maDJawa: Master Aperture Priority

Switch to Aperture Priority and start taking creative control over your depth of field

Most cameras offer a selection of exposure modes ranging from fully automatic to fully manual.
Automatic modes are best avoided unless you want your camera to make all the crucial creative decisions for you. But if you're not confident about setting both shutter speed and aperture manually, a semi-automatic shooting mode such as Aperture Priority (often abbreviated to A or Av) is the perfect solution. In Av mode you choose the aperture and the camera selects the shutter speed to match the internal light meter reading.
The main advantage of Aperture Priority is that it enables you to vary the depth of field - the zone of acceptably sharp focus that extends in front of and behind the point you focus on - with a minimum of fuss. The larger the aperture you pick, the narrower the depth of field. Conversely, smaller apertures extend the zone of sharp focus.

Maximise depth of field in a macro shot by keeping the sensor parallel with the subject.

Aperture Priority is ideal for portraits when you need mid-to-wide apertures to blur distracting backgrounds.
It's also handy for macro work because you can set narrow apertures to bring entire subjects into focus, and then quickly switch to wider apertures to create abstract compositions featuring minimal depth of field. Av mode is also popular with landscape photographers who need to maximize the depth of field across a whole scene.

Be a more effective photographer

When light levels drop it's tempting to open up to the widest aperture in order to maintain a suitable shutter speed. However, if you're shooting with macro or telephoto lenses, depth of field can become too narrow (often just a few millimeters) to ensure precise focusing.
In these conditions boosting your ISO is a more sensible way to increase shutter speed. A shot with a slight increase in noise is better than no shot at all.

Assessing depth

Depth of Field Preview can help you ascertain if your subject is 'in focus' from front to back. You'll find the button on the front of your camera next to the lens. When you press it, the lens will be stopped down to the selected aperture enabling you to preview the depth of field in the viewfinder.
This works well in bright conditions, but in lowlight levels and very small apertures the viewfinder becomes quite dark, making it hard to accurately assess the zone of sharpness.

WARNING!

One of the drawbacks of Aperture Priority is that it's all too easy to overlook your shutter speed, which can prove disastrous if you're shooting handheld.
For example, if you were using a wide-angle at f/8 and 1/125 sec, then switched to a long telephoto without selecting a wider aperture or boosting the ISO, there'd be a severe risk of camera shake.
Under and overexposure is another thing to watch out for - but your camera should flash a viewfinder or LCD warning at you, like this...

Nikon
Select an aperture that lets in too little light and the Nikon system warns you of the 'Lo' light levels.