focusblur

Focusblur is a term used to describe the visual effect in which parts of an image or scene appear blurred because they are not at the plane of sharpest focus. It is a subset of blur that arises from limited depth of field and is distinct from motion blur, which results from movement during exposure. Focusblur can occur in photographs, videos, and digital renderings when the camera’s focal plane does not align with the subject or when the lens aperture is wide.

Causes of focusblur include shallow depth of field, incorrect focusing distance, and lens characteristics such as focal length and optical design. A large aperture (small f-number) reduces depth of field and increases the likelihood of out-of-focus regions, while longer focal lengths and closer subject distance accentuate the blur. Optical misalignment, tilt or shift of the lens, and focusing errors also contribute to focusblur. The quality of the blur is influenced by the point spread function and, in imaging science, by the circle of confusion that defines what is perceived as a point.

Measurement and correction involve both optical and computational approaches. In optics, focusing techniques, accurate autofocus, and depth-of-field planning help minimize focusblur. In post-processing, deconvolution methods can partially recover detail, though true restoration is limited by information loss. Artists may also deliberately employ focusblur to guide attention or create aesthetic depth through controlled bokeh.

Applications range from artistic photography and cinematography, where selective focus isolates subjects, to technical fields such as microscopy and virtual rendering, where blur is modeled to simulate real-world optical behavior. See also depth of field, bokeh, and point spread function.