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I happened upon this video purportedly showing a coin on a horizontal table receding away from a lens would disappear from bottom up. The explanation via diffraction resolution limit is wrong. However, I would like to find the correct explanation of the phenomenon.

When the camera zooms in and focuses on the coin, the coin appears clearly. This can be explain by the fact that part of the light from the coin still enters the aperture of the lens. However, how would one explain the abrupt disappearance of the bottom of the coin below the table upper edge when the focus of the lens is on the table edge?

Hans
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1 Answers1

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The first crude sketch shows how the coin is photographed first. The optical axis of the camera is very slightly below the table top. The aperture is small. You can tell because the depth of field is large. Only rays that hit the lens near the axis are visible. This cuts off the bottom of the coin as it slide down the table.

The second crude sketch shows how things change when the aperture is enlarged. Now light that strikes the part of the lens that sticks above the table is captured. This part is above the table, and so can see the coin. The depth of field is small for a large aperture, so the focus of the camera has to be adjusted to bring the coin into focus.

enter image description here

mmesser314
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