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Outline
I. Assessment of visual function
A.
B.
C.
D.
E.
F.
Visual acuity, understand your tests
Assessment of effects of luminance, eccentric viewing
Reading acuity, thresholds for resolution and for maximum efficiency
Visual fields, understand your own goals and select appropriate tests
Contrast sensitivity, purpose and methods
Color vision, tests and their value
II. Magnifying aids for distance vision
A. Telescopes and their effect
1. Determining the magnification of telescopes
a) Image is smaller by M, but closer by M2
b) Vergence amplification. very sensitive to vergence change
2. Determining exit pupil size
a) Image brightness
b) Never brighter, dull if small exit pupil
3. Determining the field of view
a) Width of field
(1) For Keplerian telescopes = EVD
(2) Smaller for Galilean
III. Magnifying aids for near vision
A. Near vision magnifiers change Equivalent Viewing Distance (EVD)
1. EVD = distance at which original object would subtend an angle that is equal
to the angle that the image subtends at the observer's eye
2. Determining the EVD required to meet a patient's needs
3. Set a resolution goal
a) Example: to read 0.8 M print (6.3 pts or telephone book print)
4. Determine resolution limit when chart is at a known distance
a) Chart should be in satisfactory focus. (not necessarily ideal focus)
5. Note resolution limit (smallest print read) and viewing distance
a) Example. Patient with +2.50 D add, reads 4 M print (32 pts) at 25 cm
b) Focus seems OK. Is 1.50D closer than ideal
(1) Low vision patient s usually less sensitive to blur
6. Calculate required EVD
a) Reads 4.0 M at 25 cm, to read 0.8M needs to move 5 times closer
(4.0/0.8 = 5), i.e., from 25 cm to 5 cm
7. Choose most convenient system that gives required EVD (here 5 cm) or
closer
B. High addition spectacles
1. Force patient to have close working distance
2. EVD = focal length of add
3. Binocularity requires decentration
a) At least Near PD = Dist PD -1.5(add)
4. Measuring power of high add readers, Front vertex power, equivalent power
5. How to determine the EVD
C. Hand held magnifiers
1. Held remote from the eye
a) If eye-to-lens distance is longer than the focal length of magnifier,
patient should use distance glasses (or relax accommodation)
b) EVD = focal length of magnifier
(1) Need to know lens power
(2) Measuring equivalent power of hand held magnifiers
c) Field will be smaller than the magnifier lens
2. Held close to the eye
a) Patient will gain some advantage by using add
(1) If the eye to magnifier distance is shorter than the focal
length of the magnifier
b) Complete additivity of magnifier and reading add if they are in contact
c) EVD = focal length of magnifier or less
d) Field will be larger than the magnifier lens
D. Stand magnifiers
1. Fixed focus stand magnifiers give an image at a fixed distance (l’) behind the
lens
2. The image is larger than the original object by a fixed ratio (ER = enlargement
ratio)
3. Black box concept
a) Looking into a stand magnifier is like looking into a black box
b) Window of the black box is the lens area
c) Depth of black box is the lens to image distance
d) On the back wall of the black box is an enlarged replica of the
original object
e) The patient looks through the window at the enlarged replica on the
back wall
4. Image location for stand magnifiers
a) Clinician should know the image position
(1) Actual viewing distance = eye to image distance = eye to
lens distance + lens to image distance
(2) Focus should be satisfactory given the add power and image
distance
(3) Patients choose eye to lens distance for their own
convenience
(a) To achieve clear focus (depends on add or
accommodation)
(b) To achieve required resolution (closer is generally
better)
(c) To get a good field of view (closer is better)
(d) For comfort or habit (usually prefer farther away)
5. How to determine image location in stand magnifiers
6. How to measure the lens power of stand magnifiers
7. How to calculate the enlargement ratio for stand magnifiers
8. Magnification effect best expressed as EVD
a) EVD = eye to image distance/enlargement ratio
E. Choosing a magnifier that gives the required EVD
1. Need to make an estimate of how close the eye will be to magnifier lens
2. Can predict EVD, accommodation demand, and field size
a) A list with optical aids specifications will be distributed
(1) The list is arranged to facilitate the selection of magnifiers to
achieve the required EVD, accommodation demand, field
size and other features such as illumination options and
adjustability
F. Near vision telescopes
1. Can consider as a telescope with a cap on the objective
a) Cap determines the focusing distance
b) EVD = focusing distance/telescope magnification
2. Width of field of view
a) For Keplerian telescopes = EVD
b) For Galilean telescopes it is smaller than the EVD
3. Adjusting telescope length to give near focus
a) Lengthen telescope
(1) Effectively borrows plus from the objective
(2) A weaker objective gives a stronger telescope
G. Electronic display systems
1. Closed-circuit televisions (video magnifiers)
a) Enlargement
b) EVD
c) Field of view
d) Varying observation distance
e) Near vision telescopes
2. Computer displays
a) EVD
b) Screen size
c) Pixel density
d) Font size
e) Magnification programs
f) Observation distance
g) Field of view
h) Near vision telescopes
IV. Determining magnification needs when size of detail cannot be quantified
A. Examples: knitting or crocheting, cutting hair, examining collectibles
1. Recreate the task
a) Change viewing distances
(1) Maintain appropriate addition
(2) Bracket
(a) Begin with 2 fold steps or more
(3) Endpoint is the further distance that allows the task to be
performed easily and comfortably
V. Deciding the most appropriate magnifying systems(s)
A.
B.
C.
D.
E.
F.
G.
Adequate EVD
Convenience
Field of view
Cosmesis
Portability
Cost
Light control
1. Lighting
2. Tints
3. Shades
4. Typoscope
H. Monocular versus binocular
I. Head mounted magnifier systems