Does the minus lens test tend to overestimate or underestimate the amplitude of accommodation?

• A. Overestimate
• B. Underestimate
• C. Equal
• D. Not specified

Answer: (B)

Amplitude of accommodation is the maximum amount the eye can increase its lens power to shift from distance to near focus. The minus-lens test estimates this by keeping the near target at a fixed distance and adding negative lenses until the target becomes blurred or can no longer be kept clear. The amount of minus required to blur reflects how much accommodation the eye can supply from its relaxed state at that specific distance.

But this setup doesn’t engage the full, natural range of accommodation. The near working distance is fixed, and the cues that help drive the full accommodative response—such as moving the target closer, dynamic depth cues, and the interplay with vergence—aren’t as effectively used. Because of these constraints, the test tends to show a smaller, or lower, accommodative capacity than the eye’s true maximum. That’s why the minus-lens test commonly underestimates the amplitude of accommodation.