Biomedical Engineering Reference
In-Depth Information
This equation may be familiar to you. It's sometimes given in undergraduate phys-
ics textbooks for the calculation of magnification produced by magnifying lenses.
This formula assumes that the object will be in the focal plane of the lens, creating
an image at infinity.
It's not uncommon for a patient to arrive at an eye appointment with a store-
bought, handheld magnifying glass that she uses to read at near. Let's look at an
example.
A patient purchased a magnifying glass labeled 2.5
. If she can read
4 M print at a distance of 40.0 cm without the magnifier, what size print do you
expect her to be able to read with the magnifying lens?
×
Once we have determined the power of the magnifying lens, we can solve this
problem with the same strategy we have used for other problems on near magnifi-
cation. The power of the lens is determined as follows:
F
4
M
25
=
F
4
=
2.5
F
= +
10.0 D
10.0 D lens, it produces
the same angle at the eye as if held at 10.0 cm. By being able to decrease her reading
distance from 40.0 to 10.0 cm, the patient experiences an angular magnification of
4
When reading material is held at the focal point of this
+
. Therefore, she will be able to resolve 1 M print when it's held at the focal point
of the magnifier.
×
Magnification ratings can be tricky. Rather than using effective magnification,
certain manufacturers may label lenses by their so-called conventional magnifica-
tion (
M
F
/4). Moreover, magnification ratings attached to stand magnifiers
are usually based solely on the power of the lens, even though the separation
between the lens and the page can have a major effect on final magnification.
=
1
+
ELECTRONIC MAGNIFIERS FOR NEAR
Handheld electronic devices and closed-circuit televisions (CCTVs) are increas-
ingly prescribed for patients with low vision. These devices magnify the original
object, creating an enlarged electronic image on a screen. They typically have a
variable zoom feature that allows the patient to adjust the magnification, thereby
allowing her to remain at a comfortable distance from the device's screen. Patients
may also benefit from the capacity of the display screen to reverse the contrast of
the print (so that there is white print on a black background rather than black print
on a white background) or add color contrast.
