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When you press halfway on your shutter button, the camera lens adjusts the focus according to how far or close you are to the subject. Whatever this distance, the subject will most likely appear in-focus but you'll probably have objects in the periphery, whether near or far, that may or may not appear to be in focus in the shot. This is primarily due to the aperture value of the camera, which helps dictate what else is in focus besides what you focused on. Normally you may shoot in Auto or Program mode where the camera automatically chooses your aperture and shutter speed to produce the proper exposure. But by selecting the aperture priority mode, you can choose among various apertures (f-stops) to creatively control the range of subjects that will appear in focus (pros refer to this as depth of field). Aperture priority thereby lets you select an aperture value (f-stop) best suited to your subject while it automatically chooses the shutter speed for proper exposure.
The focus is on the third post from the right. Click on the photo to enlarge or click here to see a larger, side-by-side comparison of all of the photos.
Choose a low numbered aperture (large iris opening) for results with a shallow focus or shallow depth of field. Primarily only the subject that your camera lens is focused on will be in focus.
By choosing f1.8, f2.0, f2.8, or f3.5 (as examples) as your aperture value, you can restrict the area in focus in the image area. Shooting at the lowest numbered f-stop available on your camera is known as "shooting wide open," basically referring to the fact that this is the largest opening the iris within the camera can obtain (f-stop numbers vary according to camera model). Since you are letting in the most amount of light at the larger openings, the camera typically chooses a fast shutter speed or one that is better suited to freeze movement or action.
By shooting with a larger iris opening (low numbered aperture) rather than a smaller iris opening (high numbered aperture), the photographer is able to blur the background. This results in fewer distractions behind your portrait.
Choose a high numbered aperture (small iris opening) for results with deep focus or deep depth of field. Primarily the entire image area will be in focus.
By choosing f8, f11, or f16 (as examples) as your aperture value, you can increase the depth of focus throughout the image area. Shooting at the highest numbered f-stop available is known as "shooting stopped down," basically referring to the fact that this is the smallest opening the iris within the camera can obtain (again, the f-stop number varies depending on the camera model). Since you are letting in the least amount of light at the smaller openings, the camera typically chooses a slower shutter speed so as not to freeze movement or action. It is recommended to use a tripod or brace yourself on a stable surface when shooting at smaller f-stops.
After the photographer focused on the first guitar (far right), he then chose to shoot using a large aperture (low numbered aperture) instead of a small one (high numbered aperture) to provide two different visual effects.
In addition, there are other factors that can affect the range of focus or depth of field:
Shooting from a closer distance to the subject decreases depth of field.
Shooting from a longer distance to the subject increases depth of field.
Using shorter lenses or smaller focal length (ex. wide angle) lenses creates more depth of field.
Using longer lenses or longer focal length (ex. telephoto) lenses creates a shallow depth of field.
Wide angle lenses generally provide the most depth of field regardless of the aperture in use, while telephoto lenses provide very shallow depth of field even at smaller aperture openings.