Page 154 - The Indian Optician Digital Edition Jan-Feb 2020

FIGURE 6 - MINKWITZ ASTIGMATISM correction of almost +0.25 DC x 90. point, L is the dioptral distance of
FOR MODERN PROGRESSIVE DESIGNS the near point from the lens and A is
In order to fully correct the ocular any prescribed near addition. In the
astigmatism, the astigmatism in the example illustrated in Figure 5, d =
pencil arriving at the eye should be 15mm, L = -3.0 D and A = 0, since no
+2.41 DC x 90. Assuming that the near addition has been given.
eye can cope with the minimum Thus the increase required for the
accommodative demand which arises near vision cylinder is -2 x 15(-3.0) /10
in the vertical meridian, the vergence = 9%.
arriving in the horizontal meridian
should be (+2.06 + +2.41) = +4.47 9% of +2.00 D is +0.18 D which is
DC x 90. This represents a vergence very close to the result obtained by
leaving the lens along the horizontal ray tracing.
meridian of +4.19 D, so the power of
the lens which fully corrects the ocular Clearly, a change in cylinder
astigmatism for near vision at -1/3 power for near is usually only
metre is +5.00/+2.19 x 90. It can be necessary for medium to high power
seen that the distance vision cylinder cylinders. Even so, the changes
must be increased to fully correct the required are not dissimilar to
ocular astigmatism in near vision. those required for a compensated
prescription, one whose verification
The difference between the power differs from the ordered power
cylinder power for distance and near owing to changes in the as-worn
vision can be found from the simple pantoscopic and/or face form angles.
approximate relationship 5. Since free form manufacturing
methods enable surface powers to be
Increase DV cylinder by -2d(L + A)/10 %
produced in one-hundredth dioptre
cylinder which corrects an eye for where d is the distance from intervals when necessary, it is easily
distance vision does not correct the the lens to the eye’s first principal possible to adjust the near vision
eye for near vision. Owing to the
forward position of the spectacle lens
in front of the eye, the astigmatism
in the pencil leaving the lens is not
the same as the astigmatism in the
pencil reckoned at the eye’s first
principal point. For example, in the
case of the spectacle prescription
+5.00/+2.00 x 90 mounted 15mm in
front of the eye’s first principal point,
where the astigmatism measured in
the spectacle plane is +2.00 DC x 90,
the astigmatism in the pencil arriving
at the eye is +2.41 DC x 90, which
represents the actual astigmatism
possessed by the eye (Figure 5).
Tracing a paraxial pencil from the
near point at 33.33cm in front of the
lens (which is assumed to be thin),
the astigmatism in the pencil arriving FIGURE 7 - USE OF A COMBINED ATOROIDAL/PROGRESSIVE SURFACE TO EQUALISE EDGE
at the eye is +2.20 DC x 90, an under THICKNESS IN A MODERN FRAME

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