Originally posted by dosdan I've been shooting junior athletics & soccer with a K20D, K-5 & K-3 with a DA* 60-250/F4 lens since 2010. In heavy rain the deep lens hood on this lens works well as long as it's not too gusty. However, after 1-2 hrs, with the internal volume change involved in continual zooming in & out, the lens will start to mist up. (At least for me, it does.) You need to get under cover, take the lens off, wipe the rear lens element and pump the zoom in & out a number to times to reduce the amount of moist air trapped inside the lens.
I often use a DA 55-300/4-5.8 WR lens (on a K-3), and, much more often than not, it's racked all the way out to 300mm for shooting. I try to minimize zooming from 55mm (the usual "parking position") out to 300mm and back too often when it's damp/wet or dusty, simply to minimize sucking air in and out too often -- it's better, methinks, under such conditions, to walk around with the lens extended to 300mm between shots, even if it's a bit more awkward to do so.
I also like to use an old Takumar metal lens hood screwed in solidly (sorry - off the top of my head I can't remember exactly which one - it might be for the 135/2.5 and 200/4 (?) - in any event, it's similar to the shape and size of the OEM hood). While the plastic OEM bayonet-mount hood seems reasonably solid when mounted, it has wide air spaces (which are also water spaces) around its base where water can more easily get to the front element, even when just dangling from a neck strap or otherwise aimed downward -- the screw-in Tak hood requires any water that "wants' to get to the front of the lens to have to blow in, or to fall in when the lens is aimed level or higher.
Finally, "take the lens off, wipe the rear lens element and pump the zoom in & out a number to times to reduce the amount of moist air trapped inside the lens" is ~excellent~ advice, and I would just add that it is worth repeating the "pumping" action several times, not just when first getting the lens and body back inside, but every once in a while over the next few hours. [I think it's also worth "airing out" the cam body a bit (even using a blower a few times) before storing it with its body cap (or any lens) on it.]
Just my 2 f-stops worth...