Bifocal, Trifocal, and Progressive Lenses: How Multifocal Types Differ

Lined multifocals, meaning bifocals and trifocals, divide the lens into separate power areas, and an image can jump as the eye crosses a line. Progressives build power smoothly with no line, but the sides of their narrow central corridor are distorted. One published review adds that multifocal zones are smaller than single vision.

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Why multifocals exist

With presbyopia, a wearer needs extra plus power for close work on top of the distance prescription. A multifocal puts that add in part of the lens (see near point and reading add). To estimate how much add a working distance calls for, try the accommodation calculator.

Lined multifocals: bifocal and trifocal zones

Bifocals and trifocals hold areas of clearly different power, and you can see the borders between them. A bifocal has a distance area and a near segment.

Common segment styles

• Flat top: a segment with a straight top edge.
• Round: a circular segment.
• Trifocal: also lined, with visible borders between its zones.
• Progressive: no visible segment at all.

Image jump at the line

When the eye drops across a segment edge, it meets the segment’s prism suddenly, and things just under the line seem to hop up. Segment style changes how much; the math is on the image jump page.

Flat-top vs round-top segments

How much a segment jumps comes down to where its optical center sits relative to the top line, together with the add:

• Round: the optical center is at the circle’s center, a long way under the top edge, so the jump is larger.
• Flat top: the optical center sits close to the straight edge, so the jump is small.
• Optical center on the line (as in an executive style): no jump.

The distance prescription doesn’t change the jump; that comes from the segment and its add alone. Worked numbers are on the image jump page.

Progressives: gradual power and the corridor tradeoff

A progressive lens has distance, intermediate and near areas too, but none are visible. Power climbs smoothly down a corridor toward the near area. The price is unwanted cylinder, also called surface astigmatism, in the periphery. Clear intermediate vision depends on the eye tracking down that narrow corridor.

Zone size vs single vision

A single vision lens uses its whole area for one distance. A review of presbyopia treatment options (PMC8163965) says multifocal spectacles have restricted optical zones by comparison, which can matter for tasks like driving or desk work.

Measuring for multifocals

Both lined and progressive lenses depend on accurate fitting measurements. See segment height and how to measure PD.

Original practice set

Original problems.

1. A wearer wants one lens for distance, a computer screen and reading, with no visible line. Which type fits?

   Show answer

   A progressive lens.

2. Distance Rx +1.00 D sph, add +2.00. What power does the near area measure?

   Show answer

   +1.00 + 2.00 = +3.00 D sph.

3. A new progressive wearer says the lower outer edges look blurry. Most likely reason?

   Show answer

   Unwanted cylinder toward the lens edges, away from the corridor.

4. Where does image jump happen?

   Show answer

   At the top line of the segment, as the eye drops into it.

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