How is it that a camera is able to focus on a set distance, yet blurring things both closer and farther? Whats the difference between a near small image and a far large image?
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Typically there’s a circular lens and the image it forms sees the world from every point on the surface of the lens. The right edge of the lens will see the world from a slightly different point of view than the left edge. Focusing can correct the discrepancy, but only for objects at a limited range of distances from the lens. It’s fundamentally an issue of geometry. Pin-hole cameras, with just a single point of view, have infinite depth of field.
You can experiment yourself by looking at a scene where near objects appear in front of far objects. If you cover one eye and then the other then the image you see will change as each eye has a slightly different point of view. If your eyes converge to look at a near object then far objects no longer line up and you can see a double image of them.
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