possible. Step 3: Lock on the target The next step is to press the AEL (or *) button on the back of your camera so that you can take an exposure reading from the grey card. Step 4: Back to the main feature Now you need to remove the grey card, recompose the scene and take the picture of your white object -- the whites should now look bright and clean! Step 5: If you grey card isn't big enough If you are unable to fill the frame with the grey card, you may need to change your camera's metering mode, which is located in the shooting menu. Step 6: Spot treatment Using spot metering means that the grey card need only occupy the very central part of the frame when you use the exposure lock.