When you take a photograph, the objects lying within the focal length of the lens is sharper in contrast to those either further away from the lens or closer to the camera . The range between the sharpest nearest and farthest points is called "depth of field". You can creatively use depth of field in photography. Knowledge of depth of field can make your pictures interesting. For instance, by putting your subject in sharp focus, while simultaneously ensuring the background is out of focus, you will create a picture whose subject is not cluttered by the background.
Focus plays a significant part in how good a picture is. By using depth of field, you can play around with the areas in a photograph that you want to draw attention to. With creative use of defocus, you can add an aura of mystery to a picture (much like in a soft-focus photograph) or capture a glorious landscape without parts of it being blurry.
The magic of how to control depth of field lies in your ability to use the aperture of the camera. The simple rule is that opening up the aperture decreases the depth of field, making it smaller produces the opposite effect. A smaller aperture, say f11, will give you sharpness over a greater area of the picture than say f4 over the same focal length.
Depth of field can also be increased or decreased by using different focal lengths. Depth of field increases when we use wide-angle lenses and it decreases when we use longer focal length lenses, a 200mm lens for example.
However, remember that it isn't simply a matter of increasing or decreasing the aperture. Simply changing the f number can give you over or under exposed photos. The aperture and the shutter speed have an inverse relationship in continuous light, that is, the bigger the aperture, the faster should be the shutter speed and vice-versa. Suppose you have set your correct exposure for a shot to f16 aperture and 1/125 shutter speed. You then realise that you need to step down the aperture to f2.8 because you want only the subject in focus. This means that you will open up the aperture by five full stops from f16 (11-8-5.6-4-2.8). For the exposure to remain correct, you will correspondingly have to increase the shutter speed by five full stops from 1/125: 1/250-1/500-1/1000-1/2000-1/4000. At an aperture of f2.8 you will require a shutter speed of 1/4000 to maintain the original exposure.
When using flash, you will have to control the exposure by increasing or decreasing the light intensity or moving the light source away or closer to the object.
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