Hi In a recent Outdoor Photographer edition Mr. Raia tells us about using a grey ca rd instead of a split neutral densitiy filter. He says that by this way you won' t see immediately that the image is "filtered". If he needs to compensate for a two stop difference he simply holds the card in front of the lens for say three seconds over the brighter part while the exposure time is say four seconds. Is a nybody familiar with the tecnique and can tell me how I control that the card is hold and fixed in the right spot in front of the lens? Thanks Tom