Page 83 - The Indian Digital Edition September-October 2020

situation may then be selected with since this segment design exerts
the vocational requirement for the base up prism at the NVP. For the
lens taking prominence. Even with case in hand, the +2.00D addition
moderate powers, it must be the case would provide 1.2Δ base up at the
that bifocal wearers who have worn NVP so that, if an E-style bifocal was
a distance correction in the past have selected for this power, then the total
become accustomed to the prismatic prismatic effect at the NVP would
effect due to the distance portion of reduce to 5.2Δ base down, almost
their lenses, so that when faced with 50% less than is obtained with a 45
the selection of a first bifocal design, a downcurve segment!
sensible guide would be to choose a
design whose segment adds little or no It will be seen in the next section
prism to that exerted by the main lens. that a knowledge of the prismatic
effect at the near visual points
It can be seen from Table 3 that of bifocal lenses assumes great
shaped segments such as the flat importance in cases of anisometropia
(D-shape) and curved-top (C-shape) since no matter what the magnitude
segments hardly exert any prismatic of the prismatic effect, differences
effect at the centre of the near visual between the eyes, particularly in
zone and, in general, have proved to be the vertical meridian, may prevent
the most popular design. Not only do comfortable binocular vision.
FIGURE 1 SPLIT BIFOCAL they not exert much prismatic effect at
from the tables, may be summed the NVP, the jump encountered at the PRISM CONTROLLED
BIFOCALS
as follows: dividing line when the eyes transfer
their gaze from distance to near, is It is sometimes necessary to be
R eye: only about half that which would be able to alter the prismatic effect at
prism due to sphere = 2.8Δ base up exerted by a round segment of the the near visual point, for example, to
& 0.7Δ base out same diameter. provide base in prism for the reading
prism due to cylinder = 0.75Δ base portion only, or to balance the vertical
up & 0.75Δ base out In the case of high power prismatic effects at the near visual
prism due to segment = 2.25Δ base prescriptions, there is much to be points. This can be done by using
down said for using the prismatic effect a bifocal design whose method of
total prism at NVP = 1.3Δ base up & exerted by the segment to counteract construction allows independent
1.45Δ base out the prismatic effect of the main lens. centration of the reading portion. Such
Consider the case of a -8.00 D myope
L eye: who needs an addition of +2.00 D. a design is called a prism-controlled
prism due to sphere = 1.4Δ base From Table 1 the vertical prismatic bifocal which is simply a bifocal design
down & 0.35Δ base in effect due to the main lens is 6.4Δ base whose method of construction is such
prism due to cylinder = 1.0Δ base down. If the subject requires a large that prism may be incorporated in the
down & 0.26Δ base in reading zone then we might consider segment which is totally independent
prism due to segment = 2.25Δ base the use of a large diameter segment. of any prism which is included in the
down Inspection of Table 3 shows that if we main lens.
total prism at NVP = 4.65Δ base were to dispense a 45 mm downcurve
down & 0.61Δ base in There are several methods by
segment then the prism due to the which prism may be incorporated in
The jump exerted by each segment segment would be an additional 3.3Δ the segment only, some leading to
is 3.75Δ base down. base down! The total prismatic effect at designs which may be described as
the NVP would increase to 9.7Δ being fully prism-controlled and others
For low power prescriptions, the base down. which are described as being partially
prismatic effect at the NVP is not great prism-controlled.
and it is usually ignored provided that Selection of an E-style segment,
the magnitude is the same for each on the other hand would result in a A fully prism-controlled design is
eye. The best type of bifocal for a given reduced prismatic effect at the NVP one whose method of construction

SEPT-OCT 2020 | 79 Lens Talk