3" - 4" Dew strap on the 17-40 works a treat.
However be careful if using a larger one. While needing a dew strap at Duckadang I borrowed on off The h0ughinator, but it was a bit big (meant for a 10" scope
). So I wrapped it around a couple of times and set it going. It was a fairly heavy dew so I set it about 3/4 (lots of glass on the 17-40). I came back later to check only to find dew on it.
Or what I thought was dew.
The moisture was ...on the inside!!!!! I had turned the strap up too high and it had driven all the moisture in the lens onto the coolest surface, the inside of the lens itself, just like a boiling kettle and a window.
Pity I couldn't have vented the lens at the same time and then perfectly sealed it. I wouldn't have had to worry about fungus on the insides after that.
