By controlling the depth of field in your image, you can decide whether your subject is sharp and the background is blurred.
Controlling the depth of field is one of the easiest ways to make your actual subject stand out more in an image. The depth of field of an image is the amount of sharp area in front of and behind your subject. This is controlled by the exposure aperture setting.
On most cameras, you can select aperture preselection as the exposure mode, allowing you to decide what aperture you shoot at and the camera will time the exposure to get your subject correctly exposed.
With a large aperture (small aperture number) the depth of field is short; your subject is sharp but there is no depth of field in front or behind it.
A small aperture (large aperture number) has a large depth of field; that is, the front and back of your subject are also sharp.
The choice of depth of field helps the viewer to see more easily what the actual subject is in the picture.
Sometimes a wider depth of field is needed. For example, in group photos, you want all the people in the picture to be in focus.
If some people in a group are closer to the camera and some are further away, don’t use a very wide aperture so that everyone in the shot is in focus.
In this picture, when shooting with a large aperture, there is a risk that if the dog is not in line with the girl it will be blurry.
On the other hand, the subject will be more visible in a picture where the background is blurred. The risk with a wide aperture is that the exposure time will be long and the image will be blurred. In this case, you can choose a higher ISO number (a more sensitive “film”) to reduce the exposure time. With modern cameras you can use surprisingly high ISO values without the image becoming grainy.