Focus and Depth of field
This is a preliminary introduction to
the concepts of focus and depth of field. For a more
advanced and technical approach, please refer to the article
Depth of Field.
Everyone surely knows how a simple
lens, such as a magnifying glass, can focus the rays of the
sun to a tiny but intensely bright spot, and even set things
on fire, if it is held a certain precise distance away. The
sun's distance from the lens is enormous, virtually
infinite, and the distance of the lens from its point of
focus is then known as its focal length.
If you try the same experiment with a
light-bulb just a few feet away and project its image onto a
sheet of paper, you will find that the point of sharpest
focus is now a little further from the lens than it was
before. There is a scientific formula that relates this image
distance, from the point of best focus to the lens, to
the subject distance, from the lens to the source of
light. For details, refer to the article Depth
When composing a photograph, you
generally want the main point of interest in your picture to
be sharply in focus. To achieve this, you use your camera's
focusing mechanism (as described in the Start here
article) to move your lens closer to or further from the image plane. Depending on the type of camera
you are using, you can see which part of your image is in
best focus. Check the article on Camera
The point to remember here is that
there is only one subject distance at which exact focus is
achieved with any given image distance (or focus setting).
Everything that is closer to or further from the camera than
this will be more or less blurred, or out of focus.
Objects that are near to the exact focus distance will be
only a little blurred, but objects that are much closer or
further away may be very fuzzy indeed. The issue of how much
fuzziness we can accept brings us to the question of depth
of field (DOF).
Depth of field is the range of subject
distances between which the image of an object looks sharp.
This range of subject distances extends from a point
somewhere in front of the point of exact focus (i.e. closer
to the camera) to another point somewhere behind, further
away from the camera. When we say 'looks sharp', we mean
that by looking at the photograph, we cannot detect any
fuzziness. The degree of sharpness needed for this depends
on many factors: chiefly the resolving power of human
eyesight and the size of the photograph - how much it has
Depth of field depends on how quickly
fuzziness increases as an object is moved closer to or
further from the camera than the point of exact focus. This
increase is always faster when the movement is towards the
camera: there is always more depth of field behind the point
of focus, away from the camera, than there is in front of
the point of focus, closer to the camera. There will be less
depth of field when the fuzziness increases quickly, as an
object is moved away from the point of exact focus. The rate
of increase of this fuzziness depends on subject distance
and the aperture setting of the lens.
Fuzziness increases faster when the
subject at the point of exact focus is closer to the camera,
so that depth of field is often extremely limited in
close-up photography. Fuzziness increases relatively slowly
when the point of focus is at a great distance from the
Fuzziness increases faster when using
a wide lens aperture (see the Glossary
if you are unfamiliar with any of these terms). If you want
to throw the background out of focus in order to have your
main subject stand out in clear focus, then use a fast lens
with the aperture wide open. Fuzziness increases relatively
slowly when you stop down and use a small aperture.
This is frequently what you want in landscape shots. Of
course, using a small aperture requires longer shutter
speeds, which is why serious landscape photographers so
often use tripods.
Other things being equal, depth of
field is greater when using lenses with shorter focal
length, and more limited when using longer lenses.
Once again, please refer to the
article on Depth of Field
for additional information.