Question:
What are the rough probabilities involved in a routine IOL implant of
acheiving the desired focal distance?
For example, say the desired focal distance is 20 inches, what is the
probability of getting about 20 inches, the probability of getting about 10
inches, or about 30 inches ?
Answer:
My ophthalmologist says that, in very nearsighted eyes, the
uncertainty is +/- 0.5 diopter. Your target distance of 20
inches is 2 diopters, so if those uncertainties are correct
the expected range would be from 1.5 to 2.5 diopters, which
is about 27 inches to about 16 inches. The uncertainty
should be smaller if (1) you are not very nearsighted, and
(2) your eyeball was measured for the IOL before the cataract
became ripe. You should expect a noticeable amount of
astigmatism caused by the surgical wound, which could take
months to stabilize to a final non-zero amount.
Some surgeons will assume that your "dominant" eye should be fixed
at "infinity" (more than about 6 feet) and that your "non-dominant"
eye should be fixed at reading distance (about 18 inches). If you
have been very nearsighted or farsighted all your life that may not
be the best choice, especially if you get an IOL in only one eye at
a time or if you do not have one strongly dominant eye. If possible
work with an optometrist to simulate the effect of a particular choice
of focal distance (or distances, one for each eye) before surgery.
You may find that you don't like or can't tolerate the first choice
of focus distance. Or, you may find that you can't tolerate having
different focus distances in each eye, because this causes the image
sizes in each eye to differ. (I found that I could not tolerate any
image size difference; if I ever get an IOL in my other eye it will be
targetted for the same focus distance as my first IOL, to avoid this
problem.)
