What is defined as the distance between two focal lengths of a toric lens?

The term that accurately defines the distance between two focal lengths of a toric lens is the astigmatic interval. A toric lens is designed to correct astigmatism, which occurs when the eye has different refractive powers in different meridians. The lens features two principal meridians, each with its own focal point, due to its varying curvature. The astigmatic interval refers specifically to the spatial separation between those two focal points.

Understanding this concept is crucial in optometry, particularly for ensuring proper lens fitting for patients with astigmatism, as it impacts vision quality and comfort. The other options do not relate directly to this specific definition: refractive distance generally refers to measurements associated with the bending of light, the visual field concerns the entire area that can be seen by the eye, and lens thickness pertains to the physical dimension of the lens material itself rather than the optical properties.