# Optics

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```					            Optics

A Greatly Simplified Overview
 At a distance of 20 feet, light rays
become parallel.
 Placing a lens in the path of the light
results in bending of all but 1 light ray.
 This single light rays is called the
optical axis ray; it passes straight
through the center. All of the other
rays will bend.
 + (convex) lenses will cause
light rays to converge.
 - (concave) lenses will cause
light rays to diverge.
 The amount of bending that is done
by the lens = diopter value.
 Bigger lens = more bending.
 Smaller lens = less bending.
PLUS LENSES (converging)
 Rays come together at a focal point
behind the lens.
 The thicker lenses, the closer
the focal point will come to
touching the back of the lens
(See Optics Example 1).
 The distance between the back of
the convex lens and the focal point
is called the focal distance. This is
measured in centimeters and can
be divided into 100 to determine
the bending power of the lens
(diopter value).
MINUS LENSES (diverging)
 The rays will bend away from the
optical axis ray after they have
passed through the lens, the focal
point is in front of the lens and is
the place where the diverging rays
“appear” to originate from.
   The thicker the lens, the closer
the focal point will be to the
front side of the lens (See
Optics Example 2).
 The distance between the front of
the lens and the focal point is called
the focal distance. This is
measured in centimeters and can
be divided into 100 to determine
the bending power of the lens
(diopter value).
Formula for Focal Distance

 100/diopters = focal distance in cm
Refractions
 Normal eyes = 60 diopters of
refractive power (emmetropic).
 If eye has less than 60 (short eye),
it is farsighted or hyperopic. Can
make up for difference in power
with + lenses.
 If eye has more than 60 (long eye),
it is near sighted or myopic. Can
reduce power with – lenses
Myopia
 May bring the object being viewed
into focus by bringing it closer to
the eye. This is why people with
the refractive error of simple
better at close distances without
glasses. They have “built in”
magnifiers.
 When viewing at distance, person
with uncorrected myopia may
squint. This is done in an effort to
create a pinhole.
Hyperopic
 If young, accommodation may
allow for a change in the shape of
the eyeball. This will result in
person with uncorrected refractive
error being able to read without
glasses at near distances. This is
VERY fatiguing.
The Wonderful World of
Magnification
 Magnification simply makes a
bigger image on the retina and
to work with. Magnification
assumes that at least part of the
retina is viable.
4 Types of Magnification
 Relative distance
magnification…bring what you are
looking at closer, hold something ½
as far away…the size of the image
on the retina doubles.
 Relative size magnification…makes
what you are looking at bigger.
 Angular magnification…use of
lenses.
 Projection magnification…project
an image onto a screen.
 May combine the different types of
magnification to get even greater
levels of magnification.
 Keep in mind the bigger something
is, the less of it you can see at one
time.
DANGER, DANGER
WILL ROBINSON
 When working with magnification, it is very
important to keep diopters and powers (or
X’s) separate.
 There is a 4:1 difference between diopters
and powers. So…a 24D lens = 6X. The
advised that some magnifiers may be
different. (F/4 +1) (handheld magnifiers).
 Don’t always believe the manufacturers
power labels on magnifiers (often they are
wrong or misleading). Use the diopters to get
a more accurate picture.
 In low vision, the standard to label magnifiers
is based on a working distance of 25cm.
LOW VISION DEVICES
 Include:
 Telescopes.
 Telemicroscopes.

loupes, clip-on’s).
 Magnifiers (stand, handheld).
 Projection (electronic magnifiers).
 Field enhancement devices

(prisms, reverse telescopes,
mirrors).
 Non-optical aids (filters, bold

pens, bold line paper, etc.).
Computing Magnification
 Distance
work in feet
person has 20/200
person needs 20/40
will require 5X magnification
 Near
work in meters
person has 0.4/1.0
person needs 0.4/0.5
will require 2X magnification
But…alas, it’s not that simple.
Hints for Near Vision
~The standard for near is 40cm (just
like the standard for distance is 20
feet).
~Distance the student used/X =
40cm/# of the smallest line read on
the near card.
Hints for Near Vision (cont.)
~Remember though…you need to
keep the numbers on the right hand
side all in the same value. In other
words, if you are working in meters
measurements need to also be in
meters. So, we must divide the 40
by 100 to get 0.4.
Hints for Near Vision (cont.)
~Then you set up the ratio. For this
example we are saying the student
0.4/1.0 = 2 …so, he needs a 2X (8
0.5M print.
~Keep in mind, you can use whatever
distance the student needs.
Hints for Near Vision (cont.)
15cm…and he needs to be able to
read 1.0M at 15 cm. How much
magnification should you give him?
-.15/1.0 is what he needs to read.
-It is a difference of 2.4 so a 2.5X
(10 diopters) magnifier should work.
Equivalent Powers/Distance
 When magnifiers are used in conjunction
with bifocal adds, or at distances that
are closerthan infinity,the focal distance
and total power of the combined lens
Feq = F1+F2-cF1F2
F1 = spectacle; F2 = magnifier;
c = the distance between the two in
meters. (meters, meters, meters.)
Formula
Feq = F1+F2-cF1F2

