white balance vs. exposure compensation

...I've been searching the forum, reading as much as possible.....maybe I'm a little dense, but I'm more confused than ever. If I custom white balance with a grey card, am I balancing out the color (based on the color of the light being used), or am I setting an exposure compensation? I'm having a hard time with these concepts, and using the histogram for white balancing, etc. Any help would be more than appreciated!

balancing out the color (based on the color of the light being used)

To be honest though, it kind of baffles me that a white piece of paper can be used if a grey card is not available. I guess it is because they both have the same cast.

timconnelly wrote in post #10119859
...I've been searching the forum, reading as much as possible.....maybe I'm a little dense, but I'm more confused than ever. If I custom white balance with a grey card, am I balancing out the color (based on the color of the light being used), or am I setting an exposure compensation? I'm having a hard time with these concepts, and using the histogram for white balancing, etc. Any help would be more than appreciated!

....that's what confuses me so much.....if I use an 18% grey card for white balancing, which is balancing out the color for the setting, then why do so many posts mention that the grey card is used for adjusting the exposure?

For setting white balance you need something that is neutral. IOW, something that if you look at the RGB values they are all teh same. It doesn't matter if the RGB values are all 0 (aka black) all 255 (aka white) or some variation of gray that falls between. Note that a white piece of paper is often NOT a good choice because they are rarely a true white.

For exposure, the camera's metering system assumes that for any given scene the average of all colors in that scene will be about the same as 18% gray. There are plenty of threads about this, but a good example is taking a picture outdoors in the snow. The scene is very white but the camera will assume that it is supposed to be 18% gray and underexpose the shot so that the snow looks gray. If you were to hold a gray card in front of the camera while metering and then take a shot of the snow then the camera will have the correct exposure and make the snow look white.

hairy_moth wrote in post #10119871
balancing out the color (based on the color of the light being used)

To be honest though, it kind of baffles me that a white piece of paper can be used if a grey card is not available. I guess it is because they both have the same cast.

Any color-neutral item will do. White is grey, just the brightest shade of it

timconnelly wrote in post #10119964
....that's what confuses me so much.....if I use an 18% grey card for white balancing, which is balancing out the color for the setting, then why do so many posts mention that the grey card is used for adjusting the exposure?

You can use it for both. A grey card's neutral color makes it useful for white balance, but you could also use it to set your exposure by lining it up where you want it on the histogram.

As I understand it, the meter is trying to optimize the exposure for an amount of light that is similar to what is reflected from an 18% gray card. A google search will provide better explantions than mine.. this site goes into it.

http://www.the-digital-picture.com/Reviews/Gr​ay-Card-Review.aspx

I wish I had had one when I photographed a steam engine recently. The engine was flat black and grimy (very dark). It was a bright sunny day and there was a lot of bright white steam coming off of the engine. The range of reflected light may have been beyond what my camera could capture; my shots either had the engine exposed properly with a completely white (blown) sky and the steam invisible in the white sky, or the sky was blue, the steam white and the engine was just a big black blob.

timconnelly wrote in post #10119964
....that's what confuses me so much.....if I use an 18% grey card for white balancing, which is balancing out the color for the setting, then why do so many posts mention that the grey card is used for adjusting the exposure?

.....thanks, really helpful replys...I think I'm beginning to get it....