The entrance pupil is the opening in front of a camera that allows light to enter. If it is partially obscured, light can still come through the unobscured part, and the camera can still see the object.
In one of those “experiments,” flat-Earthers placed an obstacle in front of a camera, very close to the lens, so that an object is partially visible. At the widest angle setting, the person appears partially visible. But it turned out that zooming in will fully reveal the person. Flat-Earthers claim it is how objects can vanish behind the horizon if Earth is flat and how they can reappear by zooming in. In reality, zooming enlarges the camera’s entrance pupil, letting the camera to see over the obstacle.
The diameter of the entrance pupil depends on the f-number and the focal length of the camera lens. Dividing focal length by the f-number gives us the width of the entrance pupil. Increasing optical zoom increases the focal length, and consequently, the diameter of the entrance pupil.
The specification of the Nikon P900’s lens is 4.3–357 mm 1:2.8–6.5. The former is the focal length range throughout the zoom range, while the latter is the aperture’s f-number range. Dividing the former with the latter gives us the diameter of the entrance pupil. At the widest zoom, the diameter of the entrance pupil is 4.3 mm / 2.8 = 1.5 mm; while at the longest zoom, it is 357 mm / 6.5 = 55 mm. Zooming in from the widest zoom to the longest zoom will increase the entrance pupil size from 1.5 mm to 55 mm.
The Nikon P1000 is another popular “flat-Earthers’ camera.” It has a 4.3-539mm 1:2.8–68 lens. At the widest zoom, its entrance pupil size is 4.3 mm / 2.8 = 1.5 mm. While at the longest zoom, it is 539 mm / 8 = 67 mm. Zooming from the widest angle to the longest zoom will increase the entrance pupil from 1.5 mm to a whopping 67 mm.
It is how zooming in can allow the camera to see above an obstacle very close to it. It does not apply to a distant obstruction, like the Earth’s horizon, and cannot “explain” whatever these flat-Earthers are trying to explain.
The Nikon COOLPIX P900