nearparfocal

Nearparfocal is a term used in optics and imaging to describe systems in which the focal plane remains approximately constant when switching between magnifications or objectives. In a parfocal system, changing magnification requires no focusing adjustment, while nearparfocal means only a small focusing correction is needed. The degree of nearparfocality depends on factors such as mechanical tolerances, lens design, tube length, and the numerical aperture of the objectives. It is typically an informal or vendor-used descriptor rather than a rigorously defined standard.

In practice, nearparfocal behavior is advantageous in microscopy and automated imaging workflows because it allows faster transitions between objectives and smoother data acquisition. Assessing nearparfocality involves switching between objectives or zoom positions and measuring the focal offset required to restore sharpness; smaller offsets indicate higher nearparfocal quality. Common applications include compound and stereo microscopes, digital imaging systems, and automated slide scanners, where reducing focus changes accelerates scanning and reduces user intervention.

Limitations include that nearparfocal does not guarantee perfect focus across all depths of field or sample thickness. Extreme magnifications, high numerical apertures, or significant mechanical imprecision can yield larger focus offsets. The concept is distinct from parfocal, and some sources refer to nearparfocal or quasi-parfocal behavior interchangeably with limited precision.