Contact Lens Design and Application
Design fundamentals
Every lens is a compromise among four variables: geometry (base curve, diameter, optic zone, peripheral curves, edge profile), material (Dk, water content, modulus, surface treatment), power, and center thickness.
Sagittal depth is the master variable in rigid lens fitting: the height of the lens's posterior surface above its chord plane. For a spherical surface, sag ≈ R − √(R² − (d/2)²). Because diameter enters squared, changing diameter alters sag far more than the same relative change in base curve - the single most common design trap in exam questions.
Dk is oxygen permeability of the material; Dk/t is transmissibility of the finished lens and is what the cornea actually experiences. Dk/t falls as thickness rises, so a high-Dk material made thick can underperform a lower-Dk material made thin.
Soft torics are stabilized by prism ballast, peri-ballast, or dual thin zones; rigid torics either align a back toric with corneal toricity or use a bioxidal/prism design. Multifocals are aspheric (center-near or center-distance, pupil-dependent), concentric, or translating. Reverse-geometry orthokeratology lenses stack a flat base curve, a reverse curve, an alignment curve, and a peripheral curve. Scleral lenses replace corneal support with landing-zone support and are defined by diameter, sag, and clearance rather than by corneal alignment.
Rule of thumb: 0.05 mm of base curve ≈ 0.25 D of corneal power. Worked example - a 9.0 mm diameter lens on a 7.80 mm base curve has sag = 7.80 − √(60.84 − 20.25) = 1.43 mm. Widen the diameter to 9.6 mm and sag rises to about 1.65 mm without touching the base curve.