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A unit of on ―diopter‖ is the measure of power (P) in a lens. This refers to how strongly it refracts incoming light.
The focal length is in units of meters and has an inverse relationship to power. The crystalline lens of
the eye is convex and follows the same properties of convex lenses. However, the majority of the refraction in the eye is done at the cornea interface. A smaller fraction of light bending is done at the crystalline lens.
Direct Ophthalmoscope
The device is for viewing the retina through the ophthalmoscope, giving a high magnification but a small
angle of view. Looking directly into a person‘s eye, you may not see much but your own reflection on
the cornea. The human eye, unlike the eyes of cats and dogs, do not reflect much light back. Certain
animals have a layer underneath the retina called the tapetum lucidum that reflects light and is responsible for the animals glowing eyes when light is shined upon them. While, the Indirect
Ophthalmoscopy is for viewing the inverted image of the retina which is formed in front of a condensing lens, giving low magnification with a wide angle of view.
In humans, very little light is reflected back, plus if an observer were looking straight on, the observers
head would block most of the light from entering the subject‘s eye, leaving even less for reflection. This
is why the eyes appear dark. The direct ophthalmoscope uses an angled mirror, a slit with viewing window, and a diopter control to help focus the light onto the retina.
The figures 4-A and 4-B, show the
Figure 3: Light passes up from the base of the
head, and is reflected by a 45 degree mirror toward the subject. The observer can view the fundus through the view hole, without disturbing the
reflected light source. The cut-away view of the
ophthalmoscope shows the lens wheel with the
lenses adjacent to the green indicator, representing convex lenses that will as green numbers on
the reading window. The lenses adjacent to the
red indicator strip represent convex lenses and
will show up on the reading window as red. As
shown in figure 1-A
right and left direct ophthalmoscope view of the retinas. In order to understand some basic pathologies
of the eyeball, for the purpose of this article, the inner layer of the eyeball, we should review some basic
anatomy of the eye.