Why image jump in bifocal glasses ?
When looking from distance portion to the near portion of a bifocal lens, the sudden change in the prismatic effect because of the introduction of the base down prism by the segment causes the world to “jump”.
This displacement of the patient’s visual world can be very disturbing and often dangerous particularly with steps and kerbs.
It has both vertical and horizontal element. Jump occurs all around the edge of the bifocal segment, although it is at the top part which is most frequently noticed.
The base of the prism lies at the optical centre of the segment Os. Consider the eye viewing through the distance portion. As the gaze is lowered, the eye encounters an ever increasing prismatic effect as it rotates away from the optical centre of the distance portion.
When the eye enters the near portion, it suddenly encounters the base down prism exerted by the segment at the segment top.
The effect is twofold. Firstly, object that lies in the direction of AT, appears to lie in the direction BT. Apparently, they have jumped to a new position.
Secondly, light from the angular zone BTA, around the edge of the segment, cannot enter the eye.
The segment dividing line causes an annular scotoma within which the object is completely hidden until the wearer moves his head to shift the zone in which jump occurs.
The amount of jump is simply the magnitude of the prismatic effect exerted by the segment at its dividing line that is the product of the distance from the segment top to the segment optical centre, in centimeters and the power of the reading addition.
For a round bifocal the distance from the segment top to the optical centre of the segment is simply the segment radius and therefore, for circular segments:
Jump = Segment radius in cms × reading addition.
But for shaped bifocal like B, C or D segments, the jump will be less as it is:
Jump = Reading addition × Distance to the centre of the circle of which the segment is part from the top edge.
For most of the E-style bifocal, with their distance and near optical centres coinciding at the dividing line, the jump is purely horizontal at points away from the common centre.
Clearly the jump is completely independent of the power of the main lens and the position of the distance optical centre.
Jump increases as the distance from the segment top to the segment optical centre increases, i.e. in case of round segments as the segment diameter increases.
For example, if the reading addition is + 2.00D, the jump exerted by a 24 mm segment is 2.4 Δ BD in case of round segment bifocal.
But in case of D segment where segment size is, say, 28 x 19 and the segment centre lies just 5 mm below the segment top, the jump will be only 1.00Δ BD. Reduced jump is probably the one reason why shaped bifocal have proved to be so popular.
To eliminate the jump effect in a bifocal lens, it is necessary to work the segment in such a fashion that its optical centre coincides with the segment top.
OPTOMETRY-SHARP VISION
Optometrist