A test of the E-TTL system on my old 10D, showing the effects of flash output when the active focus point is set over a mid-tone (tan hat), a dark object (dark jacket), or a light object (white door). The net effect was to show that adding +1 stop of FEC to your base setting with white objects (wedding dress, e.g.) and subtracting -1 stop FEC to your base setting with dark objects (dark suit, e.g.) will net a reasonable exposure.

Each image's histogram is immediately to the right of the image.