The Bifocal Lens: Two Pairs of Spectacles Cut in Half

Stand a digital camera on a table with its focus locked at infinity: that is the aged eye, which can no longer focus near. Put a newspaper page 40 cm in front of it and a poster across the room behind. From a convex lens set pick a weak lens for 'distance' — about +0.5 D (f = 2 m) — and for 'reading' a lens 2.5 D stronger than that, about +3.0 D (f ≈ 33 cm): the 2.5 D difference is the reading add for 40 cm. Cut a card frame, mount the weak lens in the top half of the window and the strong one in the bottom half, each masked with tape so only its half shows: Franklin's split. Photograph through the top half: the distant poster is sharp, the page blurred. Lower the frame and shoot through the bottom half: the page is sharp. Move the frame slowly across the dividing line and watch whether the scene jumps.

**Benjamin Franklin** described his "double spectacles" in a letter to **George Whatley** dated 23 May 1785. Fused and one-piece bifocals, in which the reading segment is melted into or ground from the main lens, replaced the split lens in the nineteenth and twentieth centuries, and progressive lenses later blended the zones together. **Honest limits.** Hofstetter's formulas describe averages; an individual's amplitude varies widely, and prescribing is for an optometrist. The bench bifocal shows the principle with a camera, not a prescription for anyone's eyes.