Most AC systems have a built in De-humidifier, which is why it is best to leave them on auto. In my last car (A mitsti FTO) you could also run the AC in ECO mode which switched off the de-humidifier and therefore misted up the windows in damp conditions, if yours mists on AUTO maybe you have a problem with the de-humidifier, could be worth getting it checked by a dealer.
My experience with the IS is that if you turn off the AC or swith off AUTO mode it can start to mist up, (worse if you've had a shower or if the other half has washed her hair!) but switching to AUTO clears rapidly.