Hi,
Most times you would be better to forget the grey card and use an incident meter reading whether using flash or available light. The problem with the gray card is that you have to make sure that you're reading the reflected light only from the card and that the meter isn't reading other obejects in its field of view. You can only really do this if your camera or meter has spot metering.
If you use the white dome on the meter and go up to your subject, pointing the dome towards your shooting position, the meter will read the light falling onto the subject very accurately.
If you're doing all of this and are still getting odd readings then check the battery!! You never know??
Paul
|