Few months ago i Ordered for a Grey Card,they came in 3 numbers one card is Fully dark grey,second one is Light Grey,one is Like dull white,i read about Grey Cards and there uses for white Balance, to get correct colors I mean true natural colors in a photograph.what is confusing me is which one should i use and if all are worthy.why they differ in different shades of grey..so i am Little bit confused with the word,that the correct exposure should have 18% grey tone.i know that camera cannot see colors RGB and it can measure light white and black or lets say shadows all the coloring work is done by the Processor inside the camera...i hope now my question is understood.
I can't know for sure but my guess is that the "dark gray" card is supposed to be black, the "dull white" is supposed to be white and the "light gray" is the 18% gray card.
It's actually really easy to test what card to use an if it's worthwhile.
Get some family member to pose for you and take 3 photos, in each lock exposure and white balance using a diffrent card - the card that gives you the best exposure and white ballance is the right card to use, if using the card gives you better exposure and white ballance than whatever you used before it's worth using.
You probabyl should repeat the experimant in both normal and somewhat-difficult light (something that messes up auto white ballance will be good) before deciding.
A white card, or even an 18% grey card, is very useful to ensure consistent white balance across shots. Many cameras offer the ability to shoot a white card and then use that for custom white balance, thus saving a step in post production. Of course, its easy to use a grey or white card in Lightroom or Photoshop. (see my blog post on White Balance in Lightroom) However, the three cards are most likely intended to be used entirely with post shot edits, in Photoshop.
In Photoshop, Exposure, Levels and Curves provide the ability for you to choose a sample of Black, mid-grey and White, to tell the software what ranges exist in the image. Of course, you do not always have a white or even a good black in every image, so by including your cards in a sample shot on location, you can set the top, bottom and middle exposure, and levels for each image, even copying them to other images in the same conditions. This is a great way to add 'pop' to your images among other things.
You don't need a gray card anymore with a modern camera. There were two things going on with the standard 18% gray card.
First, it was a shade of gray, so could be used for color balancing. Back in the wet silver days, you could pick only one shade to match between the real world and a print. 18% gray was a good choice for that. You can think of it being sortof in the "middle" of the conceptual color space (remember that humans perceive light lograrithmically, and the film and paper weren't linear either),
Full white is easier to make neutral, and provides color balance too. However, that could either saturate the emulsions (negative) or form so little density (positive) that it wasn't a good way to measure color in the result. Also it was at one corner of the perceptual color space. Even if you got the shade of white right in the final result, other shades could be off considerably due to the non-linearities of the process.
The second purpose of a 18% gray card was for adjusting exposure. Again, that is near the middle of the conceptual gray scale, and was supposedly the reflectance of a average scene. By adjusting exposure so that the response to 18% gray ended up near the middle of the film's response, you didn't bury the shadows or clip the highlights too much.
Nowadays, we have built in exposure meters, so a gray card isn't necessary for measuring exposure. Electronic sensors have a linear response to light level, so balancing the colors can be done anywhwere on the linear scale and the rest will be correct. In that case, we might as well use white, since that is usually easily available and we can see for ourselves that a white object is pretty much white. The more overall incident light a material reflects, the less opportunity there is for reflecting it differently accross the spectrum.
I have measured several digital cameras, and all the ones I have measured have been quite linear from light intensity in to raw sensor value out. There will be some variations between sensors, but the relative sensitivity of red, green, and blue remains fixed for any one sensor. As a result, you can measure it once for each known repeatable lighting condition and compute the color correction for that lighting for that camera once. For example, I have measured my camera with a white target in full sunlight and found the relative response of the sensor to be .5406, 1, .6954. I then use this for color balance without requiring further measurement on any picture I know was taken in full sunlight.
This is to explain why you may still see gray cards and separate light meters used in some circumstances by professional photographers.
A separate light meter is usually used for measuring different points of a scene that are in different locations, oriented differently so therefore illuminated by different lights, or are a different color. This is not so much about setting the overall exposure as balancing the lighting accross a scene.
To measure just the lighting at different locations and orientations, you need some sort of standard diffuse reflecting surface. With today's linear sensors, that can be something white. However, photographers that have been around for a while probably got used to using gray cards for this. They continue to use them because they still work (even though white would now work too) and because they are used to interpreting the result from gray. There is nothing wrong with that, and no reason to introduce something that may cause you to make a mistake because you're used to something else.
However, I still stand by my answer that you don't need a gray card today. Anyone asking this question here wouldn't have 20 years of professional studio photography and using gray cards behind them.