It is a characteristic of any photographic lens, that only a limited field in front and a limited field behind the focus point will also be in focus on the negative and photograph.
Importance of depth of field
If a subject that has depth is photographed, like a wall that stretches away from the camera, the whole subject will be in focus only if it fits into the available depth of field. The depth of field must be more (longer) than the subject photographed. Normally the depth of field cannot be seen through the viewfinder, but on some modern cameras a depth of field preview is available.
The depth of field zone
The depth of field covers a zone of one third in front of the focus point and two thirds behind the focus point. In the figure subjects no.2 and no.4 fall inside the zone and will be in focus on the photograph. Subjects no.1 and no.5 fall outside the zone and will be out of focus.
If the depth of field zone is enlarged or reduced the relation of 1/3 and 2/3 remains unchanged. Because of this phenomenon, any subject or scene that stretches away from the camera should be photographed by focusing on the end of the first third of the subject or scene.
The quantity of depth of field
The depth of field is influenced by the following factors:
The diaphragm or lens opening
The focal length of the lens used
The distance between the lens and the focus point
The influence of these factors on the depth of field is as follows:
The smaller the diaphragm the more depth of field (Larger f-STOP number)
The shorter the focal length of the lens used the more the depth of field
The shorter the distance between lens and subject the smaller the depth of field
On lenses with a fixed focal length, there will usually be a depth of field scale and the amount of depth of field can be read from this scale. This is done by focussing the camera on the subject and then reading the distances corresponding to the applicable f-stop setting.
To make a portrait photograph stand out it is preferable to decrease the depth of field (lower f-stop value) so that the background “blurs” and does not distract from the main subject.
To get more of your landscape photograph in focus, use a higher f-stop value.