Page 104 - The Indian Optician Digital Edition November-December 2021

astigmatism in the near portion FIGURE 2 -
of a single vision lens designed TSCHERNING’S
for distance vision will benefit ELLIPSES FOR
DISTANCE AND
the wearer. NEAR VISION
It is well known that the
best form of a spectacle lens
for a given power intended
for distance vision is different
from the best form for near
vision. For example, in the case
of a +4.00 D lens made in CR
39 material and intended for
distance use which employs
purely spherical surfaces, the
point focal form (the form free
from aberrational astigmatism)
for distance vision, requires
a front curve in the region
of +9.62 D (Figure 1). The off-
axis performance of the lens
when used for distance and
near vision is shown alongside
the trace of the lens surfaces
in the form of field diagrams.
A field diagram plots either
the tangential and sagittal
oblique vertex sphere powers
of the design, or, as in the
case depicted here, the
errors in terms of the oblique FIGURE 3 - OFF-AXIS
astigmatism exhibited by the PERFORMANCE OF POINT
lens and the mean oblique FOCAL +4.00 D LENS WITH
SPHERICAL SURFACES
power error as the eye rotates DESIGNED FOR NEAR VISION
away from the optical centre
of the lens. The ocular rotation The performance of this lens when used for near vision at
is plotted on the y-axis and -33.33cm is shown in the field diagram of Figure 1(b). The +9.62
the power error on the x-axis. D bending is too steep for near vision, the astigmatic error for
When viewing a distant object a 40º rotation of the eye reaching -0.35 D. The error in mean
as shown in Figure 1(a), the oblique image vergence (MOIV) which is the power error for
aberrational astigmatism, or near vision, however, is now zero.
the oblique astigmatic error The point focal form for this working distance requires a
(OAE), which is represented by front curve of +7.50 D. These conclusions can be reached by
the red line, remains zero for inspection of Tscherning’s Ellipses drawn for distance and near
the entire field of vision as the vision as shown in Figure 2 which compares the distance and
eye rotates out to 40º from the near vision ellipses drawn for point focal lenses made with
optical axis. The power error spherical surfaces with the parameters shown in the Figure. It
(mean oblique error - MOE), can be seen that over the Ostwalt range of the ellipse, the point
represented by the blue line, focal near vision form is about 2.00 D shallower than the form
reaches nearly -0.50 D at 40º. employed for distance vision.

| NOV-DEC 2021 | 100 LENS TALK