I heard someone say that after using a parfocal lens for the first time, they would never switch back. What is a parfocal lens, how is it different from any other lens, and what are the advantages and disadvantages for photography? Would you pay extra for this or prefer it over varifocal?
Parfocal lens is a lens which remains in focus when you change the focal length. The non-parfocal lens is called varifocal.
It is very convenient to focus at the maximum focal length and change the zoom afterwards. It is more important for manual-focus lens because a well functioning auto-focus can quickly adjust the lens to keep it in focus.
Parfocal lenses remain in focus when you change the focal length as previously stated. They are most useful for movie/video work - it's awkward to keep having to pull focus as your zoom changes.
See also 'focus breathing', where a change in the focus changes the apparent focal length slightly. Again this is something it's nice to avoid in movie/video work. In general the more you pay, the less they breathe...
Lens design is always a series of trade-offs, if money, size and weight are no object you can build a lens that will be parfocal, fast and have high quality optically. So in the movie business they end up with lenses like this: http://www.flickr.com/photos/mrmitch/2185341989/in/photostream
A valid use case rom this article seems to be to shoot landscapes at wider aperture:
The key though, is, if you have a parfocal lens, use it to shoot at the hyperfocal distance (again, good old Wikipedia can help with an explanation of this - http://en.wikipedia.org/wiki/Hyperfocal_distance ) at a wider aperture than usual in landscapes. This gets us away from f22 and f16 down to sharper apertures like f13 and f11 - while still maintaining front to back sharpness. Many pro's, like David Noton, use this. Look at the technical info on many of his landscapes and they are shot at f11.
I have noticed that most of the parfocal lenses have zoom ratios of 3:1. Maybe this holds the answer: in my study of photography, lenses that have a zoom ratio of 3:1, 2:1, and 1:1 have better quality compared to higher lens ratios, because they produce less chromatic aberration. In my opinion, parfocal lenses have are better since their aberration is less compared to lenses that have zoom ratios of 4:1 or higher.