What it means and why
A sphere has one radius; an aspheric surface changes curvature according to its design. This gives the designer more freedom to balance the lens profile and the optics when the wearer looks away from its centre. “Double-aspheric” describes optimisation on both surfaces, not two prescriptions in one lens.
A flatter-looking lens is not automatically better for every wearer. The intended design must work in its fitted position. Material, frame size, prescription and the previous pair all affect the comparison; a surface label cannot replace those details.
Three different questions
- What correction is prescribed?
- SPH describes the spherical power component; read it with any CYL and AXIS.
- How is the surface shaped?
- Spherical and aspheric describe surface geometry, not frame outline.
- Which viewing tasks are covered?
- Single vision and progressive describe different viewing-distance arrangements.
A worked example
In a teaching comparison of plus lenses, a flatter design may reduce the bulging profile. To make the comparison meaningful, use the same prescription and frame. Comparing a small round frame with a large wide frame changes the amount of lens retained, so the difference cannot all be credited to asphericity.
What it does not tell you
Asphericity and refractive index are independent choices. A high-index lens need not have the same surface design as another high-index lens. Ask which measurements and local specifications accompany the quoted product.
THINK IT THROUGHDoes “double-aspheric” tell you the lens index?
No. It describes the surfaces. The material and its refractive index must be stated separately.
Sources & reading notes
Page updated:
- ABDO · Lens materials and design (OA Corner 40) ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.