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Bifocal Lenses 119

Putting up the values:

 5   2× 4      2 [4 +1]  
 
 
TCA = 0.8    ( 06)    + ( 0.6)   
 58    58     36  
= (0.8 × 0.086) – {(0.6) (0.38)} + {(– 0.8) (0.278)}
= (0.0688) – (– 0.0828) + (– 0.167)
= – 0.02Δ
This is negligible.
Once again the flat top fused bifocal demonstrates its superiority over
round segment, since the distance Ys is negligible in case of most shaped
bifocal.

MAGNIFICATION
Another consequence of the reading addition is the small magnification
difference between the distance and the reading portion. Patient may be
aware of this, although it is unlikely to cause serious problems as the size
change occurs in both eyes. This may give rise to change in perceived
distance rather than aniseikonia.
We have seen that a bifocal free of jump (executive type) will induce a
certain amount of prismatic effect at the reading level; while the bifocal
that induces no prismatic effect at the reading level (straight top shaped
bifocal) is subject to a certain amount of jump. For the long term spectacle
wearer who is already accustomed to a certain amount of prismatic effect,
this will change when bifocal lenses are first worn. Adaptation to this new
type of prismatic effect should not be difficult. However, jump manifests
itself each time the wearer directs his line of vision downwards across the
dividing line. Even if the patient will learn to ignore the displacement of
his environment, it will never cease to occur. Consequently, we should
expect jump to be a greater problem for the wearer than the segment induced
prismatic effect.

PRISM CONTROLLED BIFOCALS
Bifocal designs that permit control of the prismatic effects in the near portion
by the incorporation of prism in the segment are known as prism controlled
bifocals. Prismatic effect at any point in the near portion depends upon the
power of the main lens and the power of the segment lens, and the positions
of their individual optical centres in relation to the visual point at which
we wish to find the prismatic effect. Sometimes it is necessary to alter the
prismatic effect at the NVP, for example to balance the vertical prismatic
effects at the NVP or to provide horizontal prism correction in the reading
portion only. This can be achieved by using a bifocal design for which the
method of construction allows independent centration of the reading