Which of the following is NOT an optical aberration?

Refraction is not considered an optical aberration; rather, it is the bending of light as it passes from one medium to another due to a change in speed. This phenomenon occurs universally and is a fundamental principle in optics, essential for the functioning of lenses and prisms. It can be predicted and described by Snell's law, which defines how light rays change direction when they enter different materials.

In contrast, optical aberrations are imperfections in image formation that lead to blurred or distorted images. Chromatic aberration occurs when different wavelengths of light are focused at different points due to variations in refractive indices. Coma is a type of aberration that results in off-axis point sources of light being rendered as comet-shaped blur instead of as perfect points. Vignetting is the reduction of an image's brightness or saturation at the periphery compared to the image center, often caused by lens design or physical limitations in camera settings. These aberrations are all related to the failure of optical systems to produce an ideal image, unlike refraction, which is a normal and expected behavior of light.