Depth of field is an important photographic concept. You should
not confuse it with depth of focus, which is the distance BEHIND
the lens that is going to be sharp in any given circumstances -
how accurate must be the positioning of the film !

Depth of field - DOF - is that area which appears sharp in the
final picture. In general terms, for any given aperture, and any
distance on which the lens is focused, about twice as much
distance behind the focal point will appear sharp as that in
front.

So, for example, if a lens is focused on an object 20 feet away,
and full aperture is to be used, then only that at 20 feet away,
and a few inches each side may be sharp.

This is how sports photographers manage to make sorts people
stand out from backgrounds. They use long, and very expensive
lens - may be a 300 mm. f2.8 - and they use them at full aperture
in order to get minimum depth of field AND use a reasonably fast
shutter speed. The shutter speed freezes the action, but the
focusing must be critically accurate as the DOF will be minimal.

DOF increases as apertures get smaller - so, if only a few inches
are sharp at f2.8, then 20 feet may be sharp at f22 - that's 7
feet in front of the subject, and 14 feet behind.

All of this can be used to compose you own masterpieces - so
aperture is very important for more than just exposure. It is
also a fact that depth of field at any aperture and focal
distance appears greater as lens focal length gets shorter. Thus,
a 300 mm at f22 may have 20 feet sharp, but a 28 mm at f22,
focused on the same object may give apparent sharpness from a few
feet in front of the camera to infinity.

RELATIVE and APPEAR - You will have noticed the use of these
words. All of these effects are relative to each other. It is
what seems to be sharp that matters in the final picture, and
even degrees of enlargement can alter the effect. Something that
looks sharp in an enprint may be quite indistinct in a 16X12
print.


SOME GUIDE LINES....

The longer the focal length of the lens, the less is the DOF at
any given aperture.

The shorter the focal length of the lens, the greater is the DOF
at any given aperture.

Aperture controls DOF - wide apertures mean less depth of field.

Distance from the subject effects apparent depth of field. A 2
feet depth of field seems much greater when the lens is focused
on an object 6 feet away, than when focused on something 20 feet
away.

Many modern cameras have DOF preview, and all should. This is a
button that stops the lens down to the taking aperture without
releasing the shutter. The DOF that will result can thus be
viewed directly through the camera in the case of SLR's. The
image can become very dark at small apertures though.

DOF can be very restrictive in close up work. The DOF when
shooting an insect at 1:1 - that's full size on the film is only
about 4 mm. at f22. That means a spider may well have many of
it's legs out of focus and, regardless of what some judges may
say, there is nothing that ANYONE can do to correct it. A simple
law of physics !

There is a setting known as the 'Hyperfocal Distance'. At this
setting you will get maximum sharpness over the maximum depth. It
can be quite useful to read text books on these subjects as, for
example, it can sometimes produce a BETTER picture to not focus
on the main subject, as long as you know that, at the TAKING
aperture, your subject will be inside the DOF area, and thus
appear sharp. You could have a whole foreground area sharp, with
the subject right at the back of the sharp area.