Feq = equivalent lens power
F1 = magnifier
c = the distance between the two in
meters.
Tricky Part. Knowing the glasses
are the correct Rx.
 And this is what you might not know:
~The glasses Rx is no longer
correct. Providing magnification at
this time is only magnifying “blur”.
~Sometimes only partial correction
may be given. This may be done in an
effort to maintain a better working
distance or to allow the person an
opportunity to adapt to new Rx.
Tricky Part (cont.)
~You need to know this…
(Lensometer vs. Refraction!) (What
does the student need vs. what
does student have in current
glasses.).
~Partial corrections, the use of
an add and viewing distance will
influence the focal distance of
magnifiers.
Tricky Part (cont.)
~Relative distance
magnification in conjunction
angular magnification will
change total magnification
yield and focal distance.
~Some of the power of the
magnifier may be “eaten up” to
make up for refractive error.
Working w/ the Formula
+3.00D, and uses a +7.00D
magnifier. What is the equivalent
power of the system?
~If the student hold the magnifier up to
Feq = F1+F2-c(F1F2).
Working w/ the Formula (cont.)

Feq = +3.00 + 7.00 – 0(3x7) =
+10.00D.

You essentially combined the
powers of the add with the
magnifier.
Working w/ the Formula (cont.)

 Your student doesn’t want to hold
the magifier that close…so he
moves it 14cm away to use it.
What happens to the power?
Working w/ the Formula (cont.)

Feq = +3.00 + 7.00 – .14(3x7).
Feq = 10.00 - .14(21).
Feq = 10.00 – 3 (actual 2.94) = +7.00D.

Reminding you to change 14cm to
.14M before multiplying.
Working w/ the Formula (cont.)

using the magnifier. Now he is a
little uncomfortable with it. He
chooses to move the magnifier 13
inches away (33cm) so the other
kids won’t see him using it from
across the room. What happens to
the power of the magnifier now?
Working w/ the Formula (cont.)
 Answer: #3 sample. First thing to
do is change 13 inches to cm (1
inch = 2.5cm).
 Feq = F1+F2-c(F1F2).
 Feq = (3.00 + 7.00) - .33(3x7).
 Feq = (3.00 + 7.00) - .33 (21).
 Feq = 10.00 – 7 (actual 6.93) =
+3.00D.

 58 diopter eye:
2 diopters used up to bring eye
to 60 so there is only a +3
gain in power.
Total power +3 Focal distance
(100/3) = 33cm.

 60 diopter eye
Total power = +10.
Focal distance (100/10) = 10cm.
 Using +3 add in bifocal and +10
magnifier (with magnifier touching
the bifocal)

Feq =10 + 3 = 13.00D.

Total power +13 Focal distance
(100/13) = 7+cm approximately.
 Using +3 add in bifocal and +10
magnifier (with magnifier 5cm from
the bifocal)

Feq = 10 +3 -0.5(10X3) = +11.50D.

Total power +11.50 Focal distance
(100/11.50) = 8+ cm approximately.
But…
 Some practitioners suggest there is
still more to this process…they
recommend we also consider:
 Acuity reserve: This implies that
magnifier users are not able to
continually work at their threshold
acuity and will need to keep
some “in the bank” to sustain the
But… (cont.)

 Contrast reserve: This brings to
mind the need to measure
contrast sensitivity and also allow
for some contrast “stockpile”
when planning for requirements
related to near vision usage.
But… (cont.)
 Now  remember: You can’t
expect your student to use up all
of his/her visual capacities…and
be able to sustain a task. So, in
other words, if it is a struggle to
numbers would be unreasonable.
But… (cont.)

 Fieldof view: Consider the number of
letters the reader is able to take in at
one time. (The more magnification
given, the lower the number of letters
will be).
But… (cont.)
 Central scotoma size: More applicable
to the older adult population, however
many children also have central vision
loss. (This will limit the amount of
information that can be taken in at
one time).
But… (cont.)
 When these factors have been
considered, it is generally found that
higher levels of magnification are
required than have been previously
recommended.
 For further information, please see
the handout “Prescribing Near
Magnification for Low Vision
Patients”.

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