Biomedical Engineering Reference
In-Depth Information
will be focused onto the retina if the patient looks through the plus lens with the
distance portion of his spectacles.
Here's another example.
A patient is able to read 6 M print at a reading distance
of 40.0 cm when looking through her bifocal. She goes to the drugstore and pur-
chases a
10.0 D magnifying glass that she holds 10.0 cm from the page. What
is the smallest print she can resolve when looking through the magnifying glass?
+
10.0 D lens, it subtends the same angle
as if held 10.0 cm from the eye. This angle is 4
When print is held at the focal point of a
+
larger than when the print is at the
reference distance of 40.0 cm (i.e., 40.0 cm/10.0 cm
×
=
4
×
). Therefore the patient
can read print that is 4
1.5 M).
Since the material is held at the focal point of the plus lens, the rays that emerge
from this lens are parallel to each other; therefore, the patient should look through
the magnifier with her distance prescription.
×
smaller—she can read 1.5 M print (i.e., 6 M/4
=
Let's try one more problem.
A patient with age-related macular degeneration has
best-corrected
distance
visual acuity of 20/400. What power magnifying lens is
required for the patient to read 2 M print?
Based on a distance visual acuity of 20/400, we know that the patient's mini-
mum angle of resolution (MAR) is 20 min of arc.
5
Since the detail of 2 M print
subtends 2 min of arc at 1.0 m (or 100.0 cm), it will subtend 20 min of arc when
it is 10 times closer. This gives us an equivalent viewing distance of 10.0 cm (i.e.,
100.0 cm/10
=
10.0 cm). Therefore, the magnifying lens must have a power of
+
10.0 D (i.e., 100/10.0 cm
=
10.0 D).
6
When prescribing a magnifying lens for presbyopic patients, we usually start off
with the assumption that the patient will hold the reading material at the focal
point of the lens while looking through his distance correction. What happens
if the patient holds the material closer than the focal length? From Chapter 4,
we know that a magnified virtual image will be formed on the same side of the
lens as the object. For this image to be focused on the retina, the patient must
look through his bifocal add. When the distance between the magnifying lens
5. The minimum angle of resolution (MAR) is the angle (subtended at the eye) of the smallest
detail the patient can resolve. It is the reciprocal of the Snellen visual acuity. For further details
see Schwartz (2010).
6. Kestenbaum's rule, which is used commonly in low vision practice, predicts that a plus lens power
equal to the reciprocal of the Snellen fraction will allow a patient to read 1 M print (Kestenbaum
and Sturnan, 1956). For a distance visual acuity of 20/400, this rule predicts a power of
+
20.00 D
(i.e., 400/20
=
20) to read 1 M print. Therefore, to read 2 M print, the power would be
+
10.00 D.
In clinical practice, Kestenbaum's rule may underestimate the lens power that is needed.
