VIEWING ANGLES TO PERMIT ACCOMODATION

The human eye as a visual detector (in combination with the brain) is the most efficient image-processing system that has ever been encountered. The structure and operation of the eye is shown in the anatomical description in Figure (1).

Together with the muscle-adjusted lens, the curved surface of the cornea projects an optical image onto the retina (the detector). The level of incident brightness is controlled via the variable diameter of an iris (much like an optical diaphragm) under the control of specific muscles.

A sharp image is produced by the flexible lens, the focal length of which is changed by another set of muscles so that focusing is possible on any object at a distance between approximately 20 centimeters and infinity. The image itself is detected on the retina by approximately 130 million photoreceptor rod cells (responsible for recognition of grey levels) and 7 million photoreceptor cone cells (color recognition), and is then transferred to the brain along the shortest possible path through the optic nerve.

The light rays illustrated in Figure 2 form a viewing angle of 30 degrees to demonstrate accommodation of the human eye for viewing objects at varying distances. In this case, a flying duck is observed at a distance of 50 meters whereas a nearby butterfly is viewed at a much closer distance of 25 centimeters. Objects in extreme proximity to the eye cannot have their images brought into focus on the retina because of the limited ability of the eye's lens to change its shape.

Furthermore, it is not practical to get any closer than approximately 10 centimeters an object being viewed due to the fact that the viewing angle becomes extremely small, which is why many details are unrecognizable. For example, if you wanted to have a closer look at the fine capillaries residing in the stalk of a plant (see Figure 3), you would cut a wafer-thin slice from the stalk, place it on a microscope slide and protect it using a cover slip (as illustrated in Figure 3(a)).

The many intricate features you want to see have a diameter of only one hundredth or even one thousandth of a millimeter so they cannot be recognized from such a great distance because the viewing angles are too small for the details to reach different receptors on the retina.

MORALE:

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