A photographer can control these settings to adjust exposure after considering the desired effect and available light conditions. Let's take a closer look at what these settings involve. If your camera's body lacks built-in aperture control, you can change the aperture by rotating the lens aperture ring. As mentioned above, aperture diameter can be controlled to determine the amount of light entering the camera. Aperture diameter also determines depth of field.
Depth of field indicates how much of the image appears in focus.
The smaller the aperture value, the larger the aperture opening, and the shallower the depth of field. Open up the aperture for portraits, flowers, and other shots in which a foreground subject is the main focal point of the scene. The larger the aperture value, the smaller the aperture opening, and the deeper the depth of field. Stop down the aperture for landscapes, document reproduction, and other shots where you want to keep the overall picture in clear focus. A camera shutter acts something like your eyelid.