"Avogadro"
<Avogadro@Fotonet.com> wrote in message
news:d1t6lv4n936j7153hshvq23nn6ieuopfdc@4ax.com...
Everything
about depth of field was probably very well understood a
couple
hundred years ago - which is before cameras were invented. And
yet
today, DOF is poorly understood by most photographers. Why would
that
be?
Well,
DOF equations are not that complex, but most photographers are
not
really very good mathematicians, and most would hesitate to
undertake
DOF calculations. Besides, there is the difficulty of
applying
the results, because they involve concepts that can be
unfamiliar,
like the circle of confusion, resolution, subtended
angles,
etc.
Because
of these things, it is easy to make statements about DOF that
are
quite confusing. For instance, what would you think about the
following:
o Wide
angle lenses have more depth of field than telephoto lenses.
o All
lenses have the same depth of field at the same aperture.
Both
statements are quite correct when applied to many everyday
photographic
situations. Notice, though, that they disagree!
Accordingly,
statements like the above incite hours of discussion on
r.p.e.35mm,
often liberally spiced with insult, ridicule, and
diatribe.
DOF is
the zone in the picture, from near to far, that appears to be
in
sharp focus. I think most of us, when presented with an actual
photo,
would have trouble deciding exactly what is sharp and what
isn't.
Left to ourselves, most would pick different zones of
acceptable
sharpness. And it would vary by our mood too, to say
nothing
of the size of the picture, the lighting, the subject matter,
and
whether the photo is even critically sharp to begin with. So,
although
we have equations that can calculate DOF very exactly, it is
actually
a somewhat fuzzy quantity.
Although
it is fuzzy, there is good justification for understanding
DOF
well enough to be able to make decisions in our photography... to
answer
questions like, "Will I get better DOF with 35mm or with medium
format?",
"Do I have a hope of getting the front and back flower
petals
in focus at the same time?", "Should I switch to a wide angle
lens?",
and so on.
Anyone
can download DOF calculators from the Internet and do the
calculations.
Or you can punch the equations into a spreadsheet and do
the
calculations there. (Caution: many of the equations in books and
Web
sites are simplified versions and will not always give exact or
consistent
answers.)
But
when we find ourselves at a shoot with a DOF issue, there's no
time
for calculations. So here then are some RULES OF THUMB to guide
you in
your shooting. (Photographers love rules.)
1) Wide
angle lenses have more depth of field than telephoto lenses,
at
EQUAL CAMERA-SUBJECT DISTANCES. (We are assuming all other factors
are the
same.) In general use, wide angle lenses rarely present DOF
problems,
but long teles sure do.
2) All
lenses have the same depth of field at the same aperture, if
the
IMAGE SIZE IS KEPT EQUAL. Thus, if you want more DOF when shooting
a
portrait, changing focal lengths won't help. The only recourse is to
stop
down.
3) A
portion of the zone of acceptable sharpness will be on the near
side of
the plane of focus, and a portion will be on the far side. The
DOF on
the far side is always bigger, sometimes by a microscopic
amount,
sometimes by an infinite amount. The old rule that near DOF is
1/3 and
far DOF is 2/3 is ALMOST ALWAYS WRONG, but is sometimes
approached
reasonably for in-between DOF situations (subject a few
feet or
meters away, limited light).
4) When
DOF is shallow (such as in close-up photography or with long
telephoto
lenses), the near and far zones are about EQUAL in size.
5) When
DOF is deep (such as when shooting scenics with wide angle or
normal
lenses stopped down to f:8 or smaller), the far zone can extend
beyond
the horizon. The total DOF is INFINITE (but the near zone is of
course
limited).
6) You
will get more DOF with smaller image formats. The 35mm format
is
relatively small and is great for DOF. Large format cameras can
give
very narrow DOF, and this can be effective in portraits when used
skilfully.
Most film formats are larger than the retina of the human
eye, so
DOF in photos often seems shallow compared to what the eye
sees.
7) Cameras
with swings and tilts can place the plane of focus on quite
a
slant, and the DOF follows... this can allow you to get a whole rug
in
sharp focus from front to back. DOF apparently increases, but
doesn't.
8)
Comparing 35mm and 120 formats (each using lenses with similar
angles
of view), you will get about the SAME DOF if the film in the
larger
camera is TWICE as fast (because it allows you to stop down one
stop).
9) When
DOF is shallow, you can get roughly DOUBLE the total DOF by
closing
down TWO f-stops. If you want to keep the shutter speed the
same,
use faster film. Thus, quadrupling the ASA doubles the DOF.
10)
When doing close-up or macro photography, if you want to calculate
the
DOF, use the MAGNIFICATION that you are shooting at. Don't try to
use
lens-subject distance (because most people won't know where the
front
nodal point is). ("Magnification" is the size of the image on
the
film compared to the actual subject size. Thus, in 35mm
photography,
a magnification of 0.5 means the subject measures
48x72mm.
Many macro lenses have a magnification scale... if not,
magnification
can be estimated fairly easily.)
11)
With a tele-converter, the DOF will be for the focal length of the
combined
assembly.
12)
Cropping an image when printing magnifies the unsharpness, so you
should
have stopped down more when shooting to compensate.
13)
Poor quality lenses give deeper apparent DOF... the eye is more
tolerant
because of the lack of tack-sharpness. The same applies to
pictures
that are unsharp because of camera movement, poor enlarging
techniques,
etc.
14)
When DOF is shallow, you can't just keep stopping down to get
everything
in sharp focus. Macro photography is a common application
when
even at f 22, things may not all be in focus, and you will have
to
compromise. At small lens openings, sharpness suffers because of
diffraction
(due to the wave nature of light). Diffraction becomes
noticeable
at f:16-22 and gets worse as the aperture becomes smaller.
15) In
actual shooting situations at a particular f stop, you can
maximize
the total DOF by setting the infinity mark on the focus scale
opposite
the far DOF limit (assuming these are marked on your lens).
This is
the same as focusing on the hyperfocal distance. You can get
the
hyperfocal distance using those downloaded programs or equations
we
talked about before.
16)
Some lenses have DOF preview. This is a nearly useless feature
unless
you train yourself to use it... because the viewfinder image
darkens
a lot, and because the eye can often see a bit past the
viewfinder
screen to view the aerial image, making the DOF seem deeper
than it
is.
Avogadro
-------------
Nice
piece.
Thanks!