Optics and Refraction Flashcards

The vertex distance is clinically important because:

Answer: Changing it alters the effective power of high-powered lenses

The effective power of a lens changes with vertex distance; for powers >±4D, failure to account for vertex distance during refraction and dispensing introduces clinically significant error.

Snellen acuity of 20/40 means the patient can resolve at 20 feet what a normal eye resolves at:

Answer: 40 feet

Snellen notation is a ratio: the numerator is the testing distance (20 ft) and the denominator is the distance at which the letter subtends 5 arc minutes for a normal observer.

With-the-rule astigmatism is defined as when:

Answer: The steepest corneal meridian is vertical (near 90°)

With-the-rule (WTR) astigmatism has maximum corneal power vertically (90°±30°), which is the most common pattern in children and young adults.

The Purkinje-Sanson images are reflections from the four surfaces of the eye used to:

Answer: Locate the position of the crystalline lens and assess its clarity

The four Purkinje-Sanson images are virtual reflections from the anterior cornea, posterior cornea, anterior lens, and posterior lens, used historically to locate the lens and detect cataracts.

Which lens aberration causes peripheral blur and is addressed by aspheric lens designs?

Answer: Oblique astigmatism (marginal astigmatism)

Oblique astigmatism occurs when oblique rays through the peripheral lens produce different focal distances in the sagittal and tangential planes, causing peripheral distortion corrected by aspheric designs.

When transposing a spectacle prescription from plus cylinder to minus cylinder form, which step is performed LAST?

Answer: Change the sign of the cylinder while keeping the magnitude the same, and rotate the axis by 90°

Transposition steps are: (1) algebraically add sphere and cylinder to get new sphere; (2) change the sign of the cylinder; (3) rotate the axis by 90° — the axis change is the final step.