IMAGE JUMP.

Read pages (88-89 / 109) in Optical Formulas Tutorial.

When we look through the center of a glasses lens there is no induced prism. As the wearer rotates the eye away from the optical center progressively more prism comes into effect. This effect is gradual -- there is no sudden increase or decrease of prism as the eye rotates, so the glasses wearer is blissfully unaware of the changes in prism amount.

However, when we attach a second lens to the main one, as we do with traditional styles of bifocals and trifocals, this gradual increase in prism amount is interrupted. In the second diagram to the right we have a little round spherical plus lens that we are going to attach to the main distance Rx, and the result is that when the wearer is looking through the top of the added lens there is a sudden change in induced prism.

Lets look just at the little round add-on: in this case we will use an add of +2.00 for the bifocal segment. At the center of this little round lens there is no induced prism; at the edges of the little lens there is an amount of prism that depends on the power and size of the add-on lens. In this case the lens is 22 mm round so the edge is 11 mm from the center; and with a power of +2.00 Prentice's rule tells us that there is 2.2 of prism all the way around the very edge of this little lens.

Now, when we attach this little add-on lens to the distance lens we have a round area where the add starts that has a sudden change in prism as the wearer's gaze travels from the point before the segment to the point just inside the segment.

This sudden change in prism amount is called image jump. The amount of image jump for any bifocal lens depends on the power of the add and distance from the top of the segment to the optical center of the segment. The distance power and optical center placement have no effect on image jump because there is no sudden change in prism as the result of the distance portion.

In some texts and on some tests the position of the segment optical center is referred to as the pole position for the segment. In the case of the little round 22 segment the pole position is 11 mm: The optical center of the segment is 11 mm below the top of the segment. On the common segment normally called a flat top or straight-top or D segment, the pole position is determined by how much of the circle has been cut off. This bifocal design is simply a round segment that has a part missing, and this missing part is on the top of the segment as it is normally mounted in a glasses frame.

The actual position of the optical center on a straight-top segment will depend on where the top was cut off. On a particular segment you can measure the exact diameter and the exact height, divide the diameter by two, and subtract the result from the height. For example, if a segment measures 28 mm from side to side, then the optical center would be in the middle, or 28/2 = 14 mm from any of the round edges. Then, if the actual height is 18.5, the pole position is 18.5 - 14 = 4.5 mm from the top of the segment.

segment style		pole position
executive		0 mm
PAL progressive addition		0 mm
FT/ST 22/25/28/35		4-5 mm  depending on height and width
FT/ST 40/45		0 mm
round 22, etc.		11 mm  (or 1/2 of diameter)
Ultex A/AX  based on round 38		19 mm

Read page (88 /109) in the textbook and do the exercises on pages (88-89, 110).