There is an unfortunate idea in many people’s minds that somehow “digital”
photography is different from all previous types of photography, and a
completely different set of laws now prevail. Wrong! The optical principles are
just the same, with the light coming through the glass lens up front and the
light rays then striking the electronic sensitive receptor (instead of film).
A portrait with the face in focus
and the hand out of focus.
One optical principle you should try and experiment with is
DOF. What’s DOF? Quite simply, it is Depth Of Field, and mastery of DOF really
is the second rule of photography in my opinion. Before you ask, the first rule
is to walk several meters closer to the subject!
The Depth Of Field in any picture can often make or break the
entire photograph, but knowing how to manipulate the depth of field improves
your photography instantly!
The term DOF refers to an optical one and depends solely on
the lens being used and the aperture selected. Altering the shutter speed, does
not change the Depth of Field.
Depth of Field really refers to the zone of “sharpness” (or
being in acceptable focus) from foreground items to background items in any
photograph. This is different from what the eye sees, as the eye can instantly
focus on near and far objects, giving the impression that everything in your
field of vision is in sharp focus. The camera, however, gives you a slice of
time.
The first concept to remember is “1/3rd forwards and 2/3rds
back.” Again this is a law of optical physics, but means that the DOF, from
foreground to background in your photograph can be measured, and from the focus
point in the photo, extends towards you by one third and extends away from the
focus point by two thirds.
The DOF rule here is simple - the higher the Aperture number,
the greater the DOF and the lower the Aperture number, the shorter the DOF. In
simple terms, for any given lens, you get greater front to back sharpness with
f22 and you get very short front to back sharpness at f4.
For example, using a 24 mm focal length lens focussed on an
object two meters away - if you select f22, the DOF runs from just over
0.5 meter to 5 meters (4.5 meters total), but if you select f11 it only
runs from 1 m to 4 m (3 m total) and if you choose f5.6 the Depth of
Field is only from 1.5 m to 3 m (1.5 m total).
On the other hand, using a longer 135 mm focal length lens
focussed at the same point two meters away, you get the following Depths of
Field - at f22 it runs from 1.9 m to 2.2 m (0.3 m) and at f5.6 it
is 1.95 m to 2.1 m (a total of 0.15 m).
Analysis of all these initially confusing, numbers gives you
now complete mastery of DOF in any of your photographs. Simply put another way -
the higher the Aperture number, the greater the DOF; the smaller the Aperture
number the smaller the DOF; plus the longer the lens, the shorter the DOF, the
shorter the lens, the longer the DOF (just remember the ‘opposites’ - the longer
gives shorter).
Now to apply this formula - when shooting a landscape for
example, where you want great detail from the foreground, right the way through
to the mountains five kilometers away, then use a short lens (24 mm is ideal)
set at f22 and focussed on a point about 2 km away.
On the other hand, when shooting a portrait where you only
want to have the eyes and mouth in sharp focus you would use a longer lens (and
here the 135 is ideal) and a smaller Aperture number of around f5.6 to
f4 and focus directly on the eyes to give that ultra short Depth of Field
required.
Master it this weekend, and just remember that these optical
laws hold good for all cameras, be they film or digital.
