A lens that maintains the same distance of focus as it is zoomed is said to be parfocal. This is a highly desirable quality in a lens if one is shooting film or video and wishes to maintain focus while zooming in or out during a continuous shot.
Some zoom lenses that are not really parfocal, particularly those with smaller maximum apertures, can appear to act like they are parfocal if the focusing error introduced by zooming is less than the depth of field at the lens' widest aperture.
Roger Cicala recently addressed the subject in his blog entry Mythbusting: Parfocal Photo Zooms (Bold type added by me).
About twice a week I get a call from a photo-videographer wanting to know which photo zoom lenses in a certain focal length are parfocal (stays in focus while changing focal lengths). It’s understandable why they would ask; a superb photo zoom is 1/10th the cost of a good video zoom.
They get pretty mad when I tell them that none of them are. Often, they’ll tell me they know this lens is or that one is, because LensGuruGod1232 on their favorite forum shoots with this lens and says it is. They may even – if they’ve done some research – pull up an old article I wrote years ago listing some photo lenses that were parfocal and add that to the argument.
I have to tell them that article doesn’t count anymore. For one thing, parfocal is not an absolute definition. What is acceptable as parfocal to one person in one situation is unacceptable in another. Also, equipment has changed. A lens that might have appeared parfocal on a small sensor shooting standard definition video may be obviously not parfocal on a large modern sensor shooting 4k video. Finally, I’ve gotten older and wiser. Some things I wrote about years ago, I now realize are, um, well, less correct than I would have liked.
He goes on to point out that one copy of a specific lens model can demonstrate more or less parfocality than another copy of the same lens model.
Similarly, most cinema lenses also demonstrate very little or no focus breathing. Focus breathing is a change in the lens' field of view as the focus distance is changed. This is also a desirable feature for cinema lenses so that the focus distance can be changed in a continuous shot while the exact same framing is maintained.
Lenses designed for still photos are generally used to take much higher resolution photographs than lenses designed for video. Still photographers generally care about absolute image quality more than whether a lens is parfocal or doesn't demonstrate focus breathing.
The emphasis on still lenses is absolute optical image quality. The emphasis on non-Cinema quality video lenses, particularly those in the same price ranges as lenses primarily intended for still photography, is being 'close enough' to parfocal and not demonstrating focus breathing while being 'good enough' optically for HD or 4K video.
Is the Canon EF 24-105mm f/4L IS USM lens parfocal or not?
That all depends upon how one defines parfocal. To a cinematographer, the answer would be an unqualified, "No!"
To a stills or video shooter who wants to be able to focus at 105mm and then zoom out without refocusing, the answer would be, "Probably, depending upon the specific copy you are using."
By the strictest definition of parfocal, the EF 24-105mm f/4 L Is is not such a lens.
The EF 24-105mm f/4 L IS is a lens that can be "functionally parfocal."
When long-time Canon technical advisor Chuck Westfall released a list of such lenses back in 2004, he later had to explain that the list was in the context of lenses focused at their longest focal length and then zoomed out to wider focal lengths so that the focusing error introduced was smaller than the increase in depth of field due to the shorter focal length. He went on to clarify that the obverse, focusing at the wide end and then zooming in, would not work within the margin of error allowed by the narrower depth of field at the longer focal lengths with most of those lenses. The EF 24-105mm f/4 L IS was introduced in 2005, the year following Chuck's list of "functionally parfocal" EF lenses, and by most reports seems to be more parfocal than many of the lenses that Chuck included in his 2004 list.
There have been anecdotal accounts that certain specific copies of the EF 24-105mm f/4 L IS can be "close enough" if the initial focus distance is anywhere beyond about 10 feet. Even when those copies are set at 24mm and focused beyond 10 feet or so the lens can be zoomed from 24mm all the way to 105mm and the focus remains acceptably sharp for most use cases. In other words, the focus distance may shift a little bit, but that shift is small enough to not be noticeable in typical use cases. But that is far from the same thing as saying a lens is truly parfocal and does not shift the focus distance at all as it is zoomed in or out.
Officially, Canon only lists certain of their cinema grade lenses as being parfocal.