The use of dry ice to create a smoky fog effect is well known, but as the fog is actually created by the condensation of pure water vapour into tiny water droplets it can be used to soften the focus of a shot very successfully.
By directing the fog to where the soft focus is required and keeping the areas where sharp focus is needed free can create effects that are not possible with camera settings. To keep the sharp focus areas clear the use of small low velocity air jets is helpful.
These effects can be very successful when photographing food, and dry ice fog can also be used in place of steam, which is not particularly welcome in the studio. The right amount of dry ice in warmish water in a kettle or pot can easily be made to look like it is boiling or simmering without the inherent dangers of working near a hot hob.