To increase movement on a diagnostic gas permeable lens with minimal movement, which adjustment would be effective?

The effective adjustment to increase movement on a diagnostic gas permeable lens involves decreasing the overall lens diameter. A smaller overall diameter leads to a reduction in the area of the lens that interacts with the cornea, allowing for greater lateral movement or rocking of the lens during blinking or eye movement. This increase in lens movement is vital for ensuring the lens is properly centered and can provide oxygen to the cornea while also facilitating tear exchange beneath the lens.

In contrast, increasing the optic zone diameter or the overall length diameter tends to center the lens more rigidly on the cornea, which decreases lens movement. Increasing sagittal depth can also restrict movement by further anchoring the lens in place due to increased lens-to-cornea vaulting. Thus, decreasing the overall length diameter is the most effective means to achieve the desired increase in movement for diagnostic purposes with gas permeable lenses.