defocus

Defocus is the axial distance between where light converges (the focal point) and where you actually need the image to form, whether that is a camera sensor, photographic film, or a workpiece being inspected. Positive defocus occurs when the focal point sits in front of the image plane; negative defocus when it lies behind. The magnitude is measured in millimeters or microns, and even small amounts degrade image sharpness and contrast.

In machine vision systems, defocus is often the largest single cause of measurement error and false rejects. A 50 micron defocus at 10x magnification can shift the apparent edge position of a feature by several micrometers, enough to fail parts that are actually in tolerance. In lithography, defocus of just 0.1 micrometers begins to degrade pattern transfer. The relationship between defocus distance and image blur is not linear; it depends on the numerical aperture of the lens, the wavelength of light, and the allowable circle of confusion for the application.

Sources and compensation

Defocus arises from thermal expansion of optical assemblies, mechanical play in focus mechanisms, tilt of work surfaces, refractive index changes in optical media, or simply incorrect manual focus setting. In production environments, it accumulates over a shift as parts stack with varying thickness, or as machine temperature drifts. Autofocus systems, which measure defocus and drive a lens motor to null it, have become standard in critical applications; they typically use contrast detection or phase detection to determine the direction and magnitude of defocus in real time.

The term draws from optics geometry: the prefix de means away or reversal, and focus comes from the Latin for hearth or point of concentration. Unlike simple out-of-focus, which is qualitative, defocus is measurable and quantifiable, making it the preferred term in technical work where focus error must be specified or corrected to within a tolerance band.