Session outcomes At the end of this topic, learners should be able to: Calculate the necessary...
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Calculations of magnification
Session outcomes At the end of this topic, learners should be able to: Calculate the necessary magnification to achieve patient goals for both distance
Session outcomes At the end of this topic, learners should be
able to: Calculate the necessary magnification to achieve patient
goals for both distance and near Understand and be able to do all
the calculations regarding magnification as stipulated in this
topic Measure text size and calculate the appropriate magnification
to achieve that goal
Slide 3
Management options for low vision patients Reduction of working
distance Magnification Training in eccentric fixation Minification
Prisms Illumination control
Slide 4
Magnification It is essential in cases of central vision loss.
This requires few practices or training session until the patient
get it right. Magnification may be in the form of non-optical
devices such as electronic devices, and also in the form of optical
devices. Decision to use magnification method is followed by the
establishment of the power needed for magnification.
Slide 5
Calculation of distance and near magnification
Slide 6
Distance magnification Feinbloom chart is used Uses numbers:
700-10 Used at 10ft or closer Recorded: Test distance/Letter size
e.g. 10/80 Convert to 20ft Note the patient head posture : up/down
or left/right Why??
Slide 7
Calculating magnification for telescopes The commonly used
method is: M required = VA achieved / VA required The examiner
should be able to estimate the VA which will be needed (goal or
desired VA) for the particular task. For most daily distance
activities, 6/12 (20/40) are considered to be goal VA.
Slide 8
Example:1 A patient has a current corrected VA of 10/60 (3/18)
at 10 feet (3 meters), this is equivalent to 20/120 or 6/36
respectively in 6 meters. If it is estimated that the patient will
achieve his or her reading goal if he or she can read 20/40 or
6/12.
Slide 9
M required = VA achieved / VA required = 120/40 =3x The
telescope of the same power should then be tried on a patient and
refined using other power subjectively to establish the power
acceptable for the patient.
Slide 10
Example 2: Calculate magnification for the following patient.
Px S can read 2/30 prints and wishes to read 10/20, what
magnification would be required? All calculations must be
shown!!
Slide 11
Solution 2/30=20/x x = 600/2 x = 300 10/20=20/x x = 400/10 x=
40 M= 7.5x
Slide 12
Calculation of magnification for Magnifiers Best corrected near
VA values are usually used to calculate the estimated power of the
magnifier required to read at near. Different charts requires
different ways of calculating power. LogMar and lighthouse charts
to be discussed shortly
Slide 13
M-notation Its a metric notation Represents the distance in
meter. The target size and the test distance must always be noted
If the patient reads 6M at 40cm Acuity is recorded as: 0.4/6M
=40/600 =20/300
Slide 14
Determining M value Measure the height of lower case letters in
mm, and multiply by 0.7 for the M value 1mm = 0.7M 1.45mm = 1M
Slide 15
Converting between the different optotypes: M to N: x 8 N to M:
divide by 8 (1M = 8N )
Slide 16
Method 1 The lighthouse or precision vision near chart It gives
the appropriate add not the magnification required useful for
high-plus readers and hand magnifiers Snellen acuities of this
chart are calculated on a 40cm working distance
Slide 17
If one uses 40cm, then magnification is represented by the
formula M=D/2.5 Patient should hold the card at 40cm with +2.50 Add
and read the smallest line possible
Slide 18
Example 1: A patient can read 5M print and wants to read 1M
print, therefore 5x magnification is required, so the card will be
moved 5x closer to obtain magnification 40/5=8cm Questions??
Slide 19
The power needed is +12.50D,how did we achieve this power?
Lastly place this power on the trial frame and let the patient read
at 8cm
Slide 20
An add can also be obtained by multiplying the M-Value and the
D-Value 5 x 2.5 = +12.5 D then working distance can be calculated
from the power obtained: 1/12.5=8cm
Slide 21
OR It can be taken directly from the chart (during patient
examination) Remember, it still has to be refined using the trial
frame.
Slide 22
Summary of different methods of obtaining an Add using
Lighthouse 1. To bring the chart closer 2. Multiply M-Value and
D-Value 3. Read an Add directly from the chart
Slide 23
Method 2 For calculation of magnification powers using charts
which do not have powers indicated This method has to be used: M
required = VA achieved / VA required If a patient can read 6/60
(20/200) at 25cm and wishes to read 6/24 (20/80) at the same
distance Magnification will be 60/24 or 200/80 = 2.5x This is
equivalent to +10.00D lens
Slide 24
Method 3 This is called the Kestenbaums method It is based on
the dioptres of add needed to read 1M print and is approximately
equal to the reciprocal of the distance acuity.
Slide 25
If a patient can read 6/120 at 6m, the near addition required
is considered to be 120/6 which is = +20.00D This method is NOT
accurate, but may provide a basis for subjective determination of
the power of magnification.
Slide 26
Method 4 Determination based on the current add and
magnification required for goal VA Example: a patient can read 4M
print at 40cm with an add of +2.50D, what add will he require to
read 1M print? Solution?: it should be noted that 4M is 4 times as
large as 1.0 M, therefore magnification required is 4 times. The
required power will be 4x+2.5D= +10.00D Magnification is
approximately 2.5x
Slide 27
Method 5 The use of LogMAR method to calculate magnification at
near. This is applicable ONLY if the near LogMar chart is used. The
calculation is based on the current ratio of current VA and the
goal VA of the patient based on the LogMAR ratio of 1.2589 (0.1 log
unit).
Slide 28
Example 1: A patient can read 30 point print at 25cm, but his
goal is to read 12 point print. The ratio between these two reading
acuity values is 4. This achieved by dividing 30 by 1.2589 until
you get to 12, then the number of steps will be the
magnification.
Slide 29
This magnification can be achieved by increasing the Dioptric
power of 25cm for times. The Dioptric power of 25cm is +4.00D
Therefore the power needed to provide 4x magnification is 4x4=
+16.00D
Slide 30
Example 2: Also, magnification can be achieved by reducing the
working distance to a quarter of the original value. A fourth of
25cm is 6.25 cm The Dioptric value of 6.25 is +16.00D This power
can be in a form of accommodation or magnifier.
Slide 31
Poor reading performance The following factors may result in
poor reading performance: 1. Indication of need for more
magnification 2. Need for eccentric viewing 3. Resistance to close
working distance or
Slide 32
Illumination control It is necessary to try various level of
illumination that best suits each patient. High, medium or low
levels should be tried in improving the patients performance.
Slide 33
Controlling illumination can results in a significant
improvement in vision for some patient. With changes in
illumination, the power of the magnifier may be adjusted if
necessary.