If you have ever looked at the price of cinema lenses, your eyes probably watered a bit at their seemingly stratospheric cost. There are lots of reasons for those high prices, one of the most common being that they are parfocal. This interesting video explains what it means for a lens to be parfocal and why it is so desirable among filmmakers.
Coming to you from Michael the Maven, this great video details what a parfocal lens is and why it is so useful for videographers. The majority of zoom lenses are varifocal lenses. This means that they are lenses in which the focus changes as the focal length changes. On the other hand, parfocal lenses do not change focus as the lens' focal length is changed. This offers several advantages. For example, this allows one to zoom in to focus, then zoom back out to compose an image. This is sort of operation is crucial for camera operators, for whom reasonably maintaining focus manually while zooming at the same time would be very difficult. Photographers almost never need this kind of functionality, so manufacturers make varifocal lenses, which are far cheaper. Check out the video above for more on the topic.