Difference between grey card and white card?

Started Sep 21, 2011 | Discussions thread
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WFulton Senior Member • Posts: 2,689
Re: Difference between grey card and white card?

Dragonboy wrote:

Hi everyone,
I was wondering whats the difference between the grey card and the white card?

I bought the lastolite ezybalance and you can use the white side or the grey side.
Sometimes in dim conditions the the grey side is too dark for the DSLR to read.

There are two very different purposes of "cards". White Balance and Exposure. Discussions ought to mention which is involved.

Exposure, the dark 18% card has been the standard, for maybe 70 years (it is an old analog concept, not a digital concept, but all the same for exposure). For photography, we realize now it ought to have been 12%, and Kodak's instructions on their 18% cards is that we should open exposure 1/2 stop more than what the 18% card meters (which is equivalent of 12%). White cards are more like 90% (amount of incident light reflected by the card). 12% would be quite dark.

Then digital happened. We can use any neutral color to examine for color casts. Neutral color means the three RGB components are all the same, like one example of 128,128,128 (is neutral). The three RGB values being the same is also the definition of gray (no color tint), but includes white and black also, as extremes of gray. We know the card is neutral, and we can click it in a picture, and the software knows to adjust color to make the RGB components there be the same again, which removes the color cast in the entire picture.

But 18% is pretty dark for WB, counter productive, so today, there are "digital" gray cards which are much lighter in tone than the old 18% cards. These would be less useful for exposure of course.

White works perfectly fine too, the WB concept is about white. Automatic color correction methods (like Adobe auto color in the Levels control) simply adjusts the gain of the three RGB channels to align at the top end (near 255). Aligning at the top end creates white, and of course, it assumes there was something actually white in the scene - which is not always true. But including a white card makes it be always true.

The only problem with white is that we can place it too near the light, overexposing the white card, to burn it out at 255,255,255, which loses all color tint information. White is probably about 90%, which is fine. White is easier to control the printing ink color (no ink, we see the paper). But just don't place them so close to light to overexpose the white cards. Gray cards makes this over exposure clipping harder to accomplish, but it should be a light gray, to be more useful. There is normally no problem overexposing white, so long as it is not much closer to the flash than is the subject.

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