Page 90 - The Indian Optician Digital Edition January-February 2022

base curve), the asphericity of
FIGURE 7. CONSTRUCTION OF THE
+4.00 D EYEZEN START DESIGN the surface must be altered.
It is found that changing
the hyperboloid used in the
distance zone to a prolate
ellipsoid with a p-value of +0.50,
the oblique astigmatic error
can be eliminated out to 30º
where there is a residual mean
oblique error of just +0.10 D.
This form would be considered
to be point focal for near vision.
The different shapes of
the two surfaces required for
distance and near vision are
shown in Figure 9. and the field
diagrams for these two forms
when used for distance and
near vision are shown in Figures
FIGURE 8 - OFF-AXIS PERFORMANCE 10(a) and (b) respectively.
OF ASPHERIC -4.00 D POINT FOCAL
LENS DESIGNED FOR DISTANCE VISION The difference in z-heights
for the concave surface of the
-4.00 D lens is shown in Table
3 for 2 mm intervals along the
180 meridian. All z-values are
shown in millimetres.
Even with the change in
shape for this surface the
difference at 20mm from the
vertex is only 54 micrometres.

Essilor describes this
optimisation in both the
distance and near portions as
Dualoptim and rightly claims
out to 35º after which a small amount of oblique astigmatic error that this is the first lens design
is exhibited by the lens. Oblique astigmatism in a minus lens is to incorporate this feature.
normally corrected out to 30º so this design would be considered In common with the whole
to satisfy the criterion for a point focal lens. The mean oblique Eyezen™ range the Eyezen™
error at 30º is just +0.15 D. Start series also incorporates
Essilor’s blue-violet filter to filter
When this form is used for near vision at one-third metre the the waveband below 455nm,
off-axis performance is shown in the field diagram depicted in further relieving the eye of
Figure 8(b). For a 30º rotation of the eye, the astigmatism has computer vision syndrome.
increased to 0.26 D and the mean oblique error to +0.36 D, errors
which might exacerbate fatigue with long periods of close work, The description of the use
as shown by Rosenfield et al . of simple conicoids for the
5
concave aspherical surface
In order to minimise the aberrations in near vision without which is employed in the text
altering the form used in distance vision (which has a +2.00 D has been given to provide the

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