When measuring ambient light with a gray or white card for white balance, keep in mind that your camera does not need to focus on the card. In white balance ambient light reading mode—or whatever your camera calls this mode—it will not focus anyway since it is only trying to read color temperature 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 to 100 mm) away from the card is the correct distance for most lenses.
Be careful that your lens does not cast a shadow onto the card in a way that lets your camera 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. This problem is not as common because most gray cards have a matte surface; however, it can still happen. You may want to hold the card at a slight angle to the source light if the light is particularly bright and might cause glare.
Finally, when the light is dim, use the white side of the card (if your camera supports white) since it has more reflectivity. This may prevent a bad 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, and your camera supports it, a gray card is best for balancing. You might want to experiment in normal light with your camera to see which you prefer.
Keep on capturing time...
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