Biomedical Engineering Reference
In-Depth Information
100/10.00 D
+
100/
−
50.00 D
=
8.00 cm) to 7.50 cm (i.e., 100/10.00 D
+
100/
−
40.00 D
=
7.50 cm), we have
1
M
ang
=
1
−
dF
1
1
M
ang
=
= +
4.0
×
1
−
(.0750 m)(
+
10.00 D)
As you can see, less magnification is provided when the myopic patient compen-
sates for her myopia by shortening a Galilean telescope's tube length.
Now, let's consider a Keplerian telescope.
A patient with 10.00 D of myopia
looks through a terrestrial telescope that has an objective of
+
10.00 D and
an ocular of
+
50.00 D. What is the magnification when the patient wears her
−
10.00 D spectacles? What would be the tube length and magnification if she
were to adjust the tube length to correct for her myopia?
. If she
adjusts the tube length to correct her 10.00 D of myopia, we can conceptualize the
eyepiece as a combination of a
When viewing through her spectacle correction, the magnification is
+
5
×
60.00
D lens that contributes to the telescope. Our basic angular magnification formula
for a telescope gives an angular magnification of
−
10.00 D lens to correct the ametropia and a
+
+
6.0
×
. (The magnification is posi-
tive since the image is erect in a terrestrial telescope.)
The magnification can also be determined using the alternative formula for
magnification. As with the Galilean telescope, the tube length must be shortened,
this time from 12.00 cm (100/10.00 D
+
100/50.00 D
=
12.00 cm) to 11.67 cm
(100/10.00 D
+
100/60.00 D
=
11.67 cm). Substituting, we have
1
M
ang
=
1
−
dF
1
1
= −
6.0
×
M
ang
=
10.00 D)
Since we are told that this Keplerian telescope has an erecting mechanism, the
magnification is actually
1
−
(.1167 m)(
+
. As you can see, more magnification is provided
when the myopic patient compensates for her myopia by shortening a Keplerian
telescope's tube length.
15
In comparison to a myopic patient, a hyperopic patient must lengthen the tube
to correct for her ametropia. For a Galilean telescope, this causes increased mag-
nification, while for a Keplerian telescope it results in decreased magnification.
Telescope use in ametropia is summarized in Table 12-4.
+
6.0
×
15. Certain low vision telescopes cannot be shortened to correct for myopia because when focused for
infinity, they are already at their shortest length.
