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Handle
Figure 1-C
How an ophthalmoscope works:
Let‘s break the ophthalmoscope head down. Inside the head are numerous lenses both convex and
concave. Concave and convex lenses are embedded in a lens wheel, while other convex lenses are
embedded on the aperture wheel.
From the images on the front and rear view of the ophthalmoscope head, we can see several common
controls. An on/off switch is found at the base of the head, superior to the battery charging handle. By
pressing on the green release button and twisting the knob, you can turn on, increase light intensity,
and turn off the scope. A focus wheel on the side of the head is used to change the focal point. Turning
the wheel clockwise rotation moves the focal point closer to the observer, while turning the wheel
counter-clockwise moves the focal point further away from the observer. A light aperture is shown on
the back view of the head. This changes the shape and size of the aperture and usually includes different shapes such as half circles, slits as well as a blue and green filter lens. A red filter exists in some
ophthalmoscopes to view the macula, but is not common to all scopes.
The convex lens, shown in figure 2-A, focuses incoming light to the focal point, using the refractive
property of convex lenses. The concave lens, shown in figure 2-B, diverges light after passing it, by use
of the diffraction. Since the focal point for a convex lens some distance ―f‖ from the lens, we assign it a
positive number. Likewise, since the focal point of a concave lens is some distance ―f‖ prior to the lens,
we assign it a negative number.