Calculations of magnification Session outcomes At the end

Calculations of magnification

Session outcomes At the end of this topic, learners should be able to: ü Calculate the necessary magnification to achieve patient goals for both distance and near ü Understand be able to do all the calculations regarding magnification as stipulated in this topic ü Measure text size and calculate the appropriate magnification to achieve that goal

Management options for low vision patients �Reduction of working distance �Magnification �Training in eccentric fixation �Minification �Prisms �Illumination control

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.

Calculation of distance and near magnification

Distance magnification �Feinbloom chart is used �Uses numbers: 700 -10 �Used at 10 ft or closer �Recorded: Test distance/Letter size e. g. 10/80 �Convert to 20 ft �Note the patient head posture : up/down or left/right Why? ?

Calculating magnification for telescopes �The commonly used method is: VA achieved/ �M required = 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.

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.

VA achieved/ �M required = VA required = 120/40 = 3 x • 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.

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!!

Solution 2/30=20/x x = 600/2 x = 300 10/20=20/x x = 400/10 x= 40 M= 7. 5 x

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. �Log. Mar and lighthouse charts to be discussed shortly

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 6 M at 40 cm �Acuity is recorded as: 0. 4/6 M =40/600 =20/300

Determining M value �Measure the height of lower case letters in mm, and multiply by 0. 7 for the M value � 1 mm = 0. 7 M � 1. 45 mm = 1 M

Converting between the different optotypes: �M to N: x 8 �N to M: divide by 8 �(1 M = 8 N)

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 40 cm working distance

�If one uses 40 cm, then magnification is represented by the formula M=D/2. 5 �Patient should hold the card at 40 cm with +2. 50 Add and read the smallest line possible

�Example 1: A patient can read 5 M print and wants to read 1 M print, therefore 5 x magnification is required, so the card will be moved 5 x closer to obtain magnification 40/5=8 cm Questions? ?

The power needed is +12. 50 D, how did we achieve this power? • Lastly place this power on the trial frame and let the patient read at 8 cm

�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=8 cm

OR • It can be taken directly from the chart (during patient examination) • Remember , it still has to be refined using the trial frame.

�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

Method 2 �For calculation of magnification powers using charts which do not have powers indicated �This method has to be used: VA achieved/ �M required = VA required �If a patient can read 6/60 (20/200) at 25 cm and wishes to read 6/24 (20/80) at the same distance �Magnification will be 60/24 or 200/80 = 2. 5 x �This is equivalent to +10. 00 D lens

Method 3 �This is called the Kestenbaum’s method �It is based on the dioptres of add needed to read 1 M print and is approximately equal to the reciprocal of the distance acuity.

�If a patient can read 6/120 at 6 m, the near addition required is considered to be 120/6 which is = +20. 00 D �This method is NOT accurate, but may provide a basis for subjective determination of the power of magnification.

Method 4 �Determination based on the current add and magnification required for goal VA �Example: a patient can read 4 M print at 40 cm with an add of +2. 50 D, what add will he require to read 1 M print? �Solution? : it should be noted that 4 M is 4 times as large as 1. 0 M, therefore magnification required is 4 times. �The required power will be 4 x+2. 5 D= +10. 00 D �Magnification is approximately 2. 5 x

Method 5 �The use of Log. MAR method to calculate magnification at near. �This is applicable ONLY if the near Log. Mar chart is used. �The calculation is based on the current ratio of current VA and the goal VA of the patient based on the Log. MAR ratio of 1. 2589 (0. 1 log unit).

Example 1: �A patient can read 30 point print at 25 cm, 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.

�This magnification can be achieved by increasing the Dioptric power of 25 cm for times. �The Dioptric power of 25 cm is +4. 00 D �Therefore the power needed to provide 4 x magnification is 4 x 4= +16. 00 D

Example 2: �Also, magnification can be achieved by reducing the working distance to a quarter of the original value. �A fourth of 25 cm is 6. 25 cm �The Dioptric value of 6. 25 is +16. 00 D �This power can be in a form of accommodation or magnifier.

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

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 patient’s performance.

�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.