Depth of field is the range of distance around the focal plane, which is acceptably sharp. Factors affecting the depth of field are the camera and lens type, aperture and focus distance.
Depth of field is not an abrupt change, but a smooth gradual sharp to blurry transition, as can be seen in the photograph above. Everything in front or behind the focus plane begins to loose sharpness, even if it is not detectable by the camera or our eyes.
Depth of field should not be confused with Depth of Focus. Depth of Focus, or Focal Spread, describes the distance over which light is focused at the cameras sensor, as opposed to how much of the subject is in focus. It is important because it sets tolerances on how level the cameras sensor has to be in order to capture proper focus in all regions of the image.
Aperture is measured in f stops or f numbers and the numbers represent the ratio between the focal length of the lens and the diameter of the aperture. These f numbers are typically written as f/2.8 or f/22. A lens is said to be "wide open" when the aperture is at its largest, which is the lowest f number the lens is capable of. On average this is around f/5. A "closed" lens would be at the smallest possible aperture, typically around f/32.
The image below shows how the f number affects the aperture.
Aperture and focal distance are the two main factors affecting depth of field. As a general rule:
You can see in the animation below how the variation in aperture (f number) changes how much of the scene is in focus. The larger the f number the more of the scene is focused, while lower f numbers only the chess pieces focused on are in focus.
Watch the image below and see how the aperture effects the depth of field on the chess pieces.
Depending on what your subject is, depth of field can help you or hinder you. In macro photography, depth of field means that you only have a very small "sweet spot" for the subject, which means focus has to be perfect. In another example, if you are photographing a portrait of somebody, depth of field will make the person stand out from the background.
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