7.2 Neutralising Reading Adds: Bifocal, Trifocal, Progressive & Determining the Near Add
Key Takeaways
- Add power is read from the front surface of the lens by subtracting the distance sphere from the segment sphere.
- A trifocal intermediate segment is conventionally half the power of the near add.
- Progressive lenses are read at the near reference circle located by the engravings, not anywhere in the corridor.
- A useful starting add is the age-expected amount, then refine so half the remaining accommodative amplitude is left in reserve.
- The add must suit the patient's actual working distance, so the occupational task governs the final number.
Reading the add from the front surface
The distance portion of a spectacle lens is read from the back surface because back vertex power is what the prescription specifies. The add is read from the front surface. The reason is optical: the segment is a separate curve added to the front of the lens, and reading it from the front isolates the segment's contribution without the base curve of the distance portion contaminating the measurement.
Procedure:
- Neutralise the distance portion from the back surface and record the sphere.
- Reverse the lens so the front surface faces the lens stop.
- Read the distance sphere again from the front — this is the front-surface distance reference.
- Move the lens so the segment is over the aperture and read the segment sphere from the front.
- Add = segment sphere − distance sphere, both read from the front surface.
Worked example. From the front surface, the distance portion reads +1.75 and the segment reads +4.25. The add is 4.25 − 1.75 = +2.50 D.
A frequent error is subtracting the front-surface segment reading from the back-surface distance reading. In a lens with meaningful power those two references differ and the add comes out wrong.
Trifocals
A trifocal has three zones: distance, intermediate and near. By convention the intermediate segment is 50% of the near add, although 40% and 70% designs exist. Read all three zones from the front surface and record all three.
Worked example. Front-surface distance −2.00, intermediate −0.75, near +0.50. Intermediate add = 1.25 D; near add = 2.50 D — a conventional 50% intermediate.
Segment styles and heights
| Segment | Description | Typical use |
|---|---|---|
| Flat-top / D-seg (28 mm, 35 mm) | Straight top edge | The common bifocal; minimal image jump |
| Round seg (22 mm, 25 mm) | Circular | Cosmetically discreet; more image jump |
| Executive / Franklin | Full-width segment across the lens | Widest near field; heavy; used where a large near field is essential |
| Curved-top / blended | Softened transition | Cosmetic; some blur at the blend |
| Occupational (double-D, E-D trifocal) | Extra segment at the top | Mechanics, electricians, librarians working overhead |
Segment height is conventionally set at or just below the lower lid margin for a general-purpose bifocal, raised for occupational near work and lowered for patients who need more unobstructed distance field, such as habitual stair users. Image jump — the sudden prismatic displacement as the eye crosses the segment top — is smallest when the segment's optical centre is close to its top edge, which is why flat-top designs jump less than round segments.
Progressive addition lenses
A progressive lens has no visible segment line; power increases continuously down a corridor. Reading it requires finding the manufacturer's markings.
- Locate the two permanent engravings, faint circles or symbols set 34 mm apart on the horizontal reference line (17 mm each side of the fitting cross or prism reference point), one nasal and one temporal. Use oblique light against a dark background, or a specialised progressive identifier, to see them.
- Between and slightly above the engravings lies the distance reference circle, and below the corridor lies the near reference circle. Manufacturers supply an identification chart that shows exactly where each sits relative to the engravings.
- Read the distance power at the distance reference circle and the add at the near reference circle.
- The temporal engraving is usually flanked by a code for the manufacturer and design; the nasal engraving is usually flanked by the add power, printed in a form such as "25" for a +2.50 D add.
Reading a progressive anywhere else in the corridor gives an intermediate power, not the add, and misidentifies the lens.
Determining a near add clinically
The blueprint lists determining the near add for bifocals, trifocals and multifocals as a COMT-level task. Three approaches, used together:
1. Age-expected starting point. Presbyopia advances predictably, so an age-based add is the opening bid, not the answer.
| Age | Typical starting add |
|---|---|
| 40–44 | +0.75 to +1.00 D |
| 45–49 | +1.00 to +1.50 D |
| 50–54 | +1.50 to +2.00 D |
| 55–59 | +2.00 to +2.25 D |
| 60+ | +2.25 to +2.50 D |
Adds above about +2.50 to +3.00 D are unusual for standard reading and suggest either a very short working distance or an unmet low vision need.
2. Half the remaining amplitude in reserve. Measure the amplitude of accommodation, then prescribe an add that leaves the patient using no more than half their remaining amplitude at their working distance.
Worked example. Working distance 40 cm requires 2.50 D of accommodative demand. Measured amplitude is 2.00 D, so half the amplitude in reserve means the patient may comfortably use 1.00 D. Add required = 2.50 − 1.00 = +1.50 D.
3. Refine against the patient's actual task. The final number belongs to the working distance the patient uses, not to a standard 40 cm card.
- A dressmaker working at 25 cm needs 4.00 D of demand and a correspondingly higher add.
- A pianist reading music at 70 cm needs about 1.40 D and will be frustrated by a standard +2.50 D add.
- A desktop computer user at 60 to 70 cm usually needs an intermediate solution — a trifocal, a progressive, or dedicated occupational spectacles — rather than a stronger reading add.
Cross-check with range of clear vision. With the trial add in place, ask the patient to find the nearest and furthest points at which print stays clear, and centre the working distance within that range. An add that puts the working distance at the very edge of the range will feel unstable.
Binocular balance the add. Unless there is a specific reason, the add is equal in both eyes. An unequal add is a red flag that either the distance refraction or the accommodative measurement is wrong.
Troubleshooting a multifocal complaint
| Complaint | Likely cause |
|---|---|
| "I have to hold things too close" | Add too strong |
| "I have to hold things too far away / print is blurry at my usual distance" | Add too weak |
| "The computer screen is blurry but reading is fine" | No intermediate correction; needs a trifocal, progressive or occupational lens |
| "I keep hitting the step at the top of the segment" | Image jump; consider a flat-top or a progressive |
| "Everything swims when I turn my head" | Progressive adaptation, or peripheral aberration in a short-corridor design |
| "I can only read through a narrow slot" | Progressive corridor too narrow, or fitting height set too low |
| "My near vision is fine in one eye only" | Unequal add, or segment heights set unevenly |
Read from the front surface, a lens shows a distance power of −1.50 and a segment power of +1.00. What is the add?
On a trifocal, the near add is +3.00 D. What is the conventional intermediate add?
Where on a progressive addition lens should the add be measured?
A patient's working distance is 33 cm and their measured amplitude of accommodation is 1.50 D. Using the half-amplitude-in-reserve rule, what add is indicated?
A presbyopic patient reports that reading is comfortable but their computer screen at 65 cm is blurred through their new bifocals. What is the most appropriate solution?