Digital Photography Technique: Camera Focus and Depth of Field
Every focus distance ensures that one specific plane will be sharp in the photograph. Whatever happens with the rest of the planes depends on various factors. If we represent depth of field lengthwise through our line of vision, the points in a scene which are very close to the focus distance will appear imperceptibly unfocused , until a point were it becomes evident. Therefore, the focused area covers a determined interval of distances around the focus distance. This interval has a variable depth depending on:
- a) the focus distance
- b) the diaphragm's aperture, and
- c) the lens focal length (7.1.7)
The vast majority of the scenes we want to photograph have depth, and therefore sharpness is needed very frequently in some or all planes . Focus in different planes constantly comes to attention, since it is a key factor in an image. When using a camera, it comes to attention that the focused depth of field varies greatly, and also, depending on the circumstances, the field which appears focused can be reduced to only a few centimeters, or can cover an area that extends from very close to the camera to
infinity. In practice, this means that in determined circumstances, with low depth of field and a minimal margin of error, focus is critical. But in other cases we enjoy much more tolerance, with a greater depth of field and minimal risk of blurriness.
2.1.2.1 Depth of Field and Focus Distance
Sharply focused planes corresponding to different distances get closer to each other with the greater the distance: for a lens, the distances of, for example, 5m and 10m are much "closer" than 1m and 2m. The lower the focus distance is, the narrower the depth of field will be. When the lens is focused on infinity, the depth in meters extends from infinity to the cameras limited distance. From one end to the other there is a depth of indescribable largeness. If we reduce the focal distance we will confirm that depth of field is limited and can be expressed as the difference of the distance between it's ends. The closer the distance we are focusing to, the closer the ends are to each other (therefore the focused depth of field is proportionally smaller).
Therefore, the possibility to sharply focus a scene will be greater the further we are from it. The closer we are to the scene being photographed, the blurrier different planes will get.