116 Better Available Light Digital Photography
Zoom lenses for SLRs come in two varieties: fast or slow. Many
of the slower zoom lenses have a fl oating maximum f-stop. That
means the maximum opening changes within the zoom range of
the lens. A 24–105 mm f/3.5–f/5.6 zoom lens is a good example.
At the widest focal length of 24 mm, the maximum aperture is
the f/3.5. As the lens is zoomed toward the telephoto end, that
maximum aperture shifts to the f/5.6 opening. Many popular
lengths of zooms feature an f/2.8 max opening: The wide-angle
lenses with ranges of 16–35 mm, 17–55 mm fi t in here. The
24–70 mm is a workhorse mid-range zoom and the 70–200 mm
is its counterpart at the longer zoom range. Expect to see these
in the gear bags of most professional photographers. Less expen-
sive, less bulky, and lighter are the f/4.0, f/4.5, and the fl oating-
aperature zooms mentioned before; but these are best left for
outdoor, normal lighting work.These “slower” lenses work fi ne in the normal outdoor environ-
ment. They’re great for vacations, mountain and seaside hikes,
the amusement park, and general snapshot use. They fall short
when the light dips below “normal.” Using the water pipe
analogy, we have to open the valve, letting more water through,
or perhaps consider getting a larger-diameter pipe. Once the light
levels diminish, we can open up the aperture, increase the shutter
speed, increase the ISO setting, or any combination of these.
With slower lenses, we run out of apertures quickly, leaving us
with only the other two options: As ISO is increased, so, too, is
the amount of digital noise in the fi les. Once these two are maxed
out, a shutter-speed increase is all that’s left. How well can you
handhold the camera for a sharp image? How still can your
subjects pose in these low-light conditions?There’s an old rule of thumb for actually determining the best
shutter speed to use with a particular focal-length lens to get
sharp images. By taking the focal length of the lens as the
denominator in the shutter-speed fraction, we have a guideline:
a 400 mm lens delivers sharp images when shot at 1/400 of a
second; a 50 mm lens at 1/50 of a second; an 85 mm lens at 1/90
(rounding up to the nearest actual shutter speed). These types of
comparisons are not exact, but are designed to serve as guide-
lines for shooting, as well as for determining if it’s time to use
a tripod. (See Chapter 6.) These guidelines do, however, show
us that the ability to capture sharp images goes down in lower
light because it’s not always possible to use a 1/400 second
shutter speed at an indoor event.Take a look at old-time photographs. Ever wonder why they’re
usually portraits with little or no action involved? That’s because
the speed of the capture medium of the time—daguerreotypes,
tintypes, and glass-plate negatives—was very slow. The lens
openings were also very small, requiring longer exposures.