If you took a poll of portrait photographers to see which shooting mode was most often used for portraits, the answer would certainly be Aperture Priority (Av) mode. Selecting the right aperture is important for placing the most critically sharp area of the photo on your subject, while simultaneously blurring all of the distracting background clutter (Figure 6.1). Not only will a large aperture give the narrowest depth of field; it will also allow you to shoot in lower light levels at lower ISO settings.
This isn’t to say you’re stuck shooting everything at a wide angle in order to stick to the widest aperture (remember, the maximum aperture decreases when you zoom). A good place to begin is f/5.6. This will give you enough depth of field to keep the entire face in focus, while providing enough blur to eliminate distractions in the background. This isn’t a hard-and-fast setting; it’s just a good, all-around number to start with. You can change your aperture depending on the zoom level you’re using and on the amount of blur you want for your foreground and background elements.