Unsharpness

Unsharpness is a quality defect in an image where fine detail is not rendered with clarity, resulting in soft edges and reduced perceived sharpness. In photography and optics, it describes the departure from ideal sharpness and can arise from a range of factors, including focus accuracy, motion, optical limitations, and sampling or processing choices. Sharpness is often discussed in terms of acuity, edge contrast, and acutance.

Common causes include focus errors (front or back focus, incorrect focusing distance), subject or camera motion during exposure, and insufficient depth of field. Optical factors such as lens aberrations (spherical aberration, coma, astigmatism), diffraction at small apertures, and field curvature can also degrade sharpness. Sensor and processing effects, including pixel size, demosaicing, sharpening or smoothing during editing, compression artifacts, and resampling, can contribute as well.

Sharpness is influenced by both optical performance and imaging workflow. Objective measures such as modulation transfer function (MTF) quantify how well a system preserves contrast at different spatial frequencies, while perceptual sharpness depends on edge clarity and local contrast. In practice, photographers manage unsharpness by balancing exposure, focus accuracy, and depth of field, using stable support, faster shutters, appropriate apertures, and high-quality lenses. Post-processing can apply selective sharpening or deconvolution, but excessive sharpening may amplify noise or create halos.

Unsharpness can be minimized, but in some cases it is used deliberately to convey a sense of softness or motion.