My unaided vision is terrible but if I make a small gap with my thumb and index finger and look through that I can read pretty easily. Why is this?
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Objects being out of focus is due to them being viewed from multiple locations, i.e., by all the points on the surface of the lens. Pinhole cameras, which have just one tiny hole instead of a lens, see everything as being in focus but, of course, the image is extremely dim. You need a lens to gather more light but then you need to choose the distance to focus on. The bigger the lens, the smaller the range of distances that are in focus at once. Photographers call this range of distances the “depth of field” and they know that adjusting the “aperture” of their lens (the size of the hole in the lens) will control the depth of field.
This is also why focusing is easier for people in bright sunlight, because then their pupils shrink down to a small hole. In dim light with large pupils, focusing needs to be more accurate.
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