Grey Card for Raw & Jpeg Question?

Can anyone tell me why the "whibal" white balance card used for raw white balance is a lighter shade of grey than they standard 18% gray card?

For that matter, when shooting raw, can I use the eye dropper is ACR to click on something white, or do I need to click on something grey?

Thanks for your feedback.

The lighter gray is due to how digital sensors "see". Supposedly the new grey cards, optimized for digital, will not have color shifts in various lighting conditions. In my use I have used both, as well as a white piece of paper, all with similar results. In fact I just bought 2 8x10 standard gray cards from B&H to have a couple extras around.

John E wrote in post #4621127
Can anyone tell me why the "whibal" white balance card used for raw white balance is a lighter shade of grey than they standard 18% gray card?

For that matter, when shooting raw, can I use the eye dropper is ACR to click on something white, or do I need to click on something grey?

Thanks for your feedback.

As long as the card you are using is neutral in color, it doesn't really matter what shade of gray the card is.

As for using something white, yes, you can use it as long as it is not blown or clipped. If the white is white from being blown out, you're not going to get a correct white balance when you use the gray eye dropper.

PacAce wrote in post #4621284
As long as the card you are using is neutral in color, it doesn't really matter what shade of gray the card is.

As for using something white, yes, you can use it as long as it is not blown or clipped. If the white is white from being blown out, you're not going to get a correct white balance when you use the gray eye dropper.

ACR won't even let you use it for WB. A warning dialog box will pop up on the screen.

In2Photos wrote in post #4621329
ACR won't even let you use it for WB. A warning dialog box will pop up on the screen.

Good for Adobe! I didn't know that since I've never had occassion to click on a blown white area. But it's good to know that they put in some smarts into the app for the unsuspecting.

John E wrote in post #4621127
Can anyone tell me why the "whibal" white balance card used for raw white balance is a lighter shade of grey than they standard 18% gray card?

The more brightly exposed your reference area is, the more precise the data will be (as long as you don't hit the upper limit of the camera's dynamic range, as Leo indicated).

Here's the best analogy I can come up with: Suppose you have a digital scale that gives you readings in pounds. If you weigh something and it says 5 pounds, it could be anywhere from 5.00 pounds to 5.99 pounds. So two objects that vary in weight by nearly 20% will appear to be equal. But if you weigh something that weighs 500 pounds, then anything 2% greater than that (501 pounds) will be shown as a different weight. In relative terms, the scale is more precise when heavier objects are weighed.

Obviously a scale with a 600 pound capacity won't accurately weigh anything heavier than that. An object weighing 601 pounds and an object weighting 1,000 pounds would both appear to weigh 600. This is why an overexposed image with one or more RGB values at 255 will not be a suitable white balance reference.

Now keep in mind that an overexposed black reference could be theoretically just as precise as an underexposed white object, if the RBG values are the same. But using a light grey color allows a very precise reference without the need to adjust the camera's exposure relative to the overall scene, so the Whibal card is made that way. It's a reference that gives you precise data without the risk of having any clipped channels, provided the reference shot is more or less properly exposed.

Thanks, Guys. I appreciate your expertise!

Curtis N wrote in post #4621625
The more brightly exposed your reference area is, the more precise the data will be (as long as you don't hit the upper limit of the camera's dynamic range, as Leo indicated).

Here's the best analogy I can come up with: Suppose you have a digital scale that gives you readings in pounds. If you weigh something and it says 5 pounds, it could be anywhere from 5.00 pounds to 5.99 pounds. So two objects that vary in weight by nearly 20% will appear to be equal. But if you weigh something that weighs 500 pounds, then anything 2% greater than that (501 pounds) will be shown as a different weight. In relative terms, the scale is more precise when heavier objects are weighed.

Obviously a scale with a 600 pound capacity won't accurately weigh anything heavier than that. An object weighing 601 pounds and an object weighting 1,000 pounds would both appear to weigh 600. This is why an overexposed image with one or more RGB values at 255 will not be a suitable white balance reference.

Now keep in mind that an overexposed black reference could be theoretically just as precise as an underexposed white object, if the RBG values are the same. But using a light grey color allows a very precise reference without the need to adjust the camera's exposure relative to the overall scene, so the Whibal card is made that way. It's a reference that gives you precise data without the risk of having any clipped channels, provided the reference shot is more or less properly exposed.

Applying your analogy, Curtis, then I would tend to think that the closer to mid-gray the color, the more accurate it will be. I'm thinking this because an image sensor is linear while image tones are not. So, to convert a raw linear image to, say, a JPEG image, the highlight end of the data would get compressed and the shadow end would get stretched.

But all that is academic. For all intents and purposes, I'm sure it'll be safe to say that using any neutral gray scale tone will yield a good white balance, no?

PacAce wrote in post #4622052
an image sensor is linear while image tones are not.

I'm going on the assumption that the white balance eyedropper in RAW conversion software uses the RAW sensor data. I hope I'm right about that.