Image Jump vs Displacement
Learn to separate image jump from image displacement and calculate the prism effects that board exams commonly test.
What causes image jump in a bifocal lens?
Image Jump and Displacement in 30 Seconds
Image jump is the abrupt prismatic shift seen when the eye enters a lined bifocal segment. Image displacement is the prism already present from the distance lens at the point where the eye crosses into the segment. Board questions test whether you can separate the sudden segment-related effect from the prism already present at that point.
Two Related but Different Effects
Image displacement is the prismatic shift induced by the distance portion of the lens, calculated with Prentice's rule at the point where the visual axis crosses into the segment. Image jump is the abrupt additional prismatic shift caused specifically by the segment's own optical center being located somewhere other than at the top of the segment. Some sources describe a combined total displacement at the reading point, which sums the distance-lens component and the segment jump together, so be clear about which quantity a question is asking for. Image jump is sudden and discontinuous at the segment line. Image displacement from the distance lens is the effect already present through the distance portion of the lens, before the segment is even reached.
Which Bifocal Designs Minimize Each Effect
- Round segments produce more image jump than flat-top segments of similar add power because their effective optical center sits farther below the segment top.
- Flat-top segments have less image jump than round segments because their optical center sits closer to the segment top, commonly about 5 mm below it.
- Executive style segments produce minimal to no image jump because the segment optical center is essentially at the segment top.
- Progressive addition lenses avoid image jump entirely because they have no distinct segment line, though they introduce unwanted peripheral astigmatism from the continuous change in surface power across the lens.
4 Facts Exams Always Ask
- Image jump = segment add power x distance in centimeters from the segment optical center to the segment top.
- Image jump depends only on the add power and segment optical center position. It does not depend on the distance prescription.
- Patients adapting to a new bifocal design may report a visible jump sensation specifically when looking down through the segment line.
- Flat-top segments produce less image jump than round segments, while executive segments produce minimal image jump.
The Classic Trap
The most common mistake is treating image jump and image displacement as the same phenomenon. They are not. Calculate the distance-lens prism at the relevant point, then calculate the additional prism introduced by the segment. The sudden change at the segment line is image jump. A second common error is assuming flat-top segments produce the most jump. The opposite is true: round segments produce the most jump, flat-top segments less, and executive segments minimal to none.
Clinical Pearl
Think "jump equals add times the distance from the segment optical center to the top." A round segment may have the same add power as a flat-top segment, yet produce more jump because its optical center lies farther below the segment top.
Checkpoint 1
In a bifocal spectacle wearer, what determines the amount of image jump at the top of the near segment?
Checkpoint 2
An executive bifocal generally causes less image jump than a round-segment bifocal because:
A patient receives a +2.50 D add in new round-segment bifocals and reports that text appears to shift abruptly when gaze drops across the segment line. The segment optical center lies 10 mm below the segment top.
What is the image jump in prism diopters?
Sources & Reviewer Info
References
- American Academy of Ophthalmology. "Bifocal Add: Image Displacement vs Jump." Basic Optics, Chapter 24.
- Radhakrishnan A, Dorronsoro C, Sawides L, Marcos S. "Perceived image quality with simulated segmented bifocal corrections." Biomedical Optics Express. 2016.
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