Nonparfocal

Nonparfocal refers to a characteristic of a camera lens in which changing the focal length (for example by zooming) causes the image to lose sharpness at the previously selected focus distance, requiring refocusing. In a nonparfocal lens the focal plane shifts when the lens elements move during zoom, so subjects at the chosen distance may go soft after zooming. By contrast, a parfocal lens maintains focus across its zoom range, allowing the user to zoom without refocusing or with minimal adjustment.

Practical implications differ between photographic contexts. Many consumer zoom lenses are nonparfocal, so photographers typically refocus after zooming or rely on autofocus to reacquire sharpness. In videography, parfocal performance is often desirable because it prevents a visible focus shift during camera moves; cine lenses and some higher-end zooms are designed to be parfocal or to behave closely to parfocal standards.

Testing and use: A simple test is to focus on a distant subject at one focal length, then zoom to another focal length and check whether the image remains sharp. If not, refocus is typically required. Autofocus systems can help reacquire sharpness, but there may be a brief delay during zoom transitions.

Design notes: Nonparfocal is a descriptive term about how the lens behaves during zoom. It reflects trade-offs in optical design, cost, and size. Some lenses are designed to be seamlessly parfocal across their entire zoom range, while others maintain acceptable focus only within part of the range.