Not all cameras have an aperture-priority mode. If yours has one, you should use it. All the time. It'll give you more control over the look of your photos without having to dive into the complexities of full-manual exposure controls.
The good news is your camera may already be in aperture-priority mode instead of Auto mode by mistake. The "A" on your camera's mode dial actually refers to aperture-priority mode. Auto mode can be anything from the word "AUTO" to a picture of a camera to some sort of proprietary icon.
There are a lot of advantages to using aperture-priority mode. You'll get more-natural-looking photos that almost always look more interesting than the ones you'd get in Auto. Here are some other reasons:
1. It'll let you use your camera's widest aperture setting in any environment. Translation: Good for low-light shots, macro photography, and portraits.
2. Your camera will use the fastest shutter speed it can. Translation: Less blur and less need for a tripod.
3. Your camera will use the lowest ISO setting it can. Translation: Good for color accuracy.
3.5 Depending on the camera, aperture-priority will also power off the flash automatically. Translation: No blown-out details.
Basically, aperture-priority mode tells your camera this: Do whatever automated tricks you can to make this picture look good, but don't mess around with the aperture setting I picked.
And that's a truly useful thing to control. By being the boss of the aperture, you're controlling how much light reaches the sensor while your shutter is open. At a wider aperture, more light hits the sensor when you use a fast shutter speed. At a narrower aperture, you need to use a slower shutter speed or a higher ISO setting to get a well-exposed photo.
The obvious advantage to a wider aperture is that you can shoot decent pictures in a darker setting without a flash. The less-obvious advantage is that a wider aperture also decreases the depth of field in your shot. With a wider aperture, your subject will be in focus, while the background might be blurred out. It's a great way to enhance portraits and details in still-life photos.
Aperture settings are measured in "F-stops," and not all lenses (or sensors) are created equal. There's one counterintuitive piece of information to keep in mind: Wider/larger apertures have low numbers, while narrower apertures have higher numbers. In other words, an F2.0 lens has a wider aperture than an F4.0 lens.
Plus, the more you zoom, the more likely it is that your aperture will get narrower. Even if your lens has a maximum aperture of F2.0, you can probably only shoot at F2.0 when you're fully zoomed out. There are lenses with a fixed aperture throughout the zoom range, but they're pretty rare and expensive. So if you want to shoot using the widest aperture your lens can offer, keep the zoom at its widest possible angle.
When would you want to avoid using aperture-priority mode? When you're shooting landscapes, for the most part. But even then, there's even a good reason to use it. If you set your aperture to F16, bright lights in your photo can end up looking like star flares.
