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In the limiting case, consider that the object and your lens are finite in size and infinitely far apart. Then each appears as a point when viewed from the other. Two rays passing from the object to your lens would then follow the same path and would thus be parallel.


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The answer is because our ken (field of view) subtends an extremely small angle at the source. Even though the source may emit over a wide angular range, we can only receive a small angular range of that light if we have a limited aperture instrument and our distance from the source is large compared with the aperture. Suppose we look at Alpha Centauri ...


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Assume you are looking with just one eye (in order to isolate perspective from stereo vision effects), and choose any Euclidean coordinate system that has your eye at the origin. The light emitted from any point on a given straight line will fall onto your retina at essentially the same position, i.e. straight lines through your eye are collapsed onto a ...



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