A patient has 10/200 vision at 10 feet. What is the JND?

A patient has 10/200 vision at 10 feet. What is the JND?

• A. 0.50
• B. 2.00
• C. 1.00
• D. 3.00

Answer: (B)

Just Noticeable Difference is the smallest change in a stimulus a person can detect. For distance viewing, sensitivity follows a rough rule: the detectable change is about a constant proportion of the baseline distance. So at 10 feet, the smallest noticeable change is roughly 20% of that distance, which is about 2 feet. Therefore, the JND in this scenario is about 2.00 (feet). The smaller numbers would be changes that are typically not detectable at this distance, while a larger change would be easily noticed. This shows that JND depends on the starting distance.

Just Noticeable Difference is the smallest change in a stimulus a person can detect. For distance viewing, sensitivity follows a rough rule: the detectable change is about a constant proportion of the baseline distance. So at 10 feet, the smallest noticeable change is roughly 20% of that distance, which is about 2 feet. Therefore, the JND in this scenario is about 2.00 (feet). The smaller numbers would be changes that are typically not detectable at this distance, while a larger change would be easily noticed. This shows that JND depends on the starting distance.