ABO NOCE Basic Opticianry Ophthalmic Optics Questions and Answers — Questions and Answers
Question 1: A patient's prescription is OD: -5.00 D sphere, OS: -2.00 D sphere. If their pupils are decentered 4 mm inwards from the optical centers of the lenses, what is the total amount and direction of induced horizontal prism?
- 0.8 prism diopters Base In
- 2.8 prism diopters Base Out (Correct answer)
- 1.2 prism diopters Base In
- 2.0 prism diopters Base Out
Correct answer: 2.8 prism diopters Base Out
Prentice's Rule (Prism = Power x Decentration in cm) is used for each eye. OD: 5.00 D x 0.4 cm = 2.0 p.d. For a minus lens, decentering IN induces Base OUT prism. OS: 2.00 D x 0.4 cm = 0.8 p.d. For a minus lens, decentering IN induces Base OUT prism. Since both eyes induce Base Out prism, the effects are additive: 2.0 p.d. BO + 0.8 p.d. BO = 2.8 p.d. Base Out total.
Question 2: A patient with anisometropia experiences vertical imbalance when reading. The prescription is OD: +1.00 D sphere, OS: +4.00 D sphere, with a +2.00 Add. Assuming a reading level 10 mm below the optical centers, which of the following is the most appropriate corrective action?
- Prescribe single vision reading glasses only.
- Order a slab-off (bicentric grind) on the right lens. (Correct answer)
- Increase the pantoscopic tilt to 15 degrees.
- Order a slab-off (bicentric grind) on the left lens.
Correct answer: Order a slab-off (bicentric grind) on the right lens.
Vertical imbalance occurs when there's a significant power difference between lenses in the vertical meridian. Slab-off, a form of bicentric grinding, adds prism to correct this. The rule is to apply slab-off (Base Up prism effect) to the lens with the most minus or least plus power in the vertical meridian. In this case, the OD (+1.00 D) is the least plus lens and requires the slab-off correction.
Question 3: Which of the following lens materials has the highest index of refraction, resulting in the thinnest and lightest lens for a given high prescription?
- Crown Glass
- CR-39 Plastic
- Polycarbonate
- High-Index Plastic 1.74 (Correct answer)
Correct answer: High-Index Plastic 1.74
The index of refraction measures how efficiently a material bends light. A higher index means the material bends light more, allowing for a thinner and lighter lens design. High-Index Plastic 1.74 has one of the highest refractive indices available for ophthalmic lenses, significantly higher than Crown Glass (1.52), CR-39 (1.50), and Polycarbonate (1.59).
Question 4: A lens aberration that causes light of different colors (wavelengths) to focus at different points, resulting in colored fringes around objects, is known as:
- Spherical Aberration
- Coma
- Chromatic Aberration (Correct answer)
- Distortion
Correct answer: Chromatic Aberration
Chromatic aberration is an optical effect where a lens fails to focus all colors to the same convergence point. This occurs because lenses have a different refractive index for different wavelengths of light. This dispersion of light results in colored halos or fringes around images.
Question 5: Using the 'minus cylinder' form, what is the correct transposition of the prescription +1.00 +1.50 x 090?
- +1.00 -1.50 x 180
- +2.50 -1.50 x 180 (Correct answer)
- -0.50 +1.50 x 180
- +2.50 +1.50 x 090
Correct answer: +2.50 -1.50 x 180
To transpose a prescription to minus cylinder form, follow three steps: 1. Algebraically add the sphere and cylinder powers to get the new sphere (+1.00 + 1.50 = +2.50). 2. Change the sign of the cylinder (+1.50 becomes -1.50). 3. Change the axis by 90 degrees (90 + 90 = 180). Therefore, the transposed prescription is +2.50 -1.50 x 180.
Question 6: A plus power lens is essentially two prisms oriented in which configuration?
- Apex to apex
- Base to apex
- Base to base (Correct answer)
- Apex to base
Correct answer: Base to base
A plus lens, which converges light, can be visualized as two prisms with their bases joined together at the optical center. Light rays entering the lens are bent towards the thickest part (the base), causing them to converge. Conversely, a minus lens is like two prisms joined apex to apex.
ABO NOCE Basic Opticianry Ophthalmic Optics Questions and Answers
A patient's prescription is OD: -5.00 D sphere, OS: -2.00 D sphere.
If their pupils are decentered 4 mm inwards from the optical centers of the lenses, what is the total amount and direction of induced horizontal prism?