parfocal การใช้

A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes.
Note, many of today's wide range zoom lenses are not " parfocal "; that is, not true zooms.
A suggestion by K鰄ler led to the development of parfocal lenses which allow the specimen to remain in focus when changing objectives on a microscope.
Many zoom lenses, particularly in the case of fixed-lens cameras, are actually " varifocal lenses ", which gives lens designers more flexibility in optical design trade-offs ( focal length range, maximal aperture, size, weight, cost ) than parfocal zoom, and 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.
Many zoom lenses, particularly in the case of fixed-lens cameras, are actually " varifocal lenses ", which gives lens designers more flexibility in optical design trade-offs ( focal length range, maximal aperture, size, weight, cost ) than parfocal zoom, and 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.