When a contact lens is in placed on the cornea, most of the refraction occurs at the interface between air and the tear film on the anterior surface of the contact lens. Corneal contact lenses are only about 3 mm in front of the eye’s first principal plane, where refraction may be considered to occur. The lenses turn with the eye, so that compared to a spectacle lens 12 mm in front of a rotating eye, there is less aberrational blur, distortion, and magnification/minification. Spectacle lenses have different optical advantages, offering stable vision and the ability to design more useful bifocals and to apply prism when needed. Table 5-1 summarizes optical considerations of contact lenses.
Anisometropia and Image Size
When one eye is much more myopic or hyperopic than the other, spectacle correction creates retinal images of unequal size, unless the difference is caused primarily by unequal axial lengths (Clinical Example 5-1). Also, as the eyes look off-axis, through parts of the spectacle lenses farther from their optical centers, the eyes encounter unequal prism, following the Prentice rule (see Chapter 4). This is particularly disturbing, looking down to view through bifocal segments, as it does not take much unequal vertical prism to cause discomfort or diplopia. To deal with this, it may be necessary to “slab-off” prism on one of the bifocal segments (see Chapter 4).
On the other hand, contact lenses of unequal powers yield retinal images that are of almost equal size, if the eyes are of similar length. The lenses move with the eyes, so there is little induced prismatic effect. Unilateral aphakia is an extreme example of anisometropia, with the aphakic eye usually much more hyperopic than its fellow eye. Typically, the aphakic spectacle lens magnifies the retinal image about 25% larger than it would be in an emmetropic eye of the same length, whereas a contact lens magnifies it about 7%. To understand this “relative spectacle magnification,” imagine starting with emmetropic eyes and instead of removing the natural lens of one eye, neutralize the lens’s power with a minus lens just in front of the natural lens, and place a correcting plus-powered spectacle lens 12 mm in front of the eye. Those 2 lenses create a magnifying Galilean telescope effect, which enlarges the retinal image.
Table 5-1 Optical Considerations of Contact Lenses
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.