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CHAPTER 1 : BEFORE YOU SHOOT ■
Aperture and Depth of Field
It’s not only focal-length numbers that are different in the digital and film worlds.
Aperture comparisons between a digital camera and a 35mm film camera—at least
when it comes to the depth of field—won’t always give you the same results either.
Apertures, which are found in most lenses, work in tandem with shutters to
control the amount of light that passes through to the sensor or film. Aperture size is
expressed as f-stops, which represent the focal length (f) of the lens divided by the
diameter of the aperture (the number following the slash). The notation f/2 therefore
represents an aperture diameter that is 1/2 the focal length of the lens, and f/16 is an
aperture diameter that is 1/16 the focal length of the lens. f/2 is wider than f/16 and
allows more light to pass through the lens. Theoretically, an f/5.6 setting on any lens,
on any camera—digital or film—will always let in the same amount of light.
Depth of field is the range of acceptable sharpness in front of and behind the
plane of focus. As you’ll see throughout the book, the ability to control depth of field—
either to isolate a subject from its background or to put more objects in focus—is an
important creative technique.
As most of you probably already know, wider f-stops yield a limited depth of
field, while narrower f-stops increase the depth of field.
But depth of field is not determined only by f-stop. It’s also determined by the
focal length of a lens (longer lenses produce less depth of field); the distance from the
lens to the object (the closer the lens is to the subject the less depth of field); and, just
as important for the purpose of this discussion, by the size of the sensor—or for that
matter, the film. (For a very technical discussion of this topic I invite you to visit my
website:
www.shooting-digital.com.)
From a purely practical point of view, just as with small film formats, the
smaller the sensor your digital camera uses, the more depth of field is produced for
any given lens, f-stop, or distance. Depth of field, therefore, varies from one digital
camera model to another—and rarely will you get what you might expect without
some experimentation.
Later in the book, I’ll give you a way to test your digital camera and lens for
depth of field, and I’ll show you ways to deal with more depth of field than you want
(

“Know Your Camera: Controlling Depth of Field” in Chapter 3). For now, just
know that this is a potential issue with huge implications, especially, for example, if
you are shooting a portrait and you want the background to go more out of focus and
it doesn’t.
Bottom line: The amount of depth of field produced by your digital camera might not be
what you expect. You’ll need to experiment.
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