Calculations of magnification

Calculations of magnification Session outcomesAt the end of this topic, learners should be able to:Calculate the necessary magnification to achieve patient goals for both distance and nearUnderstand and be able to do all the calculations regarding magnification as stipulated in this topicMeasure text size and calculate the appropriate magnification to achieve that goal

Management options for low vision patientsReduction of working distanceMagnification Training in eccentric fixationMinification Prisms Illumination controlMagnification 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 magnificationDistance magnificationFeinbloom chart is usedUses numbers: 700-10Used at 10ft or closerRecorded: Test distance/Letter sizee.g. 10/80Convert to 20ftNote the patient head posture : up/down or left/rightWhy??

Calculating magnification for telescopesThe commonly used method is:M required = VA achieved/VA requiredThe 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:1A 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.M required = VA achieved/VA required= 120/40=3xThe 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!!Solution2/30=20/xx = 600/2x = 300

10/20=20/xx = 400/10x= 40M= 7.5xCalculation of magnification for MagnifiersBest 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

M-notationIts a metric notationRepresents the distance in meter.The target size and the test distance must always be notedIf the patient reads 6M at 40cmAcuity is recorded as: 0.4/6M =40/600 =20/300

Determining M valueMeasure the height of lower case letters in mm, and multiply by 0.7 for the M value

1mm = 0.7M1.45mm = 1M Converting between the different optotypes:M to N: x 8N to M: divide by 8(1M = 8N)Method 1The lighthouse or precision vision near chartIt gives the appropriate add not the magnification requireduseful for high-plus readers and hand magnifiersSnellen acuities of this chart are calculated on a 40cm working distanceIf one uses 40cm, then magnification is represented by the formula M=D/2.5Patient should hold the card at 40cm with +2.50 Add and read the smallest line possible

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 magnification40/5=8cm Questions??

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

An add can also be obtained by multiplying the M-Value and the D-Value5 x 2.5 = +12.5 D then working distance can be calculated from the power obtained: 1/12.5=8cmORIt 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 LighthouseTo bring the chart closerMultiply M-Value and D-ValueRead an Add directly from the chart

Method 2For calculation of magnification powers using charts which do not have powers indicatedThis method has to be used:M required = VA achieved/VA requiredIf a patient can read 6/60 (20/200) at 25cm and wishes to read 6/24 (20/80) at the same distanceMagnification will be 60/24 or 200/80 = 2.5xThis is equivalent to +10.00D lensMethod 3This is called the Kestenbaums methodIt is based on the dioptres of add needed to read 1M print and is approximately equal to the reciprocal of the distance acuity.If a patient can read 6/120 at 6m, the near addition required is considered to be 120/6 which is = +20.00DThis method is NOT accurate, but may provide a basis for subjective determination of the power of magnification.

Method 4Determination based on the current add and magnification required for goal VAExample: 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= +10.00DMagnification is approximately 2.5xMethod 5The 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).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.

This magnification can be achieved by increasing the Dioptric power of 25cm for times.The Dioptric power of 25cm is +4.00DTherefore the power needed to provide 4x magnification is 4x4= +16.00D

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 cmThe Dioptric value of 6.25 is +16.00DThis power can be in a form of accommodation or magnifier. Poor reading performanceThe following factors may result in poor reading performance:Indication of need for more magnificationNeed for eccentric viewingResistance to close working distance or Illumination controlIt 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.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.