A camera is focused on an object at a distance of 1.0 m. Now, the object is moved
closer, to a distance of 0.50 m. How must the position of the lens be changed to keep the
image in focus?
Move lens back, farther from object
Most of the refraction occurs at the cornea’s surface, where Δn is the largest.
Figuring the Near Point
People in their early 20s have about 10 diopters of accommodation. For this range of
accommodation, what is the near point?
If s = ∞
Far point: f = 60
70 = 60 + s
s = 10 cm
A woman who has never needed vision correction finds that, as she ages, her near point
grows to 1.0 m – too distant for comfortable reading. Her distant vision is still quite
If she would like to read at the more reasonable distance of 25 cm, what power lens
does she need in a pair of reading glasses?
What she has
n.p. = 60 +
f s s = 1 m
What she wants
f = 60 + .25 = 64
What she needs
Converging lens, +3 D
What she gets – with this lens in place, what is her