Biomedical Engineering Reference
In-Depth Information
13
Retinal Image Size
The size of the image that falls upon the retina is influenced by the nature of
the patient's refractive error and the manner in which it is corrected. Retinal
image size is of clinical importance because clear, comfortable, and functional
binocular vision requires the fusion of the images formed on the two eye's
retinas. When the images are sufficiently unequal in size, fusion becomes dif-
ficult and the patient may manifest asthenopic and other symptoms. A differ-
ence between the retinal image size (or shape) of the two eyes is referred to as
aniseikonia
.
SPECTACLE MAGNIFICATION
Both the refractive power and shape of a spectacle lens may affect retinal image size.
Plus lens power causes angular magnification, while minus lens power causes mini-
fication. The change in retinal image size due to lens refractive power is referred to
as the
power factor
.
Lens shape also affects retinal image size. For our discussion, shape is defined as
front surface power, thickness, and index of refraction. All these contribute to the
shape factor
magnification produced by a lens.
The power and shape factors are independent from one another. Consider two
equally powered lenses (e.g., two
5.00 DS lenses). Because of their equal pow-
ers, both have the same power factor. If refractive power was the only consider-
ation, both lenses would produce the same magnification. But if the two lenses
had different shapes, meaning that the front surface powers, thicknesses, or indices
of refraction were different, they would cause unequal magnification because they
have unequal shape factors.
In trying to understand the shape factor, it may be helpful to recall that a tele-
scope, when focused for infinity, has no refractive power, yet produces angular
magnification. It's possible to make a miniature Galilean telescope from a spectacle
lens that has a plus front and minus back surface. Since this spectacle-telescope does
+
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