Kestenbaum's rule
ADD = (acuity denominator ÷ numerator) ÷ target print in M
Magnification = ADD ÷ 4
Working distance (cm) = 100 ÷ ADD
- Acuity
- Best-corrected distance acuity as a Snellen fraction, 20/200 or 6/60.
- M
- Print size in M units. 1M is roughly newspaper print.
For 1M print the ADD is simply the Snellen fraction turned upside down: 20/200 gives 200 ÷ 20 = +10.00 D.
Distance acuity to starting ADD, for 1M print
| Acuity | ADD | Magnification | Working distance |
|---|---|---|---|
| 20/40 | +2.00 D | 0.5x | 50.0 cm |
| 20/80 | +4.00 D | 1.0x | 25.0 cm |
| 20/100 | +5.00 D | 1.3x | 20.0 cm |
| 20/160 | +8.00 D | 2.0x | 12.5 cm |
| 20/200 | +10.00 D | 2.5x | 10.0 cm |
| 20/400 | +20.00 D | 5.0x | 5.0 cm |
Smaller print needs more. For 20/100 aiming at 0.5M print the calculator gives +10.00 D, 2.5x, at 10.0 cm.
A worked case
A wearer has 20/160 in the better eye and wants to read the small print on medicine packets, about 0.8M. Entering 20, 160 and 0.8 M, the calculator gives +10.00 D, 2.5x and a working distance of 10.0 cm. For newspaper print at 1M the same acuity needs only +8.00 D at 12.5 cm, which shows how much the target decides.
Using the result
- It is a starting point for a trial, not a prescription. Wearers with reduced contrast sensitivity often need more.
- The reading material must be held at the working distance shown; at +10.00 D that is 10 cm.
- High adds worn with both eyes make converging hard; base-in prism is often added. That is the practitioner's decision.
- Above about +20.00 D, electronic magnifiers are usually easier to use than lenses.
The calculator estimates; it does not assess vision. A low vision assessment decides what is prescribed. See the medical disclaimer.
Questions
Why does a magnifier's label not match the magnification here?
The calculator uses ADD ÷ 4, the conventional magnification for a 25 cm reference distance. Many hand magnifiers are labelled with ADD ÷ 4 + 1, so a +12.00 D magnifier may say 4x where this calculator says 3.0x.