Image Jump

Since the reading segment may be thought of as a small plus lens which has been added to a larger lens, we may also think of it as a set of prisms placed base-to-base. If we consider only those prisms which are vertically oriented, it will look something like the illustration below. 

This example shows a 22 mm round bifocal segment where the distance from its center to the periphery is 11 mm. The instant the eye enters the segment area (point A above) there is some base down prism induced.

According to Prentice’s Rule, the amount of prism can be calculated as:

Δ = F × dec (mm) / 10

Δ = 2.00 × 11 / 10 = 2.2Δ base down

We know that light is always bent toward the base of a prism and the image is displaced toward the apex. In this case, 2.2 diopters of prism are induced, displacing the image upward, toward the apex. This is known as image jump since the image appears to “jump” upward the moment the eye enters the segment area. The greater the power of the add and/or the farther the optical center of the segment is from the transition line, the greater the amount of image jump. Since excessive image jump is not desirable, one of the objectives in the development and improvement of multifocal designs has been to minimize image jump.

Image jump in a lined bifocal

ABO/NCLE Exams and Image Jump

The American Board of Opticianry (ABO) and the National Contact Lens Examiners (NCLE) exams often include questions on multifocal lens designs and image jump. These exams test knowledge of optical principles, lens types, and their effects on vision. Understanding image jump is crucial for those studying for these certifications, as it directly impacts the wearer’s comfort and visual experience.

Common Questions and Doubts About Image Jump

1. What causes image jump in bifocal lenses?
   Image jump is caused by the prismatic effect of the reading segment, which displaces the image upward as the eye moves into the segment area.
2. How can image jump be minimized?
   To minimize image jump, opticians may design lenses with a lower add power or position the segment closer to the transition line. This reduces the prismatic effect and the amount of image displacement.
3. What impact does image jump have on the wearer?
   Excessive image jump can cause discomfort and disorientation, as the sudden change in image position can be visually jarring. This is why it’s important to carefully design multifocal lenses to minimize this effect.

Key Pointers

• The degree of image jump increases with the power of the add.
• The position of the segment in relation to the transition line also affects image jump.
• Opticians must balance the need for reading correction with the comfort and visual stability of the wearer.

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