Corrective Eyewear
Corrective Eyewear
• Normal Eyesight
Near point
Far point
Corrective Eyewear
• Everything you look at between these points
are in focus
Near point
Far point
in focus
Corrective Eyewear
Everything you look at outside these points are
not in focus
Near point
Far point
not in
focus
in focus
not in focus
Near Sighted
If you are near sighted, then your far point is
closer than normal
Near point
Far point
in focus
Near Sighted
If you are near sighted, then your far point is
closer than normal
Near point Far point
in focus
Near Sighted
Corrective eyewear for near sightedness, moves
the image of a distance object inside the far
pt.
Near point Far point
object
Near Sighted
Corrective eyewear for near sightedness, moves
the image of a distance object inside the far
pt.
Near point Far point
object
image
Near Sighted
The image MUST be virtual (upright) so the distance
image must be (-Di)
Near point
Far point
object
Di
image
Near Sighted
The object distance (Do) is infinity (a very large
number) because you want to see as far away as
possible
Near point Far point
object
image
Do
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
Do =
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
Do =
Di = -60 cm
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
Do =
Di = -60 cm
1/Do + 1/Di = 1/f
1/large # + 1/ -60 = 1/f
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
Do =
Di = -60 cm
1/Do + 1/Di = 1/f
1/large # + 1/ -60 = 1/f
f=-60 cm
Near Sighted
A nearsighted person has a far point of 60 cm.
What focal length lens should be prescribed?
For a nearsighted prescription, it is the same as the
person’s far point (but negative!)
Since the ‘f’ is a negative, it is a diverging lens!
Farsighted
• If you are farsighted, then your near point is
farther away than normal (happens with age)
Near point
Far point
in focus
Farsighted
• If you are farsighted, then your near point is
farther away than normal (happens with age)
Near point
Far point
in focus
Farsighted
• Corrective eyewear for farsightedness take an
object and moves it past the near point
Near point
Far point
object
Farsighted
• Corrective eyewear for farsightedness take an
object and moves it past the near point
Near point
Far point
object
image
Farsighted
• The Di is the near point but must also be
negative since we want an upright image!
Near point
Far point
object
image
Farsighted
• The Di is the near point but must also be negative
since we want an upright image!
Near point
Far point
object
image
Di
Farsighted
• The Do is how far away the person holds their
reading material.
Near point
Far point
object
image
Do
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
Do = 30 cm
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
Do = 30 cm
Di = -50 cm
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
1/Do + 1/Di = 1/f
Do = 30 cm
1/30 + 1/-50 = 1/f
Di = -50 cm
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
1/Do + 1/Di = 1/f
Do = 30 cm
1/30 + 1/-50 = 1/f
Di = -50 cm
f = 75 cm
Farsighted
• A person holds a book 30 cm away from their
eye. Their near point is 50 cm. What focal
length should be used to correct for this
problem?
The focal length is 75 cm then that means it is a
converging lens (+ f).
Corrective Eyewear
Both of these problems were worked for
someone wearing contact lenses. If the
problem gets changed for someone who
wears glasses, then just adjust the Do and Di
by how far away the glasses are from the eye
(usually 2 cm).
Corrective Eyewear
Both of these problems were worked for
someone wearing contact lenses. If the
problem gets changed for someone who
wears glasses, then just adjust the Do and Di
by how far away the glasses are from the eye
(usually 2 cm).
Ex) If Do= 30 cm…….then with glasses Do=28 cm
If Di = -50 cm….then with glasses Di = -48 cm
Homework
• Do the worksheet in your mini-packet