3. Predicting the magnification required
We can get the magnification using:
Required magnification= denominator of present Visual acuity to
denominator of desired visual acuity.
For example :if actual visual acuity=10/60
And desired visual acuity=10/20
Then
60/20=3x magnification.
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4. Magnification required = required VA
present VA
In Snellen notation to improve from 6/60 to 6/6
Magnification required = 6 x 60
6 x 6 = 10 x
Watching TV 6/18
Reading Bus Numbers 6/6
Predicting the magnification required
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5. Magnification used = achieved VA
present VA
So achieved VA = magnification x present VA
For example a Px with a VA = 6/36, using a telescope with a
Magnification = 4x would achieve a VA of:
4 x 6/36 = 6/9
Predicting the magnification required
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6. If VA is measured in a LogMAR notation:
Magnification = (1.25)n
Where n = number of steps
If the present acuity = 0.5 and the required acuity = 0.1
Then Magnification = (1.25)4
= 2.44x
Predicting the magnification required
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OPTHALMOLOGY
8. Trial with Distance-Vision Aids
Distance vision aids for low-vision patients are
usually afocal telescopes.
An afocal telescope is designed in such a
way that if parallel rays of light enters the
objective, parallel rays will emerge from the
eyepiece.
However, if an afocal telescope is used at a
distance closer than infinity, the divergence of
the rays of light entering the objective will be
greatly increased when the rays emerge from
the eyepiece.
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9. Trial with Distance-Vision Aids
Vergence amplification effect- increase in
the divergence of light that occurs when an
afocal telescope is used to view an object at a
finite distance as referred by Bailley. (1978)
The value of vergence amplification
effect- square of the magnification brought
about by the telescope in question.
For a 3x telescope, the vergence amplification
effect is equal to 32
or 9.
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OPTHALMOLOGY
10. Trial with Distance-Vision Aids
When afocal telescopes are used as trial aids
with low-vision patients, the vergence
amplification effect can be avoided by holding
in front of the afocal telescope a lens of the
power indicated by the object distance.
Jose et al have suggested that only two
telescopes are needed to asses a low-vision
patient’s prognosis for successful telescope
use:
A 2.5x monocular telescope
A combination of 6x/8x monocular telescope
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OPTHALMOLOGY
11. Trial with Distance-Vision Aids
Best CorrectedBest Corrected
Distance AcuityDistance Acuity
MagnificationMagnification
20/100 2.5x
20/100-20/300 6.0x
20/300-20/600 8.0x
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OPTHALMOLOGY
12. PROCEDURES FOR THE USE
OF TRAIL TELESCOPE
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OPTHALMOLOGY
13. Procedures for the use of trial telescopes
If the best corrected distance acuity is poorer
than 20/600, Jose et al. state that the
prognosis is poor for the use of a telescope
without extend training. They suggested
procedures for the use of trial telescopes:
1. Doctor should focus the telescope initially
for a 13-foot viewing distance.
2. The patient first should be asked to locate
a large object at the end of the examination
room.
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14. Procedures for the use of trial telescopes
3. The test should be discontinued if patient
cannot find the chart within a few minutes.
4. For the 6x telescope, the successful patient
is the one who quickly finds the chart at an
acuity level six times that of the best
corrected distance acuity (e.g. from 20/300
to 20/50).
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15. Points to Remember!
Magnification by telescope provide
only large image rather than the clear
image.
Patient having cloud media(i.e
corneal opacity) will still have hazy
vision even through optical devices.
Optical devices are recommended
according to required magnification
and patient ADL.
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16. Points to Remember!
Utility of optical devices should be
demonstrated before prescribing to
patient.
Emphasize and instruct the key role
of illumination to patient.
Patient will be on follow up as
recommended by low vision
practionar.
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OPTHALMOLOGY
18. Types of Afocal Telescopes:
Types of Afocal Telescopes:
Galilean or Keplerian telescopes.
GalileanGalilean KeplerianKeplerian
*Convex objective and a*Convex objective and a
concave ocularconcave ocular
*No Erecting prism*No Erecting prism
* Magnification 1x to 3x,4x* Magnification 1x to 3x,4x
*head-borne telescope of*head-borne telescope of
low powerlow power
*Convex objective and*Convex objective and
convex ocularconvex ocular
*Requires erecting prism to*Requires erecting prism to
erect inverted image.erect inverted image.
* Powers ranging 3x to 15x* Powers ranging 3x to 15x
* Bulky and hand-held* Bulky and hand-held
telescopes for specific taskstelescopes for specific tasks
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OPTHALMOLOGY
20. Factors determined the use of
distance magnification system.
Ease of use(e.g field of
view,spotting,focussing,scanning etc.)
Requirement for free hand magnification.
Requirement for mobility.
Contrast or image brightness.
Weight,cost,cosmesis etc.
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22. EXAMPLES OF TELESCOPES
ON THE UTILITY BASIS
On the basis of utility factors
telescopes may classified as
hand held,
spectacle mounted,
bioptic,
Max T.V
ocutech etc.
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23. HAND HELD TELESCOPES
Hand held telescopes are most
appropriate for short term viewing or
sporting activities.
For example :
Reading bus numbers or street signs ,
or viewing a chalk board in the
classroom.
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27. Max T.V
max tv are hands-free, head-mounted glasses provide
2.1x magnification AND can be adjusted so that each
eye lens can be focused separately [±3 diopters],
making the system adaptable to most patients!
The glasses will focus on objects from distances of 3m
[10 feet] to infinity, and are perfect for watching TV,
sporting events, movies, theatre, bird watching, or any
other distance viewing activity in which a magnified
image would be helpful.
With flexible temples, MAXTV telescopic glasses are
the most comfortable television viewing device ever.
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OPTHALMOLOGY
29. Bioptic Telescope
Bioptics, also known as a bioptic in the
singular, and sometimes more formally
termed a bioptic telescope, is a term forA
bioptic is a special pair of glasses with a
telescope permanently mounted in the
glasses' lens. While looking straight ahead,
a bioptic user sees a normal, unmagnified
image through the glasses. Then by dipping
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30. Bioptic Telescope
one's head slightly, the bioptic user
instantly sees a magnified image through
the telescope. This "bi-optical" system
allows the user to rapidly switch between a
normal view and a magnified view without
ever using his or her hands.It is used to
improve distance vision for those with
severely impaired eyesight, especially those
with albinism.
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OPTHALMOLOGY
32. Selsi Sportglasses
Selsi Sport Glasses are made for
distant viewing. They are like
binoculars where you turn the knobs
on the lenses to adjust the focus.
Each eye focused individually. These
glasses are worn like a pair of
eyeglasses. Good quality sports
binocular are with coated achromatic
lenses.(Magnification 2.5x,8x,6x,4x)
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OPTHALMOLOGY
34. Ocutech VES Telescope
Ocutech is the low vision telescopic
aids, including the AutoFocus, which
focuses automatically and provides
natural, hands-free magnification.
Ocutech is a leading developer of
optical low vision solutions for
patients suffering from macular
degeneration and other low vision
impairments.
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36. REFERENCES
Bennett and Rabbetts' clinical visual
optics
A guide to low vision practitioners
(American Academy)
Slide show.net
Wikipedia
Theory and practice of optics and
refraction by A.K.Khurana
NOAH-low vision aids
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