Have you ever looked at someone’s eyes when bright light suddenly hits them? The pupils quickly become much smaller, a reaction that’s keeps too much light from entering the eye. In our camera, it’s the diaphragm or aperture that closes down to limit the amount of light. Control of the aperture opening means more than just adjusting light amounts. It also has a close relationship with something called depth of field. This refers to the area in front and back of the subject that is in focus at the some time as your subject. Along with the kind of lens and positions o the subject, the aperture you select has a definite bearing on whether the depth of field is great (a lot of the picture in focus) or shallow ) only the subject and immediate area in focus).
When a lens focuses on a subject at a distance, all subjects at that distance are sharply focused. Subjects that are not at the same distance are out of focus and theoretically are not sharp. However, since human eyes cannot distinguish very small degree of unsharpness, some subjects that are in front of and behind the sharply focused subjects can still appear sharp. The zone of acceptable sharpness is referred to as the depth of field. To be technically correct, DOF is the zone of acceptable sharpness, the area in front of, and behind, a focused subject that appears in focus. Thus, increasing the depth of field increases the sharpness of an image. We can use smaller apertures for increasing the depth of field.
Depth of field is the range of distance within the subject that is acceptably sharp. The depth of field varies depending on camera type, aperture and focusing distance.
The depth of field does not abruptly change from sharp to unsharp, but instead occurs as a gradual transition. In fact, everything immediately in front of or in back of the focusing distance begins to lose sharpness-- even if this is not perceived by our eyes or by the resolution of the camera.
What we really want to know as photographers is what affects depth of field so we can control depth of field in our pictures. For a long time, photographers have gone with the following three criteria:
- lens aperture
- distance from subject
- focal length
The aperture is simply the size of the opening that allows light to go through the lens. It is expressed in f/stops (also referred to as f/value or aperture value), and a typical aperture range is f/2.8 - f/8, giving the range from maximum (large at f/2.8) to minimum (small at f/8) aperture. A small f/value (e.g. f/2.8) indicates a large aperture. For instances if we use an aperture of f/2, we may be surprised to find that although we have focused on the subject properly, not much else is very clear. That’s because large aperture sizes equal shallow depth of field. If possible, changing to a smaller aperture and corresponding slower shutter speed is advisable if we want some of the background in focus.
Distance from subject
When we focus on a subject close to the camera, the DOF is less than when you focus on the subject farther away from the camera. We can also control depth of field b y changing distance from the subject. Even with the same aperture and distance, the depth of field effect will be different. Step away from our subject to obtain greater depth of field, or move in closer to decrease depth of field.
A wide-angle lens has greater depth of field than a telephoto lens. Most consumer digital cameras have very short focal lengths and that is why it is so difficult to obtain shallow depth of filed , even with the aperture opened up wide. In another case even with the same aperture and distance the depth of field gets shallower the longer the focal length of the lens.