Page 105 - AlertDiver_Winter2013
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On a more recent project at the Georgia Aquarium
I knew enough to kick the ISO up to 640, use f/8 and a
half-powered strobe for the whale shark and porkfish
in the foreground and to go with a relatively slow 1/40
shutter speed to reveal divers, nurse sharks and other
background details. The “vertical” light in the background
was challenging that day; the light streaming through
the skylights was that of overcast skies, but, gratefully,
the workflow enhancements of the digital realm made
adjustments on the fly (swim) much easier.
Motion Blur
Slower shutter speeds to increase light penetration in
backgrounds is an oft-used arrow in my photographic
quiver. Sometimes, though, I aim to portray motion in
underwater subjects. This can be accomplished by using
a very slow shutter speed (usually 1/20 or slower) and
panning in the direction of the motion.
Let’s use the photo of the competitive swimmer as an
example. I wanted to portray her speed and athleticism,
but it was midday in very shallow water. To get my shutter
speed to 1/25, I had to reduce my ISO to 100, my F-stop
to f/22 and use a neutral-density filter (a polarizer, in
this case) over my lens to reduce the ambient light on
the scene. Because I was at f/22, I had to kick my strobe
up to full power for it to have any significant effect. This
meant my recycle time would be slow enough that I might
only get one shot per pass as the swimmer practiced her
racing starts. Again, the goal was to blend the vertical light
from the sun with the power of my strobe, the horizontal
illumination.
This can work just as well on the coral reef, of course,
as can be seen in this shot of a French angelfish in
Bonaire. I chose 1/10, f/22, and, again, a full-power strobe
for this shot, but you may notice the motion that’s implied
by the blur behind the eye follows the right-to-left motion
of the swimming fish. This is as it should be, and it was
enabled by my strobe being set to fire in “rear-curtain
sync” (sometimes known as “second-curtain sync”).
A strobe can be set to trigger at the beginning of the
shutter travel or the end. In almost any underwater
exposure there are two forms of light striking the subject:
the available light from the sun and the strobe light. If the
shutter speed is slow enough, there will be a difference
between the distances traveled by the subject as frozen by
the strobe and as blurred by the ambient light. The blur can
be intentionally exacerbated by panning in the direction of
the motion. This makes the background indistinct, but if
properly timed it can minimize the motion of the fish. The
flash will freeze the action, of course, but with front-curtain
sync the flash goes off, and the fish continues to swim (and
be recorded). The motion will be shown forward of the fish.
With rear-curtain sync, the fish moves through the slow
shutter speed and at the end, the strobe goes off and freezes
action. That’s aesthetically preferable and perceptually
more realistic.
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