The telescoping design means that the lens sucks in air when zoomed from shorter focal lengths to longer focal lengths. Unfortunately, with the lack of anything to prevent it, it also sucks in any airborne dustCanon published lens data – The EF 70–200mm f/2.8L IS; EF 70–200mm f/4L IS USM and EF 70–300mm f/4.5–5.6L USM have dust sealing. The published data shows that the 100-400 mm lacks this sealing.
