The short story is, always carry several Ziploc Bags in your camera bag. You never know when you might need them.
I was well over a mile from my car when I was
caught in a heavy downpour. There was no question about whether or not I would get soaked - I would. I had no rain gear or umbrellas.
The major concern was my
equipment. I was using a Lowe-Pro backpack that is not on of the AW (all weather) versions but I know from past experience that it will keep your gear dry unless you are in the rain for an extended period of time. I had
a 30 minute walk to the car with little or no cover, short enough time that everything should be OK.
My backpack was full and I had a camera and lens around my neck.
I tried but there was no way I could stuff them into my already bulging backpack (a Micro-Trekker with one body, five lenses, flash, and more stuffed inside).
So I opened my backpack, took out a gallon size Ziploc Bag
and put it around my camera and lens. I zipped the bag shut except where the camera straps came out. If need be, I could have put the straps in the bag and zipped it entirely shut.
When we got back to the car I checked my gear. The heavy downpour was enough that my bag had a tiny amount of leak-through, but not enough to damage my gear. Of course, it
isn't supposed to be waterproof, but it did very well under the conditions.
Had I been a lot farther from my vehicle, I had enough Ziploc Bags in my backpack to bag
camera bodies, lenses and flashes INSIDE the backpack for additional protection.
I have also used Ziploc Bags as rain protection while taking pictures in the rain. Poke a small hole in one corner of the bag, push the
front lens element through the hole, put on the lens hood for protection and you are ready to go.
("Ziploc" is a trademark and its use is subject to legal limitations).