1. A zoom lens that remains in focus as you zoom from one focal length to another.
-Why you should care
2. If you manually focus by zooming in on a subject and then zooming out to take a picture.
3. If you want to zoom while shooting a video segment.
4. Some feel the 70-200mm L IS lens is not parfocal at distances less than 20 feet from the camera 
5. I want to get a parfocal zoom lens with the longest zoom range possible.
6. par + focal (par = a level of equality)
7. DPReview.com: Not all zoom lenses are parfocal
8. DVInfo.com: Is a parfocal lens necessary?
9. Doug Chinnery Photography: Canon Parfocal Lenses
10. Wikipedia: Parfocal lens
Canon does not officially list lenses as being parfocal, but these L lenses are considered by most to be parfocal. You can find a list of other lenses at .
- Canon EF 16-35mm f/2.8L USM [B & H $1,520.00 (take $100 off until 7/10)]
- Canon EF 24-70mm f/2.8L USM [B & H $1,399.00 (take $90 off until 7/10)]
- Canon EF 70-200mm f/2.8L IS USM [B & H $1899.00. There is also a IS II model for $2,499]