Parfocal Zoom coupler
Parfocal Zoom coupler（Continuous Endoscopic Zoom coupler F20-F35）
A parfocal lens is a coupler that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but too small to be considered significant.
Parfocal Zoom coupler (sometimes referred to as "true" zoom) are ideally parfocal, in that focus is maintained as the lens is zoomed (i.e., focal length and magnification changed), which is convenient and has the advantage of allowing more accurate focusing at maximal focal length then zooming back to a shorter focal length to compose the image.
Many zoom coupler, particularly in the case of fixed-lens cameras, are actually varifocal coupler , which gives lens designers more flexibility in optical design trade-offs (e.g., focal length range, maximal aperture, size, weight, cost) than parfocal zoom, which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming"), making operation practically indistinguishable from a parfocal zoom.
