In lensometry, if the target is moved away from the observer, the displacement x is assigned which sign?

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (B)

In lensometry the displacement of the target along the optical axis is given a sign based on a simple direction rule: positive displacement is toward the observer, while negative displacement is away from the observer. So when you move the target away from the observer, you’re moving opposite to the positive direction, and the displacement is negative. This convention helps interpret how changes in target position relate to the measured lens power through the observed shift. If you moved the target toward the observer, the displacement would be positive.