what is the concept of the 18% gray card used for exposure do?

Answer 1

The mid tone of an "average" scene is 18% gray and that is what your camera's meter bases exposure on. It doesn't matter if your subject is backlit or frontlit against a dark background or if your scene has sunlit highlights and dark shadows your camera meter wants the exposure to render 18% gray.

If your subject is backlit and you depend on the meter, your subject will be a silhouette. If your subject is frontlit against a dark background and you depend on the meter your subject will be "washed out". If you have a strong area of sunlight with dark shadows and depend on the meter the sunlit part will be washed out.

Exposure for frontlit or backlit subjects:

Meter off the gray card, holding it in the same light as that illuminating your subject. That exposure is then manually set by you or locked-in with the AE Lock on the camera.

Another way to overcome the scenes described is to actually meter off the subject. You can do this by zooming in on the subject's face, setting that exposure and then re-composing and shooting. You can also use the EV Compensation on your camera and try one shot at +1 and one at +2 for a backlit subject or for a frontlit subject, one at -1 and one at -2.

Areas of strong sunlight and deep shade:

Here, you need to average the meter reading. This isn't as easy as it may sound unless you have a hand-held light meter with the capability of computing an average of 2 or 3 readings. In some instances it isn't too difficult but in some situations computing an average for exposure requires a scientific calculator.

Suppose the brightest area calls for an exposure of f11 at 1/125 sec. and the darkest area calls for an exposure of f2.8 at 1/125 sec. This is a simple example since all you do is count from f2.8 up and down f11 and you'll "meet" at f5.6. Make an exposure at f5.6 at 1/125 sec. You can also calculate this using the geometric average - multiply 2.8 by 11 to get 30.8 and then take the square root of 30.8 which equals 5.55 or 5.6 rounded up. You'd use this method if the readings were, say, f2 and f11. Using the geometric average would give you an f-stop of 4.7 so you'd try one at f4 and one at f5.6. If your lens allows you to set the f-stop at half way between f4 and f5.6 give it a try.

Answer 2

An 18% gray card is the center of the zone system in photography. 18% gray represents the exact balance between pure white and pure black.

In theory, camera meters and hand-held meters are calibrated to expose an image so a gray card will always be reproduced at the exact same shade.

In reality, most manufacturers meters are calibrated for 12% gray.
http://www.bythom.com/graycards.htm

Here are some discussions about the grey card and it's use.
http://www.acecam.com/magazine/gray-card.html
http://www.cambridgeincolour.com/tutorials/camera-metering.htm

I hope this helps you.

Answer 3

No, I wish it were that easy, but you still have to take into account consistency of lighting, reflections, setting exposure to take advantage of highlights or shadows, etc. A gray card helps in every situation, but it doesn't ALONE fix exposure problems every time. That's the difference between taking good photos and being a professional photographer.