Copyright Michael Karbo, Denmark, Europe.


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    Chapter 33. Depth of field

    Images must be sharp Ė they must be correctly focused. But there is less sharpness in an imageís foreground and background. Sometimes thereís almost no depth of field, at other times it is very large. This can be used creatively, if you are aware of it.

    Depth of field tells you how large an area in front of and behind the main subject will be sharp in a particular exposure. Everything outside of this area will be unsharp. There is a big depth of field in some exposures, in others a little depth of field.

    Figure 157. The depth of field is the area around the focus point, where the image is sharp.     

    Sharpness changes

    With the term depth of field, we try to describe, how sharpness changes in a photograph. An image has a focus point, which is found at a certain distance from the lens. At this focus point an image is sharp. But the area between the camera and towards the focus point (foreground) isnít sharp and this is the same for the area, which lies further away from the focus point (background).

    So the foreground and the background are less sharp and focused than the area around the focus point, but there is a borderline between the sharp and the unsharp.

    There are several conditions in the lens, which determine the size of the field of depth:

  • Diaphragm

  • Focal length

  • Macro

    In practice, the diaphragm is a very important factor. The smaller the hole in the aperture, the bigger the depth of field. So, to get a big depth of field, use the smallest diaphragm step (typically F8 and F6,7).

    Figure 158. The camera here was focused on the card in the middle of the row. Note how unsharp it is in the foreground and the background. The image has been taken with diaphragm 2,0.

    Figure 159. There is a very small depth of field in the example at the top, which is concentrated close to the focus point. This is the case if you photograph with the biggest aperture. In the example at the bottom, the depth of field reaches from maybe just a meterís distance to the infinite Ė i.e. a very large field of depth. The aperture here is small.

    Figure 160. The foreground is sharp, but the background certainly is not. If the whole image is to be sharp, a wide-angle lens will have to be used focusing at a distance of maybe one meter in front of the plants. With the lensí smallest aperture, there is hope that the field of depth will reach all the way out, from the foreground to the infinite.

    The area with depth of field can be very small, so there is, in fact, only a few cm in front of and behind the focus point that are focused. This is often taken advantage of with portrait photography, where it looks good, when the face itself is in focus. The picture mode Portrait, which is found on many cameras, is thus set to select the biggest possible aperture to ensure the smallest possible depth of field.

    The sharpness zone can also be extensive, so that, for example, everything from 1 meterís distance to the infinite is sharp. This can be of great help to a photographer in many situations because the risk of taking unsharp images is much smaller.

    Focal length is relevant

    Generally speaking, the depth of field is larger, the smaller the aperture. But the focal length is also relevant. The shorter the lensí focal length, the bigger the field of depth.

    This means that if you photograph with a wide angle setting in the zoom lens, then you can, in fact, avoid unsharp images by taking advantage of the large depth of field. With diaphragm 8, the depth of field covers all of the focus area, if the camera is set to a focal distance of 1 meter and with zoom factor of 1X:

    Field of depth with wide angle setting (1X zoom)

    Diaphragm

    Focus

    Sharpness zone

    2

    4,1 m

    2 m to infinite

    4

    2 m

    1 m to infinite

    8

    1 m

    Minimum distance to infinite

    The depth of field is reduced with longer focal lengths. A telephoto with, for example, 7X zoom hasnít much depth of field left. It is important here to focus on the required distance, everything in front and behind the focus point will be unsharp.

    Finally, you have to be careful with macro exposures where the depth of field is very limited. You might have to focus manually here because it is very difficult to get the cameraís autofocus to focus at the required distance.  In some cameras you can select focus bracketing, which consists of 5 exposures, the camera takes itself, where the focus is altered a little bit with every shot.

    This focus bracketing is, among other things, meant for use with macro exposures, where the poor depth of field makes it difficult to achieve the right focus.

    Both foreground and background are unsharp in this picture taken with macro. There are only a few centimeters between the three figures but it is still only the big one, which is in focus.


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