Distance of virtual image
(Science: physics) When a simple lens is used as a magnifier for visual observation the eye becomes part of the optical system. A virtual image can be formulated by construction and its apparent distance will vary with the focus of the eye. This will vary among individuals. In a rather arbitrary but standardised assumption, the normal distance for close observation, or reading has been set at 10 inches (250 mm).
The optics for the compound microscope have been designed to furnish parallel light from the eyepiece so that the eyes are relaxed for distant viewing. This makes the virtual image lie at infinity. Tests showed that the average observer accommodates somewhat, placing the virtual image rather variably, often about 20 - 25 feet.