At a recent corporate shoot, I had to photograph company employees having fun as they rode down a zip line. Not too difficult, you might think, except the sun was bright and in the wrong direction. I wanted to show the faces of the zip line riders, but of course they were in shadow.
If I used my camera in the normal matrix/evaluative metering mode, the result would have been as shown in the first pricture below. There is a nice blue sky, but the figures in shadow are virtually sillouettes.
To get any kind of usuable picture of the people, I would have had to increase the exposure, so that the faces were no longer in deep shadow. I could do this by spot metering on the person, or I could simply shoot in manual and add some exposure compensation. The result would look like the picture below, I have captured the figures, but the sky has turned almost pure white in an ugly blown-out way. I think it's prefereable to the first picture, but not great.
The third alternative uses the computer to fix the image. Instead of worrying how it looks in the camera, I would simply make sure that the histogram shows that all the image data has been collected with no clipping. I can then do noise reduction on the image and run it through Photoshop to selectively brighten the dark areas, as shown in the image below.
Some cameras are now attempting to achieve this result by including an HDR function in the camera, which takes several images quickly at different exposures and combines the image for the best result. I have found that moving images taken using this technique don't seem sharp enough, but images exposed in camera like the third example will probably soon be a matter of routine.