As I mentioned in my “Photography Basics” post, aperture is the size of the opening in the rear of the lens that allows light to pass through to the camera sensor. Aperture is described in f stops. The higher the number of f stop, the smaller the opening at the rear of the lens is, and therefore, less light is transmitted to the sensor. The lower the number, the larger the opening at the rear of the camera, therefore, more light is transmitted to the sensor.
When shooting in aperture priority mode, you select the aperture to control your depth of field and the camera will calculate the remaining parts of the photograph (shutter speed and ISO) based upon the ambient light where you are shooting.
Shooting with an Open Aperture:
If you are shooting in low light, as in the photo of my Diet Coke bottle below, you would select an open aperture to compensate for the low light. As you can see in that photo, because I used a wide-open aperture, there is shallow depth of field where the background is blurred.
An open aperture is also good for shooting fast moving subjects such as sporting events or auto racing. The open aperture allows the camera to use a fast shutter speed to freeze the action without creating blur in the photograph. That technique is demonstrated in the soccer player photograph below.
Finally, when making portraits an open aperture will place your subject in focus with the background blurred so that the portrait subject stands out and your viewer’s attention is not distracted by elements in the background of the photograph.
Shooting with a Narrow Aperture:
A higher aperture allows for greater depth of field. This is used in landscape shots so that the entire landscape remains in focus throughout the photographic image. You’ve seen this technique in any landscape you’ve ever seen such as the photograph below where I used a higher aperture (smaller opening) so as to keep all of the photograph in focus.
Using a higher aperture will have the camera slow the shutter speed. Knowing that, it will not work well for fast moving subjects such as sports or auto racing because the moving subjects will be blurred in your photo. However, a higher aperture is great when you want to use a slower shutter speed to intentionally blur motion such as shooting light trails or wanting to smooth water. Generally, however, when I am trying to shoot light trails or blur water I will shoot in shutter priority to set the length of the exposure to properly blur the motion and allow the camera to select how high of an aperture to use. (For more on that, please see my post “Shooting in Shutter Priority”.)