Do you have to recalculate JND during the subjective refraction?

• A. Yes
• B. No
• C. Only if changes occur
• D. Never

Answer: (A)

Just-noticeable difference (JND) is the smallest refractive change a patient can reliably detect under the current viewing conditions. It isn’t a fixed number; it shifts with how the eye is relaxed, how bright the scene is, how much the pupil is dilated, and how good the retinal image quality is with the current correction.

During the subjective refraction you’re continually altering the lens power and axis, which changes the retinal image and the patient’s comfort and clarity. As these conditions change, the perceptual threshold for noticing a difference in blur or sharpness also changes. That means you should reassess or recalculate JND as you progress, selecting test steps that are meaningful to the patient at that moment. If you keep using the same step size regardless of conditions, you might miss a real improvement or stop too soon.

In practice, this means adjusting how finely you diferencia lenses as you approach the final prescription. In bright, well-illuminated conditions with good contrast, the patient may notice smaller differences, so smaller steps help. In dim light or with anxious accommodation, the threshold may be less sensitive, so larger steps might be appropriate to avoid endless testing. By recalculating JND throughout, you ensure each step tested is perceptible and relevant, guiding you toward an accurate, comfortable final refraction.