For any camera, not just an istD,
You want to move the snow from Zone V (middle gray) to Zone VII (white with plenty of texture). How to do that (in decreasing levels of desirability)?
1. Spend the time to learn and understand the Zone System.
2. Use manual exposure, open the lens two stops.
3. Use auto-exposure, set the compensation dial to +2.0
4. Use auto-exposure (no comp dial), set the film exposure -2 stops (for example: set 100 speed film at EI-25).
5. (Camera with no adjustments) Use something like Kodak Max (and hope the lab people know snow is white).
Two stops is the old rule-of-thumb exposure compensation for snow. With experience you may use anything between +1.5 and +3.0 depending upon the effect you want.
Of course you can go buy an incident meter (folks who have been here awhile know I am an incident meter advocate) or a gray card, but realise you have to know how to use those to get things right just like with everything else. The methods I mentioned above do not require spending extra money (well except #1 which requires an investment of money and time). I usually use #2 myself.
graywolf http://www.graywolfphoto.com "Idiot Proof" <==> "Expert Proof" -----------------------------------
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