Does The Lens Breathe?
A lens that breathes changes the size of the image slightly when your focus from near to far or vice-versa. Most photography lenses do. If you look at the Specs we post for various lenses, any lens that has “rear element focusing groups” will definitely breathe, sometimes dramatically. Lenses with a “floating element” (used to improve close up sharpness) will also breathe.
Is The Lens Parfocal?
Some lenses are parfocal, meaning that zooming does not change the focus point. All cine zooms are parfocal, but very few photography zooms are -- for the same reason that they breathe, and for lower costs. If you want to zoom while filming, its important to make sure the lens you choose is parfocal, and remember, most are not.
The following lenses are constant focus while zooming. If its not on this list, it is not parfocal.