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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
 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
 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
 myopia (no astigmatism) may read
 better at close distances without
 glasses. They have “built in”
 When viewing at distance, person
 with uncorrected myopia may
 squint. This is done in an effort to
 create a pinhole.
 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 simply makes a
 bigger image on the retina and
 gives the person more information
 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
 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
 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
  formula is F/4 (high-add reading glasses. Be
  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.
 Include:
     Telescopes.
     Telemicroscopes.

     Microscopes (reading lenses,

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

  (prisms, reverse telescopes,
 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
~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
 for print size, your distance
 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
  read 1.0M and he wants to read 0.5M.
~The fractions should read:
0.4/1.0 = 2 …so, he needs a 2X (8
0.4/0.5       diopters) magnifier to read
              0.5M print.
~Keep in mind, you can use whatever
  distance the student needs.
 Hints for Near Vision (cont.)
~Suppose your student read 2.4M at
 15cm…and he needs to be able to
 read 1.0M at 15 cm. How much
 magnification should you give him?
 -.15/2.4 is what currently reading.
 -.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
 systems require additional computation.
    Feq = F1+F2-cF1F2
    F1 = spectacle; F2 = magnifier;
    c = the distance between the two in
    meters. (meters, meters, meters.)
Feq = F1+F2-cF1F2

Feq = equivalent lens power
F1 = magnifier
F2 = spectacle add
c = the distance between the two in
 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
    ~Partial corrections, the use of
an add and viewing distance will
influence the focal distance of
Tricky Part (cont.)
    ~Relative distance
     magnification in conjunction
     with reading add or other
     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
 A student has a reading add of
  +3.00D, and uses a +7.00D
  magnifier. What is the equivalent
  power of the system?
  ~If the student hold the magnifier up to
     the spectacle add:
     Feq = F1+F2-c(F1F2).
Working w/ the Formula (cont.)

 Answer: #1 sample.
 Feq = +3.00 + 7.00 – 0(3x7) =

 You essentially combined the
 powers of the add with the
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.)

 Answer: #2 sample.
 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.)

 Somebody asked the student about
 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) =
It’s All About Placement

 58 diopter eye:
    Add +5 diopter lens.
    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.
It’s All About Placement (cont.)

 60 diopter eye
    Add +10 magnifier.
    Total power = +10.
    Focal distance (100/10) = 10cm.
It’s All About Placement (cont.)
 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.
It’s All About Placement (cont.)
 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.
 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
    task for longer periods.
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
  read one phone number…
  reading a page of phone
  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
 For further information, please see
  the handout “Prescribing Near
  Magnification for Low Vision

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