To decrease the depth of field and make less of the scene in front of and behind the
subject sharp, use a wider aperture, such as f/2.8 or f/4.
There are other ways to control depth of field. You can increase the depth of field by
changing to a shorter focal-length lens or stepping back from the subject, although
both of those choices will change the picture in other ways as well.
To decrease the depth of field and make less of the scene in front of and behind your
subject sharp, you can use a longer lens or move closer to the subject. These alterna-
tives will also change the composition of the picture.Lens Focal Length, Aperture, and Light
Why does a lens of longer focal length produce less depth of field than a shorter lens
at the same f-stop (aperture)? The answer relates to the diameter of the aperture
opening. The relative aperture (the same f-stop setting for lenses of different focal
lengths) is a larger opening on a longer lens than it is on a shorter lens (see
Figure 4.14).60 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
FIGURE 4.14
f/4 on a 50mm
lens and a
300mm lens.
The smaller
focal length has
greater depth of
field at the
same aperture
as a 300mm
lens.Both lenses were set at f/4, let in the same amount of light, even though the actual
opening in the 50mm lens is physically smaller than the opening in the longer
300mm lens.
The longer the focal length, the less light that reaches the film (or CCD chip on a
digital camera), therefore a long lens will form a dimmer image than a short lens
unless more light is admitted by the aperture.
The sizes of the aperture openings are determined so that at a given f-stop number
the same amount of light reaches the film, no matter what the focal length of the
lens. The f-stop number, also called the relative aperture, equals the focal length of
the lens divided by the aperture diameter.