If JND is 1.25 D, what is the cylinder power for a patient with 10/100 vision?

• A. 1.25
• B. 2.50
• C. 3.75
• D. 0.50

Answer: (B)

JND is the smallest change in cylinder power a patient can perceive. If that detectable change is 1.25 diopters, choosing a cylinder correction that is a multiple of that detectable step helps ensure the patient notices an improvement. For someone with very reduced vision (10/100), a practical, noticeable correction is often a modest step beyond the minimal detectable change. Doubling the JND gives 2.50 diopters, which provides a clear, meaningful improvement without overshooting. That’s why 2.50 diopters is the best choice.

The smaller increment (1.25 D) is detectable but may not yield a substantial improvement for very low vision; a 0.50 D change is below the detectable threshold, unlikely to be perceived; a larger step (3.75 D) could overcorrect and cause new visual discomfort.