What is the power adjustment when fitting a lens closer than the vertex distance specified?

When a lens is fitted closer to the eye than the specified vertex distance, the effective power of the lens increases. This phenomenon is due to the thin lens formula and the relationship between lens power and vertex distance. When you move a lens closer to the eye, the light rays entering the lens converge more sharply, resulting in a greater effective power.

In terms of optics, the lens's power (measured in diopters) is inversely related to the distance at which it is placed. Therefore, as the vertex distance decreases, the power must also increase to maintain the focus at the correct distance. This adjustment is essential for ensuring that patients achieve optimal vision, especially in cases where the lens is positioned at a shorter distance than anticipated.

The reasoning behind the other options can be summarized: the power does not remain the same because the geometry of light changes as the position of the lens changes relative to the eye, and it certainly does not double; power changes incrementally based on vertex distance adjustments, not by such a dramatic factor.