Why the correct answer is right
Image jump is an inherent property of lined bifocals and is primarily caused by the prismatic effect at the top edge of the segment. The amount of jump is determined by the add power and the design of the segment. However, a patient's *perception* of this jump can be made much worse if the segment height is incorrect. If the seg is too high, the line is in their way; if it's too low, they have to make a large, conscious eye movement to get into the reading zone, which makes the jump more jarring. Therefore, checking that the seg height is correct is a key troubleshooting step.
Why the other options are incorrect
The distance PD, add power, and base curve are determined by the prescription and lens design. The segment height is the primary fitting variable that the optician controls and is a common source of adaptation problems.
Memory aid
If the patient is having trouble with the **jump**, check the **height** of the hurdle.
Real-world application
When a patient returns complaining about their new bifocals, after verifying the prescription is correct, the next step is to re-check the segment height. It is a common error, and if it is off, remaking the lenses at the correct height will often solve the patient's complaint.
