Virtual Image Tutorial
The virtual image concept can be hard to grasp. Basically, underwater the camera "sees" the image much closer than it really is, because of the optics of the dome port in water.
The Virtual Image
The optics required for an over/under must be housed in a dome port. When a dome port is used under water (in this case half the dome is under water), the dome creates a mysterious image known as a "virtual image.
HOWEVER, the lens focuses not on the subject, but on a "virtual image" projected on the dome port.
The lens behind a dome port does not focus on your chosen subject as it does with your terrestrial ones. It is a virtual image about 9 to 12 inches in front of the port. What the heck is a virtual image?
Camera lucida
Lens and prism system through which a virtual image was seen.
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Information provided by: SWPP BPPA
More Photographic Terms ...
A 35mm lens set to f/8; the diameter of the seven-sided entrance pupil, the virtual image of the opening in the iris diaphragm, is 4.375mm ...
when the screen is removed, it is an aerial image. The image seen through a telescope optical viewfinder, etc. cannot be focused on a surface without the aid of another optical system and is a virtual image.
 See also: Lens, Image, Camera, Light, Subject
  
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