How to Determine Your Custom Magnification Needs

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ErgoPractice News – September 2013

How do we know which magnification power we need?

For most hands-on work, our eyes are the main sensory system for detecting critical information which is needed to perform procedures. Although the human eye is a marvelous optical instrument, its visual acuity is inadequate for many complex clinical procedures.

Normal visual acuity is commonly referred to as 20/20 vision, the metric equivalent of which is 6/6 vision. Under an optimum illumination condition, a human eye with 20/20 vision is able to recognize two points or lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart at 20 feet or 6 meters away). The vision of 20/40 is considered half as good as normal performance. As we get older our vision will naturally deteriorate and become far less than 20/20 vision.

Our visual acuity is significantly affected by illumination. Under bright clinical illumination, which is brighter than optimum illumination for best visual acuity, our iris naturally closes tighter making our pupil smaller. So, the effective area of our eye lenses will become smaller than the optimum size. Most clinicians can recognize lines spaced about 100 µm to 130 µm at 35 cm (about 14 inches) away. If we want to see beyond the vision of our unaided eyes, we have to use magnification devices such as loupes or microscopes.

If we use the average line space of 115 µm [= (100 + 130)/2] which can be recognized by most clinicians, the minimum magnification power (MP) as a function of working distance (WD in cm) and line space (LS in µm ) to be recognized will be given by MP = [(115/35) x WD]/LS. Using this equation, we can easily find the magnification power needed to recognize lines spaced LS µm. This equation shows that longer working distances require larger magnification powers. So, in general, taller clinicians need higher magnification power than shorter clinicians to achieve the same result.

Let’s consider several examples. If WD = 40 cm (about 16 inches) and LS = 50 µm, MP = 2.6x. If WD = 50 cm (about 20 inches) and LS = 50 µm, MP = 3.3x. If WD = 35 cm (about 14 inches) and LS = 25 µm, MP = 4.6x. If WD = 50 cm (about 20 inches) and LS = 25 µm, MP = 6.6x.

From these examples, you can see that you cannot always use the same magnification loupes which your friends use. Your friends may swear there is no need for magnification higher than 2.5x or 3.5x or 4.5x, et cetera, for a certain procedure. But if your working distance is longer than your friends you will need a higher power magnification than what your friend uses. Using higher than the minimum magnification will significantly improve your motor skills and work performance. Note, however, adapting to higher power magnifications may require a longer learning period.

To learn more about how this concept may be impacting you and your work contact your local SurgiTel representative at Find a Rep or at 1-800-959-0153.