Unclear whether grey card can be used for exposure in post production?

Question

I only use a grey card a little, but need to use it more to get more consistent product photo's. I understand how to use it to adjust white balance in camera, in Photoshop and Camera Raw, however I'm unclear whether it can be used to adjust exposure?

Several posts online say that grey card can be used for exposure adjustment, however I can't find any that actually provide details on how to adjust in post. When I use the White Balance tool in Camera Raw, it only change the White Balance temperature and tint. And in-camera, I believe it only changes white balance.

Can someone clarify if a grey card can be used to adjust exposure across a set of images to result in even exposure, and if so, how can it be done, either in camera or (preferably) in post-production.

Many thanks for your time in advance.

Answer 1

To use a photo of a gray card to adjust white balance you need to adjust white balance (color temperature along the blue←→amber axis and tint along the perpendicular magenta←→green axis) until the values of the red, green and blue components of the gray card are all the same. When you hover the cursor over the gray part of the photo the values should be something like (128, 128, 128). Pure green, for instance, would be (0, 255, 0), pure red would be (255, 0, 0), and pure blue would be (0, 0, 255).

When we use a "color picker", the program tends to average the three values for the R, G, and B components and set all three to that value. Sometimes the automated routine will take other factors into account as well and bias the results based on them. That's why we can get really wacky colors if we click on an area that is totally blown out in all three channels.

To use a photo of a gray card to adjust exposure when post processing photos you just need to change the brightness of the image until the area of the gray card has a value of 128 on the 0 to 255 luminance scale.

When you have properly adjusted both white balance and exposure, you should wind up with an RGB value of (128, 128, 128) for the area of the image with the gray card.

Keep in mind that using only an 18% gray card to adjust exposure does nothing to set the brightest highlights at or just below 255 and the darkest shadows at or just above 0. It only sets an 18% gray card to 128/255. To set blacks and whites you need to use a black and white card and adjust using the shadows and highlights adjustments (or the ends of the "curves adjustments) after setting the mid-tones using the gray card.

Answer 2

You're asking two questions here:

1. How can I use a grey card to get the proper white balance?
2. How can I use a grey card to get a proper exposure?

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1: To get proper white balance with a grey card you have two options: You can adjust in camera or in software (provided you shoot in RAW). To adjust in camera you want to find the proper method based on the camera you have. For instance, with my Canon 6D I would take a picture of the grey card where it takes up the whole frame. Then I would go to settings and select custom white balance and select that photo. The camera would calculate what it needed to do to properly adjust the white balance.

If you want to make the change afterwards it depends on the software you use. In Lightroom there is an eyedropper tool for white balance that you can use. It allows you to mark a point on the picture that is fully neutral. You would take a photo of the grey card and then select it with the eye dropper. Then you want to sync the change across all photos in the set. Unfortunately, Photoshop (and Adobe Camera Raw in Photoshop) doesn't have that option. Instead you would have to use dead reckoning.

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2: Automatic exposure metering will meter perfectly off of a grey card. Place the card where you want to have it perfectly exposed, and then see what your camera selects for the auto exposure. Dial that in manually and you should have a perfect exposure.

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You can find more resources here:

• Digital Photography School
• Digital Photo Mentor

Answer 3

Two photo scientists, in the 1890’s devised a way to precisely expose films and papers and how to measure the resulting blacking induced when the material is developed. The measure they use is called “density”. This is a numerical value assigned to how much light the blacking retards. The inverse is how much light the blacking allows to pass, this measure is the percent transmission.

As you know, films and papers display a range of tones. Then center of this range is a battleship gray. The gray card is a placard that reflects 18% of the light that is incident to it. Incident is old French meaning, about to happen.

In the mid 1930's, Messrs Jones and Condit at the Kodak Laboratory determined that statistically, a typical sunlit scene integrated to a reflectance value of about 18%. About this time, the Western Electric Company brought to market the first light meter. Kodak Labs publish a recommendation; place a Kodak film box in the scene. Seems the box reflected 18% of the ambient light. Now measure the reflected light from the box top and use this reading to set your exposure.

In 1941, Ansel Adams, a prominent landscape photographer, and his friend, Fred Archer, a photo magazine editor, jointly published the Zone System which provided photographers with a method to precisely fine-tune exposure. Their zone system revolves around the use of an 18% placard (battleship gray). This card replaces the Kodak box top. The 18% gray target became the de facto standard. Today film and paper speed as well as the digital chip are calibrated, and film and digital ISO is established using the 18% gray card.

This shade is unique because if an object that reflects 18% is photographed, the resulting correctly exposed and processed film image of this object will measure 0.75 transmission density units. Additionally, if this film is correctly printed and processed, that object on the print will also measure 0.75 reflected density units. The uniqueness is, object and film and print all measure the same density when exposure, processing, and printing are spot on.

Again, the 0.75 density value is well-thought-out to be the center of the scale of film and paper. This value is the defacto standard used to calibrate instruments used to measure photographic films and papers and this value transfers to light meter calibration also.

The scale of the digital image in 8-bit ansi terms = 128. We can also call this value Zone V in the Ansel Adams Zone System.

If the same scheme is applied, an image of a gray card, if properly repented would likely have a value of approximately 128.

Answer 4

If you were to photograph just the grey card at a perfect exposure, you'd see a line straight in the middle of the histogram.

To use a grey card to adjust exposure in post, you would need to see how far over or under exposed your gray card image is. Let's assume that the histogram of your card is ever so slightly to the left of center.

This means that you could bump the exposure up by 1/3 ~ 2/3 stop for every photo where the exposure settings and lighting were the same as in your grey card photo.

And here's the rub - your lighting and exposure settings would have to be consistent to apply any sort of mass exposure compensation in post.

tldr; check the gray card histogram - and make an adjustment to get it to read dead center. Apply that adjustment across all similar shots.

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You won't find much literature on using a grey card for exposure compensation in post simply because if you've missed your exposure, there could be nothing worth editing.

The real value in measuring a gray card for exposure is in dialing in an exposure before you need to take the shot. Using the technique above, you can do some finite adjustments in post - but if your shot is completely underexposed, or it's clipped in a channel or two...the technique above isn't going to help.

So, if you're going to take the time to photograph a grey card for WB - you should be looking at dialing in an exposure for the given light situation at the same time.