ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 2 — Questions and Answers

Question 1: What is the prismatic effect at a point 4 mm below the optical center of a -3.00 D lens?

1.2Δ base up (Correct answer)
1.2Δ base down
0.75Δ base up
0.75Δ base down

Correct answer: 1.2Δ base up

Using Prentice's Rule: P = d × F = 0.4 cm × 3.00 = 1.2Δ. For a minus lens, the prism base is toward the optical center (base up when point is below OC).

Prentice's Rule states P = d × F, where P is prism in prism diopters (Δ), d is distance from the optical center in centimeters, and F is the lens power in diopters. Here: d = 4 mm = 0.4 cm, F = 3.00 D (absolute value), P = 0.4 × 3.00 = 1.2Δ. For a minus lens, the base of the prism is always directed toward the optical center. Since the point is 4 mm below the OC, the base points up (toward the OC, which is above the measurement point). Result: 1.2Δ base up. This calculation appears frequently on the ABO exam, especially for determining differential prism in bifocal dispensing.

Question 2: What is the power cross for the prescription +2.00 -1.50 × 090?

+0.50 D at 090 and +2.00 D at 180 (Correct answer)
+2.00 D at 090 and +0.50 D at 180
-1.50 D at 090 and +2.00 D at 180
+2.00 D at 090 and -1.50 D at 180

Correct answer: +0.50 D at 090 and +2.00 D at 180

The axis is 090, so no cylinder acts at 090. Power at 090 = sphere = +2.00 + 0 = +2.00. Wait — at axis 090, cylinder = 0, so power = +2.00. At 180 (perpendicular to axis), power = +2.00 + (-1.50) = +0.50.

To build a power cross from a spherocylindrical prescription: (1) Draw a cross aligned with the two principal meridians. (2) Axis of cylinder = 090 — at this meridian, no cylinder power exists, so total power = sphere = +2.00 D. (3) At the meridian 90° away = 180 — full cylinder acts here, so power = sphere + cylinder = +2.00 + (-1.50) = +0.50 D. Power cross: +2.00 at 90° and +0.50 at 180°. This can be verified by transposing: +0.50 -(-1.50) × 180 = +0.50 +1.50 × 180. The power cross is a fundamental tool for understanding spherocylindrical lenses.

Question 3: Transposing the prescription +3.50 -2.00 × 045 to plus-cylinder form gives:

+1.50 +2.00 × 135 (Correct answer)
+1.50 +2.00 × 045
+3.50 +2.00 × 135
+5.50 -2.00 × 135

Correct answer: +1.50 +2.00 × 135

To transpose to plus cylinder: new sphere = old sphere + old cylinder = +3.50 + (-2.00) = +1.50. New cylinder = -old cylinder = +2.00. New axis = old axis ± 90° = 045 + 90° = 135°.

Prescription transposition converts between minus-cylinder and plus-cylinder form (or vice versa). Steps: (1) New sphere = old sphere + old cylinder: +3.50 + (-2.00) = +1.50. (2) New cylinder = reverse sign of old cylinder: -(-2.00) = +2.00. (3) New axis = old axis ± 90° (whichever keeps axis between 001–180): 045 + 90 = 135°. Result: +1.50 +2.00 × 135. To verify: power cross should give same values. At axis 135: power = +1.50. At 045: power = +1.50 + 2.00 = +3.50. Original: at axis 045: +3.50; at 135: +3.50 + (-2.00) = +1.50. Confirmed. Transposition is a mandatory ABO competency.

Question 4: When two thin lenses are placed in contact with each other, the combined power is:

The algebraic sum of their individual powers (Correct answer)
The product of their individual powers
Always less than either individual power
Determined by the separation distance only

Correct answer: The algebraic sum of their individual powers

For thin lenses in contact (zero separation), the combined power equals the algebraic sum: F_total = F1 + F2.

When two thin lenses are placed in contact (with negligible separation between them), the vergence formula simplifies to simple algebraic addition: F_total = F1 + F2. For example, a +4.00 D lens in contact with a -1.50 D lens gives +2.50 D total. When lenses are separated by a distance d (in meters), the combined power formula becomes: F_total = F1 + F2 - d × F1 × F2 (thick lens / Gullstrand equation). For contact lenses, the zero-separation assumption is valid. For spectacle lenses mounted on an optical bench, the separation distance must be accounted for. This principle is used in calculating the power of bifocal adds and doublet lens systems.

Question 5: What is a prism diopter (Δ) and how is it defined?

A unit of prism power equal to 1 cm of displacement at 1 meter; P = d × F (Correct answer)
A unit equal to 0.57° of angular deviation
The displacement of light in millimeters per diopter of lens power
The angle of minimum deviation through a prism in degrees

Correct answer: A unit of prism power equal to 1 cm of displacement at 1 meter; P = d × F

A prism diopter (Δ) is defined as 1 cm of linear displacement of a ray at a distance of 1 meter from the prism. Equivalently, P = d(cm) × F(D) by Prentice's Rule.

The prism diopter (Δ) is the unit used to measure prism power in opticianry. It is defined geometrically: a prism has a power of 1Δ if it displaces a ray of light 1 centimeter at a distance of 1 meter from the prism. Mathematically: P(Δ) = 100 × tan(θ), where θ is the angle of deviation. Approximately 1Δ ≈ 0.57° of angular deviation (since tan(0.57°) ≈ 0.01). In opticianry practice, prism power is calculated using Prentice's Rule: P = d × F, where d is in centimeters and F is lens power in diopters. Prism is used in spectacle lenses to correct binocular vision disorders, and its measurement and verification is a core ABO competency.

Question 6: What is the difference between back vertex power and front vertex power of a thick lens?

Back vertex power is measured from the back surface; front vertex power from the front surface; they differ for thick lenses due to lens thickness (Correct answer)
They are always equal regardless of lens thickness
Front vertex power is always greater than back vertex power
Back vertex power applies to plus lenses only; front to minus lenses

Correct answer: Back vertex power is measured from the back surface; front vertex power from the front surface; they differ for thick lenses due to lens thickness

For thick lenses, back vertex power (neutralizing power) and front vertex power differ because the lens thickness creates a gap between surfaces. Prescriptions are specified as back vertex power.

For a thin lens, front and back vertex powers are equal. For a thick lens (or a lens system with separated elements), they differ. Back vertex power (BVP) — also called neutralizing power or back surface vergence power — is measured from the posterior lens surface and is what a lensometer measures when the lens back surface is placed against the lens stop. Front vertex power (FVP) — also called front vertex neutralizing power — is measured from the anterior surface. The difference arises because the lens thickness creates a path length between the two refracting surfaces. By convention, ophthalmic prescriptions are specified as back vertex power, which is what the lensometer reads in standard back-surface-down position. This distinction is important for very thick lenses and aphakic corrections.

ABO NOCE Basic Opticianry Ophthalmic Optics and Principles Questions and Answers 2

What is the prismatic effect at a point 4 mm below the optical center of a -3.00 D lens?