When AP (Aperture Priority) Fails
Digital SLRs' AP (Aperture Priority) mode can be a boon or a blessing, if you know what you're doing. With the right eye, it can help you blur a background to focus on your subject. Or, it can overexpose a shot as you'll see below.
These 2 photos were shot in AP mode. In AP mode, the user selects the aperture and the camera adjusts the shutter speed to compensate for an evenly exposed photo. So as the aperture is increased, so is the shutter speed and vice versa. This is useful for adjusting DOF (Depth of Field). In the case of the photos below, I wanted a narrow DOF so the background would be blurred behind the subject. Unfortunately when I opened up the aperture to narrow the DOF, the camera choose a lengthy shutter speed. This led to the overexposure. As you can see in the properly exposed shot, the DFO is deeper. That means the background is less blurred.
Be careful when opening up your aperture and using a flash on a close subject. The flash mode used here was TTL slow sync flash for daytime use. It's when there's enough ambient lighting (middle of a sunny day), but the subject's face contains strong shadows. Daytime flash helps bring the level of exposure for both your subject and background the same. Lest your subject be dark while the bright sunny background is perfectly exposed.