Depth of Field
Depth of field is the transition from sharp focus to soft, out of focus areas in the image. The zone of sharp focus, and how much is in focus, is known as the depth of field. You will hear photographers speak of a shallow depth of field or a deep depth of field. The latter means that most or all of the image is in sharp focus. A shallow depth of field means that a small portion of the image is in focus. The biggest reason is that your eye will naturally go to whatever is in focus in a photograph. So, if you are shooting a wedding and you have a photo of the bride and groom, but they are out of focus and the people behind them are in focus, where do you think viewers will look? They will look at the people behind the couple of course. We assume that if something is out of focus, that we aren’t supposed to look at it. So you can use shallow depth of field to force your viewer to look at your subject. Make sure that whatever your subject is, that it is sharp and in focus. If the rest of the image is out of focus, that part of the image will stand out and people will immediately look there.
The ball of this pen is in focus because there is a shallow depth of field. The photo wants to draw your attention to the ballpoint.
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In this image there are two subjects but the one in front is out of focus. This is because the shot has a larger depth of field. The viewers eye automatically will go to the second subject because it is in focus.
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The blurred out image in the back or fore ground is known as boca. The desired effect of boca is to have an image the appears creamy like butter. There is no definite lines or distinct colors, just a smooth blend of the image.