I have a really quick and easy 16-35 question. Is the rear element of the 16-35 "open" to the inside of the lens like the 17-40 or is it sealed off? This has always seemed to me the weak link in the 17-40's sealing -- there is no doubt that my 17-40 has accumulated dust inside it, probably through the rear element since I've always had a filter on the front.
It is the same. The rear element needs to get back as close to the mount as possible at the wide side so block glass like telephoto is not possible. Dust can come in and out via lens element movement even with a block glass so there is no silver bullet to prevent it.
One thing to prevent dust from entering from the rear is to constantly clean your lens end cap as it seems always attracts dust bunny.
