ABO Optician — All Questions

Using Prentice's rule, how much prism is induced when a +4.00 D lens is decentered 5 mm?

• a.1.0 prism diopter
• b.0.5 prism diopters
• c.2.0 prism diopters✓
• d.20 prism diopters

Prentice's rule: prism (prism diopters) = decentration (cm) x power (D). 5 mm = 0.5 cm; 0.5 x 4.00 = 2.0 prism diopters.

A -6.00 D lens must produce 3 prism diopters. How far must it be decentered (Prentice's rule)?

• a.18 cm
• b.3 cm
• c.0.5 cm (5 mm)✓
• d.6 cm

Decentration = prism / power = 3 / 6.00 = 0.5 cm, which equals 5 mm of decentration from the optical center.

Transpose -1.75 +1.25 x 060 to minus-cylinder form.

• a.-0.50 +1.25 x 150
• b.-1.75 -1.25 x 060
• c.-3.00 -1.25 x 060
• d.-0.50 -1.25 x 150✓

New sphere = -1.75 + 1.25 = -0.50; change cylinder sign to -1.25; rotate axis 90 degrees (060 to 150). Result: -0.50 -1.25 x 150.

A frame has an A measurement of 52 mm, DBL of 18 mm, and the patient's PD is 62 mm. What is the total decentration for both lenses combined?

• a.8 mm total (4 mm per lens)✓
• b.0 mm
• c.16 mm per lens
• d.8 mm per lens

Frame PD (geometric center distance) = A + DBL = 52 + 18 = 70 mm. Total decentration = frame PD - patient PD = 70 - 62 = 8 mm total, split as 4 mm per lens.

With a 34 mm effective diameter and 4 mm per-lens decentration, what is the approximate minimum blank size?

• a.34 mm
• b.About 42 mm✓
• c.30 mm
• d.52 mm

A common working rule for minimum blank size is effective diameter + (2 x per-lens decentration). 34 + (2 x 4) = 42 mm, before adding any small manufacturing tolerance.

A patient is refracted at a 12 mm vertex with -10.00 D, but the frame sits at 16 mm. Which way does the effective power change, requiring compensation?

• a.No change occurs
• b.It becomes plus power
• c.Toward more minus, so less minus is ordered
• d.The lens must gain minus power✓

Moving a minus lens farther from the eye (12 to 16 mm) increases its effective minus power, so a slightly weaker (less minus) lens is ordered to keep the correction accurate at the new vertex distance.

What is the combined horizontal prism when the right lens gives 2 prism diopters base out and the left lens gives 1 prism diopter base out?

• a.3 prism diopters base out total✓
• b.0 prism diopters
• c.1 prism diopter base out total
• d.2 prism diopters base in

Horizontal prisms of the same base direction in the two eyes add together. 2 base out + 1 base out = 3 prism diopters base out total split across the two eyes.

Compute the spherical equivalent of +3.00 -2.00 x 090.

• a.+5.00 D
• b.+2.00 D✓
• c.+4.00 D
• d.+1.00 D

Spherical equivalent = sphere + (cylinder / 2) = +3.00 + (-2.00 / 2) = +3.00 + (-1.00) = +2.00 D.

How much vertical prism imbalance exists if the reading point is 8 mm below the OC in a +2.50 D right lens and a +1.00 D left lens (Prentice's rule)?

• a.0.8 prism diopters
• b.0 prism diopters
• c.1.2 prism diopters✓
• d.3.5 prism diopters

Each eye's prism = 0.8 cm x power. Right = 0.8 x 2.50 = 2.0; left = 0.8 x 1.00 = 0.8. Both are base-down (plus lens, below OC), so the imbalance = 2.0 - 0.8 = 1.2 prism diopters.

A lens has a front curve of +8.00 D and needs +2.00 D total power. What back curve is required (thin-lens approximation)?

• a.+6.00 D
• b.+10.00 D
• c.-6.00 D✓
• d.-2.00 D

Total power = front + back. Back = total - front = +2.00 - (+8.00) = -6.00 D. The back surface must be ground to -6.00 D to achieve the +2.00 D lens.