What is Aperture?
Before we start working with aperture priority mode we need to understand the meaning of aperture. Aperture denotes the opening in a lens that allows reflected light, bouncing off a subject, to enter the camera and be finally converted into electronic signals in the form of a picture.
The aperture of the camera can be controlled using an electronic mechanism buried inside the camera’s menu. Some lenses also come with an aperture ring that allows you to manually adjust the aperture value.
Related Post: Wide vs. Narrow Aperture (Example Images)
Working with Aperture Priority Mode
Aperture priority mode is a manual mode that is accessible directly from the main shooting dial on the top of the camera. When this mode is selected the photographer can adjust the aperture value of the camera.
Depending on the Depth of Field that he wants to capture in his photos as well as the availability of light he can choose between a range of aperture values expressed as a fraction like f/2, f/2.8, f/4 and so on.
When the aperture value is selected by the photographer, the camera automatically selects the ideal shutter speed. What the camera does basically is, it uses the built-in light meter that analyzes the scene based on the metering mode selected (which is by default set at matrix metering in most cameras) and then selects the shutter speed that can make the scene appear 18% grey.
Low-Light and Recommended Aperture
In low light situations, a wider/faster aperture is recommended as it allows you to collect a lot of light with the same shutter speed. If you have a prime lens something like the 35mm f/1.8G it is better to shoot at its widest rather than use a longer shutter speed. At longer shutter speeds you risk capturing a lot of camera shake resulting in blurry images.
Aperture priority is the only tool that can get you the beautiful soft out of focus foreground and background effects also known as Bokeh. It is ideal for outdoor portraiture where you want to separate the subject from the rest of the scene.
To capture Bokeh you could have a lens with a fast maximum aperture, or place the subject at a considerable distance from the background or do both at the same time.
Depth of Field
Aperture controls the Depth of Field in a picture. If the scene has almost everything in sharp focus then you are using a very small aperture something like f/11 or f/16 making this an example of big Depth of Field.
If on the other hand only the spot where you are focusing is sharp and the rest of the photo is out of focus then you are using a big aperture like f/2.8 making this an example of shallow DoF.