What aspect does double refraction affect in terms of visual performance?

Double refraction, also known as birefringence, occurs when light passes through certain types of materials, such as crystals, and is split into two separate rays, each traveling at different velocities and in different directions. This phenomenon leads to a separation of the light beams, which can cause variations in the appearance of the image being viewed through the material.

Specifically, double refraction introduces distortion in the way that images appear, making them look blurred or distorted. The effect manifests in visual performance as it alters the way we perceive shapes and outlines of objects, which can significantly impact clarity. For instance, when viewing an object through a birefringent substance, one might see multiple images or a shift in the position of the object, thereby contributing to image distortion.

This impact on visual performance is crucial in applications such as optical instruments, where precise image clarity is necessary. Thus, the correct answer focuses on the inherent property of double refraction that most directly relates to how images are perceived and understood visually.