A big part of understanding how to user your DSLR properly is understanding the aperture. The aperture is an opening in the lens, which allows for a certain amount of light to reach the camera's sensor.
The aperture in a lens is typically controlled by the camera. You can make the aperture larger or smaller which has two main effects.
- Allows more or light to reach the camera's sensor.
- Changes the depth of field
The aperture in a lens works the same way our pupils do. If you shine a bright light into your eye, your pupils constrict. This is to prevent too much light entering into your eye, which would be blinding. If you sit in the dark for a while, your eyes will dilate. This is to allow more light to enter your eye, so that you can see better in the dark. The aperture works in a similar fashion.
You can make the aperture smaller, which will allow less light to reach the camera's sensor. This also will make the depth of field larger. An example of a small aperture is f/22.
You can make the aperture larger, which allows more light to reach the camera's sensor. This will also make the depth of field narrower. An example of a small aperture is f/2.8.
Aperture works in conjunction with the camera's shutter speed and ISO setting to properly expose an image. However, often times it's best to start with setting your aperture setting first, and then changing the shutter speed and ISO settings afterwards.
When should I use a shallow depth of field or deep depth of field?
Ultimately, you have creative control to answer that question. However, typically a shallow depth of field is used for shooting a portrait of a person and a deep depth of field is used to photograph a landscape. Use a shallow depth of field when you really want to have a single subject be in focus. Use a deep depth of field when you have multiple subjects you want to be in focus, or you are photographing a large area like a landscape.