Biomedical Engineering Reference
In-Depth Information
TABLE 13-1.
RETINAL IMA
GE SIZE IN AMETROPIA
Retinal Image Size When the Ametropia Is
Corrected with a
Contact Lens (mm)
Corrected with a
Spectacle Lens* (mm)
Condition
Uncorrected (mm)
Emmetropia
x
x
x
Axial myopia
>
x
>
x
x
Refractive myopia
x
x
<
x
**
Axial hyperopia
<
x
<
x
x
Refractive hyperopia
x
x
>
x
**
*Spectacle lens located at anterior focal point of eye.
**The retinal image size in uncorrected refractive myopia and hyperopia is the same as in emmetropia. Correction
of refractive myopia with a spectacle lens causes minifi cation, while correction of refractive hyperopia with a
spectacle lens causes magnifi cation.
What does this imply about retinal image size in
corrected
ametropia?
We can assume that the retinal image size will be about the same as in emmetropia
when:
• Axial ametropia is corrected with spectacles (A minus lens minifies the enlarged
image found in uncorrected axial myopia and a plus lens magnifies the dimin-
ished image found in uncorrected axial hyperopia.)
• Refractive ametropia is corrected with contact lenses (The retinal image size in
uncorrected refractive myopia and hyperopia is the same as in emmetropia, and
a contact lens doesn't change this.)
For the retinal image in corrected axial ametropia to be exactly the same size as in
the emmetropic eye, the spectacle lenses should be positioned at the anterior focal
point of the eye, which is 16.7 mm (i.e., 1000/60.00 D
16.7 mm) anterior to the
reduced eye's front surface. This is referred to as
Knapp's law
. In clinical practice,
it is not necessary for the vertex distance to be exactly 16.7 mm. Retinal image size
in uncorrected and corrected ametropia is summarized in Table 13-1.
What are the clinical implications of all this? Consider
anisometropia
, a rela-
tively prevalent condition in which the two eyes have different refractive errors.
Depending on the magnitude and nature of the anisometropia (axial or refractive)
and the manner in which it is corrected (spectacles or contact lenses), the retinal
images in the two eyes may be different sizes, a condition we previously referred to
as aniseikonia.
=
Let's look at an example.
A patient has the following refractive error:
OD
−
2.00 DS
OS
−
5.00 DS
