Image Jump in Bifocals: Cause and Calculation

Image jump is the sudden shift a bifocal wearer sees as the eye crosses the segment line. It comes from the segment’s own prism at its top edge. By Prentice’s rule it equals the add times the distance in centimeters from the segment top to the segment’s optical center. A segment centered at its top has none.

Last checked Nov 2, 2026. Confirm fees and dates with the certifying body. optical.education is independent and is not affiliated with or endorsed by the American Board of Opticianry and National Contact Lens Examiners (ABO-NCLE). Disclaimer.

What wearers notice

As the eyes drop from the distance part into the segment, an object near the line can seem to leap up. The prism changes suddenly at the edge of the segment instead of building gradually.

Calculation

Think of the segment as a small lens with its own optical center. At the top line, its prism follows Prentice’s rule: image jump (Δ) = distance from segment top to segment OC (cm) × add (D). Check answers in the Prentice’s rule calculator.

24 mm round segment, +1.75 add: the OC is 12 mm below the top, so 1.2 × 1.75 = 2.1Δ.
Flat-top whose OC is 5 mm below the line, +2.50 add: 0.5 × 2.50 = 1.25Δ.
Segment with its OC on the line: 0Δ.

Segment styles compared

Round: OC in the middle of the round, far from the line, so the most jump.
Flat-top: OC close to the line, so jump is small.
Executive (full-width line): OC on the line, essentially no jump.

Image jump vs image displacement

Image jump happens at the line. Image displacement is the total prism at the reading position, from the distance lens and the segment combined. The distance prescription changes displacement, not jump. For unequal powers between the eyes, see slab-off and vertical imbalance.

Practice set

Original problems.

22 mm round segment, +2.25 add. Image jump?
Show answer
OC 11 mm below the top: 1.1 × 2.25 = 2.475Δ (about 2.5Δ).
28 mm round segment, +1.50 add. Image jump?
Show answer
OC 14 mm below the top: 1.4 × 1.50 = 2.1Δ.
A flat-top with OC 4 mm below the line, +3.00 add. Image jump?
Show answer
0.4 × 3.00 = 1.2Δ.
Same add, two patients: one −4.00, one +4.00 distance. Whose image jump is larger?
Show answer
Neither. Jump depends on the segment and add, not the distance power.
Which style removes image jump?
Show answer
A segment with its optical center on the top line, such as an executive style.

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Standard text used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 19 (image jump, segment optical center). The Iowa optics review is linked under sources.

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Frequently asked questions

What causes image jump in bifocals?

At the segment line, the eye suddenly meets the prism of the segment itself. Objects just below the line appear to leap upward as the eye moves down into the segment.

Sources: University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), pp. 39, 79–80 (opens in a new tab) ; Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 19, p. 442 (image jump) (opens in a new tab)

How do you calculate image jump?

Use Prentice’s rule with the add power and the distance from the top of the segment to the segment optical center, in centimeters: Δ = c × add.

Sources: Brooks & Borish, System for Ophthalmic Dispensing, 3rd ed. (textbook), ch. 19, p. 442 (image jump) (opens in a new tab) ; University of Iowa EyeRounds — Wilkinson & Shahid, Optics Review (PDF), pp. 39, 79–80 (opens in a new tab)

Which bifocal segment has the least image jump?

One whose optical center sits at its top line, as in an executive style, gives essentially none. Flat-tops keep the segment center close to the line and give little. Round segments, centered in the middle of the round, give the most.

Does the distance prescription affect image jump?

No. Image jump depends on the segment style and the add. The distance power does affect total image displacement at the reading level, which is a separate effect.

Official sources

Last reviewed Nov 2, 2026. Requirements, fees, and dates change — always confirm with the official source before you register or apply. ABO-NCLE and IJCAHPO are the only authorities for their own exam facts, so those facts rest on a single source.