Instrumentation, Adjustment, and Repair
What this domain tests
This domain tests safe and accurate use of lensmeters, PD instruments, and alignment tools, plus frame adjustment and minor repair. Expect practical sequencing questions and measurement verification.
Core rules
- A lensmeter measures sphere, cylinder, axis, and prism. Focus the eyepiece first, center the target, and verify the lens orientation before reading. Calibrate according to the manufacturer's instructions.
- A pupillometer or PD ruler measures interpupillary distance. Keep the patient's head straight and eyes fixated on a distant target; record monocular values when possible.
- Frame adjustment follows a sequence: align the bridge and nose pads, adjust temple length and bend, then set pantoscopic tilt and face form. Recheck symmetry on a flat surface.
- Heat only frame materials that are heat-adjustable; avoid overheating acetate or damaging coatings. Use correct screwdrivers and avoid over-tightening.
- Repair includes replacing screws and nose pads, tightening hinges, and aligning temples. Structural cracks or soldered joints may require referral rather than a quick fix.
Worked example
A lensmeter reads +2.00 −1.00 × 180 in the right lens. The target cross is centered, but the axis drum is slightly off. Rotate the axis control until the cylinder lines align with the target, then recheck sphere and cylinder. If the reading is inconsistent, confirm that the lens is not upside down and that the lensmeter is calibrated. Record the final values and compare them with the prescription before dispensing.
Traps
Reading a lens from the wrong surface; forgetting to focus the eyepiece; over-tightening screws; and heating a frame before checking whether it is heat-adjustable. Also avoid bending temples before the bridge is level.