What is the Just Noticeable Difference (JND) for a patient with 20/400 visual acuity?

What is the Just Noticeable Difference (JND) for a patient with 20/400 visual acuity?

• A. 1.00
• B. 2.00
• C. 3.00
• D. 4.00

Answer: (D)

The question is testing how to interpret the smallest noticeable change in visual acuity, using how Snellen/logMAR steps relate to acuity levels. On a logMAR chart, each line represents a fixed step of 0.1 in logMAR. The Just Noticeable Difference (JND) in acuity is typically about four such steps for someone with very poor vision, meaning four Snellen lines. For 20/400 acuity, that translates to a JND of four lines. That’s why the correct value is 4.00. The idea is that a change of roughly four lines is the smallest change the patient is likely to notice when their acuity is this reduced.

The question is testing how to interpret the smallest noticeable change in visual acuity, using how Snellen/logMAR steps relate to acuity levels. On a logMAR chart, each line represents a fixed step of 0.1 in logMAR. The Just Noticeable Difference (JND) in acuity is typically about four such steps for someone with very poor vision, meaning four Snellen lines. For 20/400 acuity, that translates to a JND of four lines. That’s why the correct value is 4.00. The idea is that a change of roughly four lines is the smallest change the patient is likely to notice when their acuity is this reduced.