Tips for using a white/gray card: When measuring WB with a gray or white card, keep in mind that your camera does not need to focus on the card. In PRE mode, it will not focus anyway, since it is only trying to read light values, not take a picture. The important thing is to put your lens close enough to the card to prevent it from seeing anything other than the card. Three or four inches (about 75 mm–100 mm) away from the card is about right for most lenses.

Also, be careful that the source light is not casting a shadow from the lens onto the card in a way that lets your lens see some of the shadow. This will make the measurement less accurate. Also, be sure that your source light does not make a glare on the card, which is a little harder to do since the card has a matte surface, but it still can be done. You may want to hold the card at a slight angle to the source light if it is particularly bright and might cause glare.

Finally, when the light is dim, use the white side of the card since it has more reflectivity. This may prevent a No Gd reading in low light. The gray card may be more accurate for color balancing, but might be a little dark for a good measurement in dim light. If you are shooting in normal light, the gray card is best for balancing. You might want to experiment in normal light with your camera and see which you prefer.