Depth of field refers to the zone of acceptable sharpness in an image, both in front of and behind the point of focus. It is controlled by the aperture, lens, and distance to the subject. A smaller aperture number, shorter focal length lens, or greater distance to the subject will result in a greater depth of field, keeping more of the image sharply focused. Understanding depth of field allows photographers to control what the viewer's attention is drawn to in an image.

1. Why is this important? This is what draws the
viewer’s attention to what it is that you want
him or her to look at when they look at your
photos. It should be that which is in focus.
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2.  Depth of field is simply the zone in front of and
behind the point of focus that is acceptably
sharp. Simply, how much of your picture is in
focus. We try to impress people with the beauty
of our pictures, so we have to know what effect
focus has on the viewer. Depth of field is what
we use to control what the viewer looks at.
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3.  Depth of field is controlled by your choice of
Aperture, Lens and Working Distance. It is also
controlled by your vision of the scene.
 Aperture as you probably know is expressed in
f-stops. There are two numbers you must
memorize to master f-stops. They are 1 and 1.4.
All f-stops are then a mathematical progression
of those two numbers. It goes like this:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, and 64.
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4.  The closer you are to the subject the shallower
the depth of field.
 So if you want to shoot a bee on a flower you
could use a 28mm lens which will give you
better depth of field but not much closeness or
you could use a 200 mm lens and stand farther
away. But you would have to stand 10 feet
back. Do you know why?
 Orrrrrrrrrrr, you could use a 200 mm lens with
a 25 mm extension tube and get up about 5 ft
closer.
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5.  An aperture, usually referred to as an “ap”, is a
hole through which the light from a scene passes
to land onto the film or sensor plane. It
represents the diameter of the opening as a
fraction of the focal length of the lens. So for
example, f/5.6 on a 100 mm lens is a larger
opening, 1/5.6 100mm than, f/11 which is 1/11
the opening of a 100mm. So the larger the f/stop
number the smaller the lens opening.
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6.  In a visual sense this means that the apertures
are related by size. So, an “ap” of f/2.8 is twice
as wide as an “ap” of f/4.0 and an “ap” of
f/4.0 is twice as wide as an “ap “ of f/5.6. 5.6 is
twice f/8, f8 is twice f/11, f11 is twice f/16,f/16
is twice f/22, f/22 is twice f/32, f/32 is twice
f/64. How many times bigger is f/4 than f/16?
See if you can figure that out. The upshot of all
this is as you go from f/2.8 to f/16 you get half
as much light each time you change f/ stops.
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7.  You can check out the depth of field if your
camera has a depth of field preview button.
When you push the preview button it actually
changes what you see. When you focus a shot
before you press the shutter, the lens is actually
wide open so you can see to focus. Pushing the
preview button stops the lens down to the actual
setting you have chosen for the shot. That allows
you to see what’s in focus and what’s not.
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8.  So how does lens figure in? There are a lot of factors in
your choice of lens. What is your subject? Time of day?
Wind? Stationary object? Subject to distance? You are
standing on an overlook in Yellowstone N.P. using a
28mm lens to get a vista shot. If you choose f/5.6 as
your aperture and the closest object is 500 yards away
you may get an acceptable shot. But if you choose f/32
you’ll get a great shot, all other things being equal.
Secret: your exposure time is controlled by your
choice of aperture. Thus your depth of field is a
function of time. The smaller your aperture the longer
your shutter is open; the longer your shutter is open
, the greater your depth of field.
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9.  So if you are trying to isolate a flower you
might chose a 100 mm lens or a 200 mm lens
because you want to isolate the flower from the
other flowers or other foliage. The longer the
lens, the shallower the depth of field.
Conversely, the shorter the lens the greater the
depth of field. So if you use a Macro Lens you
will get a very small depth of field. Adding
tele-converters will cut down your depth of
field more as will the use of extension tubes.
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10.  Extension tubes are used for magnification
purposes and follow a simple formula.
 That is:
Extension
Magnification = ------------------------------
Lens length in MM
They also limit Depth of field
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11.  Speaking of lenses, an important statistic to
know is what is the focusing distance of the
lens you will use for this shot? I bring this up
because it will help visualize the shot. My 70-
200 f/2.8 lens stops focusing close up at 10 feet.
If I want to get closer I have to add extension
tubes. So read the papers that come with your
lenses. The extension tubes ad magnification.
This affects depth of field in the reverse of a
grand scenic shot. You use this to isolate a
flower, a bug, or some other small subject.
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12.  Depth of field isolates a subject for the
viewer, be it a grandios scene, a single object
within a wider view or a close up .
 You control this by a). Lens
 b). Aperture
 c). Working distance
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