The SO-2200LED binocular indirect ophthalmoscope can be used to
estimate the refractive error of an eye. Observe the image size as the hand
condensing lens is moved from a point close to the eye (say 30mm) to a
point further away (say 120mm) in a line from inside to outside the point
where the first principle focus of the lens is at the anterior focus of the eye.
If the image size remains constant, the eye is emmetropic; if the image
diminishes, the eye is hypermetropic and if the image size increases, the eye
is myopic. If the eye is astigmatic, a meridional difference in magnification
may be seen. At the point where the first principal focus of the lens is at the
anterior focus of the eye (that is, a lens to cornea distance of about 60mm)
magnification of the image is independent of refractive error. Normally
however, the lens is closer to the eye than this, so the image magnification
is greater in hypermetropia and less in myopia than emmetropia (that is, the
linear field of view varies with refractive error). The angular field however,
being limited by the condensing lens aperture, is constant.
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Condensing lenses of powers between 15D and 30D are available from
several manufacturers. These lenses have a major influence on both the
illuminating and viewing optical systems, and it is important that only high
quality lenses are used. Scan Optics offer high quality lenses in powers of
20D and 28D.
When held, the higher power surface should be towards the observer, for
correct aberration control. A higher power lens will increase the field of
view, reduce the image magnification, increase the image brightness and
reduce the pupil size needed.
