How Optical Zooms Zoom
The positive element closer to the camera end of the lens doesn’t
move. The other positive element (at the front of the lens), along
with the negative element, moves in response to the photographer’s
use of the zoom control.3
A single zoom lens lets photographers range through an infinite number of focal lengths
from wide-angle through normal to telephoto. At first, the zoom lens was scorned by
professional photographers because it lacked the image clarity of a primelens—one
that has a fixed focal length. Not surprising. Zoom lenses have as many as 30 separate
lens elements that move in complex patterns. Because advances in
optics have improved the sharpness of zoom lenses today,
zoom lenses are accepted by photo pros and are a
standard fixture on most digital cameras. The
dozens of designs for zooms are still
complex, but the concept for
many boils down to the
simplified zoom
lenses we have here.
(^56) PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
Call a Zoom a Zoom
The power of a zoom lens—the range of its focal lengths—is commonly
expressed in multiples of its widest focal length. For example, a zoom lens
ranging from 100mm to 400mm is a 4X zoom. The term hyperzoom is applied,
mostly by marketeers, to lenses with exceptionly large ranges, more than 4X,
with some as high as 12X.
Light bouncing off the
subject of your photo
begins the journey to
create a photograph
when it enters the front
of your zoom lens.
Specifically, the light first
passes through a part of
the zoom called an
afocallens system,
which, as its name sug-
gests, has nothing to do
with focusing the image.
The afocal system con-
sists of three lens
elements that are con-
cerned only with the size
of the image when it
reaches the image sen-
sor at the back of the
camera. That size is a
direct result of the
positions of the three
elements. Their positions
come from the photogra-
pher operating the lens’s
zoom control.
1
The afocal part of a zoom lens has two positive elements, which
causes light rays to converge. Between these two is a negative ele-
ment, which causes light rays to diverge and makes images larger.
The elements in this illustration are reduced to a single simple lens
for each element.
2