Depth of field is the area in front of, and behind, your
subject that is in focus.

Your subject can be anything from a mountain
to a person’s eye.

There is no sharp delineation between a
photograph being “in focus” and “out of focus.” Instead, a picture will
gradually go from sharp to blurry. What causes something to look out of
focus is often referred to as the “circle of confusion.” This is when a
point, whether it’s a pixel or a dot on a print, slide or negative,
becomes so blurry that the blur can be detected by the human eye. The
blurrier the dot is, the more out of focus it looks.

If you’re
taking a picture of a person, you can have a very shallow depth of
field, and only their right eye may be in focus. Or you could have a
deep depth of field and not only would their entire body be in focus,
but so would the mountains behind them, and most of the ground that’s
between them and the camera.

It’s important to understand depth
of field so you can get the photograph you want. Let’s say you are
photographing a person in a crowded situation, like a Renaissance Faire,
and you want them to stand out in the photograph. In this case you
would use a shallow depth of field. This would throw the crowd behind
him out of focus, and since people are drawn toward objects that are in
focus, he would stand out in the photograph. On the other hand, if you
are standing on the edge of a meadow, and beyond the meadow is the Rocky
Mountains, then you could use a deep depth of field, so both the meadow
and mountains would be in focus.

Depth of field is a tool, and
you need to learn how to use it because it’s in every picture you take,
but you can control it to meet your photographic needs.

Here’s
how you can control Depth of Field.

• Aperture – The
  smaller the aperture in a lenses diaphragm, the sharper the picture.
  Shooting at F 16 will give you a greater depth of field than shooting at
  F 2. However, you must also consider light diffraction, which occurs in
  all lenses. Light diffraction is how light scatters in the barrel of a
  lens. Most light is focused by the lens elements to go directly to the
  film or sensor in the camera, but some light does just bounce around in
  the lens. The more light diffraction you have, the less sharp an image
  will be. Other things affect sharpness too, but that’s another article.
  As you close down the diaphragm, going from F 2 to F 16, this
  diffraction decreases, because less light is going through the lens. But
  there is an optimum F stop where diffraction is at a minimum, and it
  may not be F 16, it may be F 11 or 8. You will need to test your lens to
  determine this. But as a rule, F 16 (or 22 or 32) will give you your
  sharpest image.
• Focal Distance – The closer you are to your
  subject, the shallower you can make the depth of field. Shoot a person
  that’s just a few feet away, and you could only have one eye in focus if
  you want. Shoot that same person with the same lens when they’re 100
  feet away, and their entire body will be in focus, and probably much
  more.
• Focal Length – A shorter lens will give you more depth
  of field than a longer one. Shoot with a 50 mm lens, and you can get
  more depth of field than with a 200 mm lens at the same distance.

How
do you determine the depth of field for a picture you are taking? It
can be simple or complex, but you have three options.

• Experience.
  Shoot with your camera, a lot, and you will be able to have a pretty
  accurate idea. With a digital camera you can see the results instantly.
• You
  can use the Preview Button. This button can be found on all 35 mm film
  cameras, and most 35 mm DSLRs (Digital Single Lens Reflex) cameras. When
  you push the button it closes the diaphragm to whatever F-stop you have
  the camera set for. I know people that use this technique with great
  success, but for me it only makes everything dark.
• Your
  final option is to use a somewhat complex mathematical formula. I’ve
  never know anyone to use this method, but if you want to put your high
  school algebra to use, do an online search and you will find the
  formula.

To get a feel for depth of field, put a
yardstick or tape measure on a table, with one end pointing toward you.
Focus on the mid-point of the tape measure and taking pictures at F 2,
3.5, 5.6, 8, 11 and 16. You can put a piece of paper in the picture with
the F-stop you are using so you can easily compare the images. It may
be a little hard to see the change from say F 2 to 3.5, but you will
easily see the difference between F 2 and 16. Once you have a good
handle on this, go outside and do the same thing, but use what you find
outside instead of a tape measure.

Notice how the sharpness of the 25 changes with
the f-stop?

With experience, controlling depth of field will
become second nature to you, so get out there and start shooting.

Have
Fun,

Jeff Colburn (Guest Author!)

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