varifocal

Varifocal is a term related to zoom lenses, and indicates that the zoom is constructed in such a way that the focusing distance does not remain stable while zooming. In effect, you always need to refocus after changing the focal length.

The opposite of a varifocal (zoom) lens is a parfocal zoom lens. Some consider that only parfocal lenses should be called ‘zooms’, whereas varifocal lenses should be referred to as ‘variable focal length” lenses.

While it would surely be desirable for a zoom to be perfectly parfocal, there is a trade-off as parfocal zooms tend to be bigger, heavier and more expensive.

If you want to test your zoom for whether it is varifocal, keep in mind that parfocal and varifocal are the ends of a spectrum, and while most modern still photography zooms evidence decent levels of parfocal behavior (i.e. the focus is not waay off after zooming), very few zoom lenses are truly parfocal without having been expressly designed for it.