gray cards/white cards

[vinnyveez]

ok maybe i am missing something here but how can you use either a gray card or a white card to get proper white balance? it seems counterintuitive that theywould both give good results.??

[PacAce]

ok maybe i am missing something here but how can you use either a gray card or a white card to get proper white balance? it seems counterintuitive that theywould both give good results.??
White is a color which has an equal amount of red, blue and green at their fullest intensity. Black is the absence of all three of these colors. And gray is a shade of these colors in equal amounts ranging from black (all colors = 0) to white (all colors = 255). They are all neutral in color, meaning that there is no one or two colors more intense than any of the other colors. So white, any shade of gray or black can be used very effectively for white balancing.

[vinnyveez]

White is a color which has an equal amount of red, blue and green at their fullest intensity. Black is the absence of all three of these colors. And gray is a shade of these colors in equal amounts ranging from black (all colors = 0) to white (all colors = 255). They are all neutral in color, meaning that there is no one or two colors more intense than any of the other colors. So white, any shade of gray or black can be used very effectively for white balancing.

are there instances where gray is better tahn white or vice versa? or should i just buy a gray card and use it all the time?

[PacAce]

are there instances where gray is better tahn white or vice versa? or should i just buy a gray card and use it all the time?
Most people, I imagine, would opt for the 18% gray card since it can serve two purposes, for setting the WB and also for setting exposure. You can also use a white card for setting exposure but it won't be as straightforward as using an 18% gray card.

[Jon]

My preference is for a grey card. That's what Canon's started recommending in their manuals, and it makes sense. If you use any old sheet of white paper it may not be a neutral white or it may have brighteners in it. If you try to get a correct exposure (which you should if you're dealing with mixed light situations), a white card may blow out, which will make it difficult for the camera to compute the proper correction.
Example: Grey card values R=126, G=128, B=132. The camera will boost your reds by 128/126 and reduce the blues by 128/132 in setting values.
Same conditions, white card. R=253, G=255, B=(258, but scale only goes to 255, so blue's blown out). It'll boost your reds by 255/253 and leave the blues alone. End result, there'll be a slight blue cast to everything.

[robertwgross]

You can use either a gray card or a white card for setting a custom white balance, as long as you get a normal exposure.

What is important is that the gray card or white card must be a pure gray or a pure white. If it has any tint at all, then your custom value will not be perfect. Also note that this is the desired effect for some people. They will intentionally set a custom white balance using a slight pink, slight yellow, or slight blue semi-white card, and that will set the custom value "the other way" for a special filter-like effect.

---Bob Gross---

[PacAce]

My preference is for a grey card. That's what Canon's started recommending in their manuals, and it makes sense. If you use any old sheet of white paper it may not be a neutral white or it may have brighteners in it. If you try to get a correct exposure (which you should if you're dealing with mixed light situations), a white card may blow out, which will make it difficult for the camera to compute the proper correction.
Example: Grey card values R=126, G=128, B=132. The camera will boost your reds by 128/126 and reduce the blues by 128/132 in setting values.
Same conditions, white card. R=253, G=255, B=(258, but scale only goes to 255, so blue's blown out). It'll boost your reds by 255/253 and leave the blues alone. End result, there'll be a slight blue cast to everything.
As much as I agree with you about using the gray card for both WB and exposure setting, I don't necessarily agree that you shouldn't use a white card because you might blow the white. Why? Because if you meter off the white card without adding exposure compensation, you will get a gray image of the card. None of the color will be blown and you can use this image for setting your white balance. I doubt anybody will go out of their way to make sure the white card is really white in the image before using it for setting the custom white balance because the gray (under-exposed image of the white) image will work just fine for WB, too.

[Jon]

If you're using mixed lighting, though (including flash in the mix), you really want to keep the contribution from each light source as close to your target output levels. Otherwise you risk introducing a bias due to one of the light sources contributing more to the "sample" shot than it will to any of the "actual" shots. A correctly exposed shot is safer, and using a grey card then is safer.

[PacAce]

If you're using mixed lighting, though (including flash in the mix), you really want to keep the contribution from each light source as close to your target output levels. Otherwise you risk introducing a bias due to one of the light sources contributing more to the "sample" shot than it will to any of the "actual" shots. A correctly exposed shot is safer, and using a grey card then is safer.
Sorry but I can't buy that argument. The color "cast" mix will be the same whether it's exposed properly or 1 1/2 to 2 stops under. If we're talking 3 or 4 or more stops under and going into the black areas, then I'd agree with you but not with the exposure difference we're talking about. Of course, it's just my opinion and you could very well be correct.

[Avalonthas]

Ive always used the 18% grey card as it has always been good for my purposes. It seems the grey card is the majority here for general practise so i would opt for the grey.

[PhotosGuy]

Gray Card…White Paper. What’s best?
http://photography-on-the.net/forum/showthread.php?t=58677