Stand Magnifier Technical Specifications
Douglas R. Williams, O.D., F.A.A.O.
Diplomate, Low Vision, AAO
Private Practice
Huntington Beach, CA
2007
TABLE OF CONTENTS
Stand Magnifier Technical Specifications
Using the Tables
Agfa Stand Magnifier
Bausch & Lomb Stand Magnifiers
COIL Stand Magnifiers
Eschenbach Stand Magnifiers
Optelec PowerMag™ Stand Magnifiers
Schweizer Stand Magnifiers
STAND MAGNIFIER TECHNICAL SPECIFICATIONS
Notes Regarding Nominal or Claimed Ratings:
1. The nominal dioptric power is the equivalent power, rounded up or down. Some manufacturers/distributors will
choose to label to one tenth or a diopter while others will choose to label to a 4 diopter multiple.
2. The nominal lens diameter may be rounded up or down.
3. Image location is labeled to a tenth of a centimeter and is measured with respect to the top surface of the magnifier
lens.
4. The image location is the distance from the top of the magnifier lens to the enlarged virtual image. The image
location cannot be altered because it is dependent on the lens position in the magnifier which cannot be changed
(unless the magnifier is focusable).
5. The enlargement ratio is the transverse magnification, expressing the ratio between the image height and object
height
Clinical Use of the Image Location and Enlargement Ratio:
1. Knowing the image location and the eye position allows clinicians to determine whether the patient will see the
image clearly and in good focus.
(a) For a given eye position, the total eye-to-image distance can be calculated, and the clinician can decide
whether the patient’s addition or accommodation is appropriate to give a satisfactory focus. Commonly,
patients make compromises by changing their eye-to-lens distance (z) to vary the magnification effect, the
field of view, and clarity of focus. A ± 1.00D defocus tolerance is generally acceptable in most cases.
(b) For a given addition, the clinician can determine the eye-to-lens distance for a perfect focus. A +2.50D add
requires an eye-to-image distance of 40cms. [100/2.5] for ideal focus. If the image is 28cms behind the lens,
12 cms is the eye-to-lens distance for perfect focus.
2. Knowing both the Enlargement Ratio and eye-to-image distance allows the clinician to determine the
total magnification effect. As an example, consider the Model 9527 PowerMag™ that gives an
Enlargement Ratio of 6.9X with the image located 28.3cms behind the lens.
(a) For an eye-lens position of 2.5cms (z=2.5cms) from the top of the magnifier; the eye-image
distance is 30.8cms; Equivalent Viewing Distance is 30.8/6.9 = 4.5cms. At the eye-to-image distance of
30.8cms., the accommodative demand is 3.24D and the Equivalent Viewing Power is 3.24 x 6.9 = +22.4D.
(b) Take an addition of +2.50D and assume that the patient’s position provides perfect focus. So, eye-to-image
distance = 40cms; eye-to-lens distance = 11.7cms (40-28.3 = 11.7); Equivalent Viewing Distance = Eye-to
Image distance/ER = 40/6.9 = 5.8cms; Equivalent Viewing Power = Add (or Accommodative Demand X ER
= 2.50 x 6.9 = +17.3D
USING THE TABLES
Determining the Required Equivalent Viewing Distance (EVD) or Equivalent Viewing Power (EVP)
1. The patient’s goal acuity should be set to meet the patient’s needs. For example we may state that the goal is to have
the patient read .8M print (6.3 points).
2. Next measure the near acuity using a chart which can be placed at a working distance at which the target will be in
satisfactory focus for the patient. For example, if the chart is held at 40cms away, have the patient wear a +2.50D addition
and take the acuity. Let’s say the acuity taken at 40cms (the focal length of the addition) is .4/3.2M.
3. Determine the required Equivalent Viewing Distance (EVD). The required EVD is calculated by using proportions.
Because in our example, the patient reads 3.2M print at 40cms, then in order to read .8M print, (4 times smaller), the
viewing distance must be reduced by a factor of 4.
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