Optics and Spectacles
Basic optics, transposition, vertex distance, and spectacle lens forms and fitting concepts.
Optics the Assistant Actually Uses
Lens basics — plus (convex, center-thick) lenses converge light and correct hyperopia; minus (concave, edge-thick) lenses diverge light and correct myopia; cylinders correct astigmatism along one meridian.
Transposition — convert between plus- and minus-cylinder form: algebraically add sphere and cylinder, flip the cylinder sign, rotate the axis 90°.
Vertex distance — the spectacle plane sits ~12–14 mm from the cornea; above ±4.00 D, effective power changes meaningfully with vertex distance, and conversion matters when moving between spectacle and contact lens prescriptions.
Reading an Rx — sphere / cylinder × axis, add for near, prism with base direction. Know OD (right), OS (left), OU (both) — and that a transposed Rx is the same prescription, not a different one.