Free COA Lensometry and Refractometry Questions and Answers — Questions and Answers
Question 1: While performing manual lensometry on a pair of glasses, you first focus the thin, single 'sphere' lines at -1.50 D. You then turn the power wheel further until the thick, triple 'cylinder' lines come into focus at -2.75 D. The axis wheel is set to 090. How would this prescription be written in standard minus-cylinder format?
- -1.50 -1.25 x 090 (Correct answer)
- -2.75 +1.25 x 180
- -1.50 +1.25 x 090
- -2.75 -1.25 x 090
Correct answer: -1.50 -1.25 x 090
In minus-cylinder lensometry, the first reading (sphere lines) is the sphere power, which is -1.50 D. The cylinder power is the difference between the second reading and the first reading (-2.75 D - (-1.50 D) = -1.25 D). The axis is the setting on the axis wheel when the cylinder lines are in focus, which is 090. Therefore, the prescription is -1.50 -1.25 x 090.
Question 2: What is the primary purpose of the Jackson Cross Cylinder (JCC) during a manual subjective refraction?
- To determine the initial sphere power
- To measure the pupillary distance
- To refine the cylinder axis and power (Correct answer)
- To check for the presence of prism
Correct answer: To refine the cylinder axis and power
The Jackson Cross Cylinder (JCC) is a specialized lens used to fine-tune the astigmatism correction. It is flipped and rotated to help the patient determine which of two choices provides clearer vision, allowing the examiner to precisely refine both the axis and the power of the cylinder.
Question 3: When neutralizing a lens in the lensometer, the target mires are perfectly clear and centered when the power wheel is at +3.00 D. However, to center the target in the reticle, the entire lens holder had to be moved down, causing the target to be displaced 2 units up on the prism scale. How should this prism be recorded?
- 2Δ Base In
- 2Δ Base Out
- 2Δ Base Down
- 2Δ Base Up (Correct answer)
Correct answer: 2Δ Base Up
Prism shifts the image toward its apex. To re-center the image, the lens must be moved in the direction of the prism's base. Since the lens holder was moved down to center the target, the image was originally displaced upward (towards the apex). This indicates the presence of Base Up prism. The rings on the reticle indicate the amount of prism in diopters.
Question 4: A patient with a high myopic prescription of -11.00 D has their manifest refraction performed at a standard vertex distance of 14mm. Their new glasses are fit very close to their face, at a vertex distance of 10mm. Which of the following complaints is MOST likely?
- The glasses feel too weak.
- The glasses feel too strong. (Correct answer)
- The patient notices colored fringes around lights.
- The patient feels no difference in power.
Correct answer: The glasses feel too strong.
For high minus lenses, moving the lens closer to the eye increases its effective minus power. This change in vertex distance will make the -11.00 D prescription feel stronger to the patient, which can cause symptoms of eyestrain or blurry vision.
Question 5: Which of the following describes the correct procedure for measuring the 'add' power of a lined bifocal lens using a manual lensometer?
- Measure the near segment first, then subtract the distance power.
- Measure the distance power, then turn the glasses around and measure the front of the near segment.
- Measure the sphere power in the distance portion, then move to the near segment and re-focus the sphere lines. The add is the difference. (Correct answer)
- Place the geometric center of the lens in the lensometer and take one reading.
Correct answer: Measure the sphere power in the distance portion, then move to the near segment and re-focus the sphere lines. The add is the difference.
To find the add power, first measure the spherical power through the distance portion of the lens. Then, without changing the axis, move the lens upward to center the near segment in the lensometer. Re-focus the sphere lines (single, thin lines) and note the new power. The add power is the algebraic difference between the near sphere reading and the distance sphere reading, and it is always a positive value.
Question 6: An automated refractor measures a young, asymptomatic patient's prescription as -3.00 D sphere in both eyes. However, a subsequent cycloplegic refraction reveals a true prescription of -0.75 D sphere. What is the MOST likely cause for this significant discrepancy?
- Instrument miscalibration
- Accommodative spasm (Correct answer)
- Poor patient fixation
- Corneal surface irregularity
Correct answer: Accommodative spasm
Young patients can exert a significant amount of focusing effort (accommodation), especially when looking into a machine. This accommodative spasm causes the eye's crystalline lens to increase its power, leading the autorefractor to measure more myopia than is actually present. A cycloplegic drop temporarily paralyzes the focusing muscle, revealing the true, relaxed refractive state.
Free COA Lensometry and Refractometry Questions and Answers
While performing manual lensometry on a pair of glasses, you first focus the thin, single 'sphere' lines at -1.50 D.
You then turn the power wheel further until the thick, triple 'cylinder' lines come into focus at -2.75 D.
The axis wheel is set to 090.
How would this prescription be written in standard minus-cylinder format?