What type of lens defect causes colored rings around focused objects?

The phenomenon of colored rings appearing around focused objects is primarily associated with chromatic aberration. This defect occurs due to the different wavelengths of light being refracted by different amounts when they pass through a lens. Since lenses do not focus all colors to the same point, this results in the blurring and color fringes—often seen as colored rings—around the edges of objects, particularly in images taken through lenses that are not designed to minimize this effect.

Chromatic aberration is common in photography and optics when using simple lenses and can be mitigated by using achromatic or apochromatic lenses, which are specifically designed to correct this dispersion of light. The presence of this defect highlights the importance of lens design in optical clarity and color fidelity. Understanding this concept can help in selecting the right equipment for various optical applications, as well as diagnosing issues related to image quality in photographic and viewing situations.