"Correct exposure is essential to successful photography, and an understanding of exposure and light metering is a core skill for anyone wishing to explore the art of photography. Most photographers rely almost exclusively on the automatic lightmeter in their camera to produce an accurate exposure, and under most circumstances this is not a problem. The through-the-lens (TTL) multi-segment evaluative lightmeters found on most modern good quality cameras are very sophisticated, and can cope with most situations quite adequately. However even the best meter can be fooled by unusual lighting conditions or very high contrast scenes into producing under- or over-exposed images. For example, if you’ve ever take a photograph of someone standing in front of a brightly lit window, you’ll probably have found that you can see the pattern on the curtains but that the subject is far too dark. This is because the lightmeter was fooled by the bright light from the window and the shot was under-exposed."A helpful article that explains the basics behind shutter speed, aperture, and how they relate to metering. Worth a read. Myself, I'm very pleased that my Nikon D200 has spot metering; it really helps in the situations described in this article.