version I released May 2012, this is Panasonic's 1st weathersealed lens and its first zoom lens with constant aperture throughout the zoom range
giving a 35mm equivalent field of view of a 24-70mm lens, it is half the size of its f/2.8 24-70mm full frame counterparts, with much faster and more accurate AF of still subjects, but of course does not offer the same ability to have narrow depth of field (DOF)
nevertheless, the f/2.8 aperture combined with its compact size, weatherproofing, high image quality and image stabiliser means it will be an awesome travel lens in particular.
version ii announced in Jan 2017 offers better weathersealing, and smoother aperture variance when zooming
both lenses can support Dual Sync IS (version I needs firmware upgrade)
12-35mm f/2.8 = 24-70mm field of view in 35mm full frame terms