Ametropia/Ophthalmic Optics Flashcards

A patient requires +2.50 D at the spectacle plane (12 mm vertex distance). What contact lens power is needed?

Answer: +2.44 D

F_contact = F_spec / (1 + d × F_spec) = 2.50 / (1 + 0.012 × 2.50) = 2.50 / 1.03 ≈ +2.43 D, rounded to +2.44 D.

Oblique astigmatism refers to astigmatism with principal meridians oriented at:

Answer: Between 30–60 and 120–150 degrees

Oblique astigmatism is conventionally defined as astigmatism with the principal meridians between 30–60 degrees and 120–150 degrees.

The Maddox rod test is used clinically to measure:

Answer: Heterophoria and vergence deviation

The Maddox rod converts a point source into a streak, dissociating the eyes to reveal and measure phoria or tropia.

Which lens type is most effective at reducing oblique astigmatic error for off-axis gaze?

Answer: Corrected curve (best form) lens

Corrected curve (best form) lenses are designed with base curves that minimize oblique astigmatism during off-axis viewing.

A patient's spectacle magnification is greater than 1.0. This indicates the patient is:

Answer: Hyperopic

Spectacle magnification >1.0 means the retinal image is larger than in the standard emmetropic eye, which occurs with plus (hyperopic) lenses.

The Jackson cross cylinder (JCC) is used to refine which components of the refraction?

Answer: Cylinder power and axis only

The JCC is specifically designed to refine the cylindrical axis and power without changing the spherical equivalent.

In the reduced eye model, what is the approximate refractive index used for the ocular media?

Answer: 1.336

The reduced eye model uses a single refractive index of approximately 1.336 to represent the combined ocular media.