How do you use a gray card for exposure and white balance?
Asked 1/19/2011
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I understand what a gray card is, but I’m not sure how to use one in practice. How do photographers use a gray card to set exposure and to correct white balance, either in-camera or later in post-processing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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There are a few ways. Here are the main two I use:
before you start shooting a scene/pose have someone hold the card in your primary light and hit it with your in camera meter. (in spot meetering mode, ideally with the card fully filling your spot meter area, even if that means zooming in.) set your exposure such that that is "properly exposed". Now remove the card and shoot away. Your main light source should be properly exposing subjects.
first shot of a session (or anytime you change the lighting substantially) slip the grey card into the shot in the main key light. Take a "throw away" shot with the card in it. If you have multiple light sources with radically different color temperatures, you'll want to shoot one with the grey card illuminated by each light source. When you're editing, use the grey cards in those shots to get your white balance adjustments, then mass apply them to the entire shoot.
There is also the "historical" use with dark room printing... shoot a frame with the card in it, even if it's way off to the side out of crop, then in the enlarger, slide the film so that you can print that bit, do a test strip of the card, develop, match the box on the test strip to your grey card to get the initial printing time for the negative.
/me wanders off to FIND my old dektol stained gray card... been using a modern white/grey/black target with digital lately... not sure where it is. ;)
Originally by user1872. Source · Licensed CC BY-SA 4.0
user1872
15y ago
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A gray card is mainly used for two things: exposure and white balance.
For exposure: place the card in the same light as your subject, ideally facing the camera. Use spot metering if possible and fill the metering area with the card. Adjust exposure so the meter reads the card as a proper midtone, then remove the card and shoot. This gives you a reliable exposure reference for that lighting.
For white balance: take a reference shot with the gray card in the scene under the same light as your subject. Don’t use Auto White Balance if you want consistency; choose the closest fixed WB setting first. Later, in editing, use that frame to set neutral white balance and apply it to the rest of the images taken under that same lighting.
Many digital cameras also let you create a Custom White Balance from a photo of the gray card. This is especially useful for JPEG shooting, since RAW gives you more flexibility afterward.
If the lighting changes, or if you have different light sources with different color temperatures, shoot a new gray-card reference for each setup.
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