What term describes the gradual loss of sharpness toward the edges of an image?

The term that describes the gradual loss of sharpness toward the edges of an image is vignetting. Vignetting occurs when the brightness or clarity of an image diminishes at the periphery compared to the center. This phenomenon can be due to various factors, such as lens limitations, exposure settings, or light fall-off, particularly in wide-angle or certain types of lenses.

In imaging, particularly in radiologic contexts, understanding vignetting is important because it can affect the perception of an image's detail, potentially impacting diagnostic accuracy. While blurring generally refers to an overall lack of focus rather than a specific gradation toward the edges, halation deals with the spreading of light around bright areas in an image, which is not the same as a gradual loss of sharpness at the edges. Edge enhancement, on the other hand, refers to techniques used to increase contrasts at the borders of objects within an image, aiming to provide a sharper overall appearance rather than describing a loss of sharpness.