dots where one pattern or symbol will stand out to a normal person, and another pattern or symbol stands out to a color-blind person, in the same image. In fact, it is possible to have a bunch of colored dots where one pattern or symbol will stand out to a "red-weak" color blind person, and another pattern or symbol stands out to a "green-weak" color-blind person, in the same image. This is the basis of the Ishihara">Ishihara">http://www.cleareyeclinic.com/ishihara.html">Ishihara test which is used to diagnose color blindness. The test (consisting of about 25 circles with patterns of colored dots and an explanation of who should see what in each one and why) is copyrighted but most of the images can be found online. Of course, they are not as reliable when seen on a computer screen, since they are carefully adjusted so that they give the best results when printed a certain way, but just looking at it on a computer should give you an idea of the test (and if you're color blind, you should notice some patterns can be seen far more easily than others). Here are some links to (most of) the Ishihara test online: kcl.ac.uk,">kcl.ac.uk,">http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/plate1.htm">kcl.ac.uk, toledo-bend.com,">toledo-bend.com,">http://www.toledo-bend.com/colorblind/Ishihara.html">toledo-bend.com, colorvisiontesting.com.