Corrective Lenses Flashcards

A patient has a spectacle prescription of -3.00 -1.50 x 090. What is the power in the 180-degree meridian?

Answer: -4.50 D

The power at 180° equals the sphere plus the cylinder: -3.00 + (-1.50) = -4.50 D.

Which type of bifocal segment provides the widest reading area at the bottom of the lens?

Answer: Executive (Franklin) bifocal

The Executive or Franklin bifocal has a full-width segment line across the entire lens, providing the widest reading area.

A prism of 2Δ base-in is prescribed for the right eye. In which direction will the image appear to shift for the patient?

Answer: Temporally (away from the nose)

Prism displaces images toward the apex; base-in means the apex is out (temporal), so the image shifts temporally.

What is the vertex distance, and why does it matter for high-power prescriptions?

Answer: Distance from cornea to lens; affects effective power for powers >±4.00 D

Vertex distance is the distance from the back surface of the spectacle lens to the cornea, and it significantly affects effective power for prescriptions above ±4.00 D.

Polycarbonate lenses are most often recommended for which patient population?

Answer: Children and patients requiring impact-resistant lenses

Polycarbonate is highly impact-resistant and is the material of choice for children and monocular patients requiring eye protection.

Which lens design is characterized by a gradual change in power from distance to near with no visible segment line?

Answer: Progressive addition lens (PAL)

Progressive addition lenses (PALs) provide a smooth corridor of increasing power from the distance zone to the near zone without a visible dividing line.

A contact lens prescription reads +2.50 D. When converted to a spectacle prescription for a vertex distance of 12 mm, the spectacle power should be approximately:

Answer: +2.43 D

Moving a plus lens away from the eye reduces effective power; the spectacle equivalent is slightly less than the contact lens power.