White Balance / Gray Card / Exposure

Hiya folks, long time lurker, first time poster...Thanks for all the info you've unknowingly imparted to me.
So, about the white balance...Is it correct to say that a gray card is to be used to determine the proper exposure only? It should not be used to set white balance? I'm not sure why one would photo a gray card to set white balance instead of a known white...If using a color picker in PP, should one be choosing a white or a gray? Sorry for the convoluted route to asking the question, but it's all jumbled up in my head right now.
Thanks again for all your help, you guys have gotten me started on the right foot!

You can use either a gray reference card or a white reference card as a white-balance reference, assuming that both are truly "neutral" (all colors have equal level). However.....

The best choice - white or gray - depends on precisely how you would use a reference card.

If you are doing a "custom white balance", then either is suitable because you will be taking a photo of the card with your exposure settings attempting to make either look like a mid-gray in the image.

If, however, you are placing the reference card into a scene and taking a shot with the exposure controls set for the overall scene then a gray card is usually better than white. The reason is that it would be easy for the white card to be overexposed in this situation and that would render one or more channels "blown out", essentially making the card no longer "neutral".

While we're at it, does the 18% gray card need to take up the entire viewfinder or just the center af point to set custom wb?

Matt57 wrote in post #3800856
While we're at it, does the 18% gray card need to take up the entire viewfinder or just the center af point to set custom wb?

There's a picture on page 86 of the XTi/400D manual that shows how much it needs to cover. Basically the whole area inside of the 7 central AF points.

SkipD wrote in post #3800757
You can use either a gray reference card or a white reference card as a white-balance reference, assuming that both are truly "neutral" (all colors have equal level). However.....

The best choice - white or gray - depends on precisely how you would use a reference card.

If you are doing a "custom white balance", then either is suitable because you will be taking a photo of the card with your exposure settings attempting to make either look like a mid-gray in the image.

If, however, you are placing the reference card into a scene and taking a shot with the exposure controls set for the overall scene then a gray card is usually better than white. The reason is that it would be easy for the white card to be overexposed in this situation and that would render one or more channels "blown out", essentially making the card no longer "neutral".

Is this to say that if one takes a pic of a white or gray card, there will be no difference in the outcome? Both white and gray "samples" will produce the same images? How does this affect the exposure? I would imagine a white sheet of paper would reflect more, brighter light resulting in a lower in-camera exposure setting. I would think a gray card would ask for a "longer" exposure.

EWek11 wrote in post #3801959
Is this to say that if one takes a pic of a white or gray card, there will be no difference in the outcome? Both white and gray "samples" will produce the same images? How does this affect the exposure? I would imagine a white sheet of paper would reflect more, brighter light resulting in a lower in-camera exposure setting. I would think a gray card would ask for a "longer" exposure.

If you are using a reference card for a "custom white balance", you expose for the reference card (ignoring the scene you will be shooting). That will make the image look like the card is medium gray, no matter what the card looks like to you. Thus, you won't be overexposing the reference card.

Gray Card…White Paper. What’s best?

“Film tricks” can help you out.
Need an exposure crutch?