What does it indicate if the lens clock needle moves while checking the curvature on the front surface of a spectacle lens?

When the lens clock needle moves while checking the curvature on the front surface of a spectacle lens, it indicates a potential lens warpage. A lens clock is designed to measure the curvature of a lens accurately. If the lens is warped, the surface will not conform to the expected standard curvature. This results in inconsistencies in the readings taken by the lens clock, causing the needle to move variably, suggesting that the lens is no longer retaining its intended shape.

In contrast, a plus cylinder lens design, aspheric design lens, or concave lens would not typically cause erratic readings on a lens clock unless there were other issues with the lens, such as warpage. These lens types can exhibit specific curvature profiles that should produce stable readings when measured correctly. Therefore, the movement of the needle implies a significant concern regarding the integrity of the lens, confirming that warpage may be present.