How does blurring out the background of a photo make the main subject look really sharp and stand out like it's 3D?

The blurring of a photo's background, a technique known as shallow depth of field or selective focus, makes the main subject appear exceptionally sharp and stand out with a three-dimensional quality by creating strong visual separation and utilizing specific monocular depth cues. Shallow depth of field refers to the photographic state where only a narrow range of distance from the camera is in sharp focus, while objects closer or farther than this range appear progressively blurred. This effect is primarily achieved by using a large lens aperture (represented by a small f-number), a longer focal length, or positioning the subject relatively close to the camera. When the background is blurred, it loses fine detail and texture, becoming a soft, undifferentiated area. This creates a stark contrast in sharpness between the highly detailed, in-focus main subject and its out-of-focus surroundings. The human visual system is naturally drawn to areas of high contrast and clarity, instinctively directing attention to the main subject. This visual prioritization makes the subject appear exceptionally clear and defined against the soft background, significantly enhancing its perceived sharpness. Furthermore, this technique contributes significantly to the perception of depth, making the subject appear three-dimensional. Our brains interpret the differing levels of blur as indicators of distance, which is a monocular depth cue. The sharp subject is perceived to be at a specific focal plane, while the increasingly blurred background objects are understood to be further away. This blur gradient, where sharpness transitions smoothly into blur, provides crucial information about the spatial relationship between the subject and its environment. Objects that are relatively clear are perceived as closer, while objects that are increasingly blurry are perceived as farther away; this concept is known as relative clarity. This separation of planes in terms of focus creates an illusion of depth, giving the impression that the sharp subject is physically lifted off or standing in front of the blurry background. By removing distracting details from the background, the subject's edges and form become more distinct, solidifying its identity as the figure against the indistinct ground. This reduction of visual competition allows the viewer's brain to process the subject's contours and volume without interference, reinforcing the perception of its three-dimensional form.