 While the original version has excellent optical performance, its design predated the mass-market availability of digital single-lens reflex cameras.The telescoping nature of the original design means that the lens zooms quickly, although not all users prefer this design.According to Roger Cicala, the head of the American lens rental house "LensRentals.com", 

The telescoping design means that the lens sucks in air when zoomed from shorter focal lengths to longer focal lengths.