CLSA Eye Witness Spring 2011 - (Page 27)

–4.75 D= 1 F (in meters) @ 90 +5.00D = 1 x 1000 = 200mm (Multiply x 1000 5 (0.2m) to convert to mm) –6.50 @ 180 200mm – 12mm = 188mm (Vertex Distance) Figure 4 +7.62 @ 90 (Divide by 1000 to convert back to m) 188 = 0.188m 1000 +5.25 @ 180 1 = +5.32D 0.188 Figure 5 Figure 3 diopters). There are several online resources that will calculate vertex distance based on your inputs, and most contact lens manufacturers provide vertex correction tables within their fitting guides. The process of vertexing the spectacle or manifest refraction into a contact lens prescription is not complicated. For spherical powered soft contact lenses above a power of ±4.00D, simply using a conversion table can help to determine the correct power. If a conversion table is not available, a rule of thumb that can be used is to square the spectacle plane power and divide by 100 to give a good approximate amount to vertex. For example, a 5D lens needs a 0.25D correction (52/100). A 10 diopter lens needs 1.00D of correction. This rule of thumb works well for moderate minus and plus powers, but may underestimate the amount of vertex correction needed for plus powers above +10.00D. Another way to calculate the vertex prescription would be to use the focal length (Figure 3). The focal length of a lens is the inverse of the diopter power and specifies the distance from its optical center to the point where the rays of light being imaged come into focus. Taking the straight inverse of the diopter power will give the focal length in meters. For instance, on a +5.00D lens, we know the focal length is 1/5.00 or 0.2 meters. Typically, focal lengths are specified in centimeters or millimeters, so this would be 20cm or 200mm. Vertexing a +5.00D spectacle plane power at a 12mm distance means the corrected vertex distance is 200mm – 12mm which is equal to 188mm or .188M. The new focal length would now be 1/.188 or in diopters, +5.32. For a – 5.00D lens, considerEyEWitnEss spring 2011 | ing the same conversion, the new focal length would be .212M, or in diopters, – 4.72 lens. One aspect of vertex correction that is often overlooked is in the calculation of sphero-cylindrical, or toric contact lens prescriptions. In order to vertex a toric spectacle prescription, the two principal meridians, the meridian of the shortest and the longest radii of the lens, will need to be vertexed. You can best understand the concept of vertexing both meridians by using the optical cross. The optical cross is a graphic method for specifying the powers in each meridian. For example, if there is a contact lens prescription with compound astigmatism, the prescription should be placed onto an optical cross to determine how much power in each of the meridians needs to be vertexed. To illustrate, if a patient presents to you with a compound myopic astigmatic prescription of –5.00 –2.00 x 180, the spherical power will vertex to a –4.75 but the total combined power in the vertical meridian (–5.00 (+) –2.00 = –7.00) will vertex to a –6.50 (Figure 4). The refractive power will need to be decreased resulting in a contact lens prescription of –4.75 –1.75 x 180. (Note: the cylinder power is now –1.75 rather than –2.00 because the amount of change in power from one meridian to the next is 1.75. The travel of –4.75D to –6.50D is 1.75D) If a patient presents with a compound hyperopic astigmatic prescription of +7.00 –2.00 x 090, the spherical power will vertex to +7.62 but the total power (+5.00) will vertex to +5.25 (Figure 5). The refractive power will need to be increased resulting in a contact lens prescription of +7.62 –2.37 x 90. (As noted previously, the cylinder power changes be| w w w. c l s a . i n f o c o n ta c t l e n s s o c i e t y o f a m e r i c a 27 http://www.clsa.info http://WWW.CLSA.INFO

Table of Contents for the Digital Edition of CLSA Eye Witness Spring 2011

CLSA Eye Witness Spring 2011

CLSA Eye Witness Spring 2011 - (Page Cover1)
CLSA Eye Witness Spring 2011 - (Page Cover2)
CLSA Eye Witness Spring 2011 - (Page 1)
CLSA Eye Witness Spring 2011 - (Page 2)
CLSA Eye Witness Spring 2011 - (Page 3)
CLSA Eye Witness Spring 2011 - (Page 4)
CLSA Eye Witness Spring 2011 - (Page 5)
CLSA Eye Witness Spring 2011 - (Page 6)
CLSA Eye Witness Spring 2011 - (Page 7)
CLSA Eye Witness Spring 2011 - (Page 8)
CLSA Eye Witness Spring 2011 - (Page 9)
CLSA Eye Witness Spring 2011 - (Page 10)
CLSA Eye Witness Spring 2011 - (Page 11)
CLSA Eye Witness Spring 2011 - (Page 12)
CLSA Eye Witness Spring 2011 - (Page 13)
CLSA Eye Witness Spring 2011 - (Page 14)
CLSA Eye Witness Spring 2011 - (Page 15)
CLSA Eye Witness Spring 2011 - (Page 16)
CLSA Eye Witness Spring 2011 - (Page 17)
CLSA Eye Witness Spring 2011 - (Page 18)
CLSA Eye Witness Spring 2011 - (Page 19)
CLSA Eye Witness Spring 2011 - (Page 20)
CLSA Eye Witness Spring 2011 - (Page 21)
CLSA Eye Witness Spring 2011 - (Page 22)
CLSA Eye Witness Spring 2011 - (Page 23)
CLSA Eye Witness Spring 2011 - (Page 24)
CLSA Eye Witness Spring 2011 - (Page 25)
CLSA Eye Witness Spring 2011 - (Page 26)
CLSA Eye Witness Spring 2011 - (Page 27)
CLSA Eye Witness Spring 2011 - (Page 28)
CLSA Eye Witness Spring 2011 - (Page 29)
CLSA Eye Witness Spring 2011 - (Page 30)
CLSA Eye Witness Spring 2011 - (Page 31)
CLSA Eye Witness Spring 2011 - (Page 32)
CLSA Eye Witness Spring 2011 - (Page 33)
CLSA Eye Witness Spring 2011 - (Page 34)
CLSA Eye Witness Spring 2011 - (Page 35)
CLSA Eye Witness Spring 2011 - (Page 36)
CLSA Eye Witness Spring 2011 - (Page Cover3)
CLSA Eye Witness Spring 2011 - (Page Cover4)
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