CPO CPO Basic Science 2 — Questions and Answers
Question 1: A patient with a prescription of +10.00 D sphere is refracted at a vertex distance of 12mm. If their new glasses are fit with a vertex distance of 8mm, what is the compensated power required to provide the same effective power as the refraction?
- +9.62 D
- +10.00 D
- +10.42 D (Correct answer)
- +10.87 D
Correct answer: +10.42 D
When a plus lens is moved closer to the eye, its effective power decreases. To compensate for this, the ordered power must be increased. The formula for vertex compensation can be applied, or it can be reasoned that moving a high-plus lens closer requires more plus power to maintain the same visual effect. Moving a +10.00 D lens 4mm closer to the eye necessitates an increase in power to approximately +10.42 D to ensure the focal point aligns correctly with the retina.
Question 2: Which of the following refractive errors is defined by the eye having different focusing power in different meridians, requiring a toric lens for correction?
- Myopia
- Hyperopia
- Presbyopia
- Astigmatism (Correct answer)
Correct answer: Astigmatism
Astigmatism is a refractive error where the cornea or lens has an irregular, non-spherical curvature, causing light to focus at two different points instead of one. This results in blurred vision at all distances and requires a toric lens, which has two different powers in meridians that are 90 degrees apart, to correct the two different focal points.
Question 3: A patient looking 8mm below the optical center of a progressive addition lens (PAL) with a distance power of -3.00 D and an add power of +2.50 D will experience unwanted vertical prism. What is the primary source of this prismatic effect in the intermediate portion of the lens?
- The spherical distance power alone.
- The add power creating base up prism.
- The inherent design of the progressive corridor causing base down prism.
- The combination of changing plus power and Prentice's rule. (Correct answer)
Correct answer: The combination of changing plus power and Prentice's rule.
In a progressive lens, the power gradually increases from the distance portion to the near portion. As the patient's line of sight moves down the lens, they are looking through increasingly more plus power. According to Prentice's Rule (Prism = Power x Decentration), this changing power away from the optical center induces a variable prismatic effect. The complex surface design creates this smooth transition, but a consequence is induced prism that changes along the vertical corridor.
Question 4: The AC/A ratio represents the relationship between accommodative convergence and accommodation. A clinically 'high' AC/A ratio (e.g., 6/1) implies which of the following scenarios?
- The patient converges very little for each diopter of accommodation exerted.
- The patient has a reduced ability to accommodate for near targets.
- A large amount of convergence is generated for a small amount of accommodation. (Correct answer)
- The patient is likely to be exophoric at near.
Correct answer: A large amount of convergence is generated for a small amount of accommodation.
A high AC/A ratio means that for every diopter of accommodation (focusing effort), the eyes converge a large amount (measured in prism diopters). For example, a 6/1 ratio means 6 prism diopters of convergence occur for every 1 diopter of accommodation. This can lead to esophoria or esotropia at near, as the accommodative effort brings the eyes inward excessively.
Question 5: The Purkinje images are reflections from the various optical surfaces of the eye. Which image is formed by the posterior surface of the crystalline lens and is characteristically inverted?
- P1 (First Purkinje image)
- P2 (Second Purkinje image)
- P3 (Third Purkinje image)
- P4 (Fourth Purkinje image) (Correct answer)
Correct answer: P4 (Fourth Purkinje image)
The four main Purkinje images are reflections from the anterior cornea (P1), posterior cornea (P2), anterior lens (P3), and posterior lens (P4). Due to the concave nature of the posterior lens surface acting as a mirror, the reflection it creates (P4) is inverted, a unique characteristic among the four primary images.
Question 6: For light to be considered at 'optical infinity' with negligible vergence (i.e., parallel rays), the light source must be at least how far away from the observer?
- 1 meter
- 3 meters
- 6 meters (20 feet) (Correct answer)
- 10 meters
Correct answer: 6 meters (20 feet)
In clinical optics, light rays originating from a source 6 meters (approximately 20 feet) or more away are considered to be effectively parallel and have a vergence of near zero. At this distance, the accommodative demand on the eye is minimal (about 1/6th of a diopter), which is why 20 feet is the standard distance for visual acuity testing.
CPO CPO Basic Science 2
A patient with a prescription of +10.00 D sphere is refracted at a vertex distance of 12mm.
If their new glasses are fit with a vertex distance of 8mm, what is the compensated power required to provide the same effective power as the refraction?