What change occurs to the power of a plus lens when moved closer versus further from the eye?

For a plus lens, when it is moved closer to the eye, the effective power of the lens appears to decrease relative to the eye. This happens due to the optics of how we perceive power in relation to distance from the lens. The lens creates a virtual image that moves further away from the eye as it is moved closer, effectively reducing the convergence of light rays entering the eye.

In practical terms, for a plus lens, which is designed to converge light for individuals who have difficulty focusing on near objects, bringing the lens closer means that the eye requires less plus power to focus sharply on the object's image. This reduction in effective power due to decreased distance is often referred to as “the eye’s distance gain,” and emphasizes how the lens's distance from the eye influences its perceived power.

Additionally, as the lens is moved further away from the eye, its apparent power does not increase significantly due to the fixed focal properties of the lens, affirming that the relationship between distance and perceived power is not linear.

Understanding these principles is essential for optical professionals, as they often need to provide advice on lens selection and positioning based on these optical dynamics.