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A dew shield is actually a bit of a misnomer because dew doesn't fall from the sky as commonly thought ! Dew forms as a result of the right combination of humidity and air temperature. All a shield does is stop the glass from having so much heat sucked from it by 2.7 kelvin night sky.
You could have a dew shield a meter long and still have dew forming on your optics... Obviously some scopes are more prone than others (SCT with a huge exposed glass corrector plate cooling below ambient etc)... But even my 16" dob which has a effective 1.5m long dew shield for the primary will dew up on a poor night without thermal management.
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