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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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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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  • The change of aperture is crucial to resolving my question. I was puzzled because I assumed the aperture was simply the lens size. Could elaborate on "The aperture is small. You can tell because the depth of field is large."? How do you see the depth of field is large from the video? – Hans Oct 01 '23 at 05:56
  • In addition to a widening of the aperture, this could also be explained by the entrance pupil of the lens system being pushed outward (and over the table top level) as the focal length is increased. However I don't know enough about the optical design of zoom lenses to know if this does in fact happen. It also likely depends on the lens design: some zoom lenses have a front element that moves out quite a bit at longer focal lengths, along with the entrance pupil, presumably. – Puk Oct 01 '23 at 06:08
  • "the depth of field is large." Is it because most of the objects in the video at the beginning are in focus? – Hans Oct 01 '23 at 06:20
  • Depth of field isn't really important to the solution. It is an easy way to tell that the aperture was expanded. Depth of field increases when aperture is small, and decreases when large. A large depth of field just means that objects are in focus over a large range of distances from the lens. Here is more on it. https://en.wikipedia.org/wiki/Depth_of_field – mmesser314 Oct 01 '23 at 14:01
  • So the answer to my question in my last comment is affirmative? – Hans Oct 01 '23 at 18:15
  • Yes. Objects near and far are in focus. That is what a large depth of field means. – mmesser314 Oct 02 '23 at 02:52