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Night
work has always fascinated me, as it presents a
number of artistic and technical challenges. When
working at night, a photographer starts with a
"blank canvas" of dark, and through selective
exposure and light painting reveals only those
portions of the scene that are important. This image
was taken in the Kofa Mountains of Arizona on a
clear, cool, moonless night. Before sunset, I spent
several hours scouting for a suitable shot, finally
settling on this group of ocotillo branches reaching
for the large mountain in the background. I was
forced to set up near the ground, aiming almost
straight up, to get the required angle for this
shot. Once composition, focus, and aperture were to
my liking, I sat down and waited for it to get
sufficiently dark to start my star exposure.
Usually, about 80 minutes after sunset will suffice
for the sky to be completely dark (assuming there is
no moon or nearby light pollution from cities).
I made
sure that my lens pointed north, so that the image
would be aligned with the North Star. Since the
Earth's axis points directly toward the North Star,
as the Earth spins during a long exposure, the North
Star will appear as a fixed point, whereas all other
stars in the sky will form concentric circles around
that point. It is this motion of the Earth that
causes the apparent motion of the stars in the sky,
and allows photographers to record their movement
during long exposures.
Using
my camera's Bulb setting, I left my shutter open for
an hour to create these star trails. At the
beginning of the exposure, I fired my flash several
times with a red filter over it, to light up the
graceful ocotillo branches. I had worked out the
appropriate amount of flash before I started the
main exposure, shooting a few test exposures and
reviewing the results on my camera's LCD screen. I
also "simulated" the one-hour exposure to see how
much ambient light would be revealed, using a
30-second exposure at 3200 ISO, and thus knew that
the distant mountain would stay in silhouette during
the long exposure. Because the temperature dropped
into the 40s, I knew it would remain cool enough to
avoid the build-up of long exposure sensor noise,
which results from the sensor heating up during a
long exposure. Cool ambient temperatures prevent
sensor warm-up, and thus limit the need to use
in-camera noise reduction.
Finding
my way back to camp in the dark after the exposure
was quite a challenge. My flashlight's batteries
were running low, and by its dim light I had to
navigate a series of small ravines and patches of
teddy bear cholla cacti, which have nasty needles
that seem to leap of the branch into exposed flesh.
I managed to make it back before the batteries died,
with only minimal amounts of cholla spines sticking
out of my backside.
Technical data: Canon 5D Mark II, 14-24mm
zoom lens (@14mm), Bulb mode, manually
focused using Live View, ISO 400, f/8, one hour. |
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I had
to clone out a few errant light trails caused by
planes passing overhead, but other than that the
file was very clean and noise-free. Canon sensors
often exhibit a noticeable shift to magenta during
low-light exposures, so my main processing challenge
was to restore the color of the sky to a more
natural blue. This was done using Selective Color,
and shifting the blues in the scene to get rid of
the magenta color cast. I also increased contrast
slightly to make sure that the distant mountain was
rendered as a pure black silhouette. I added some
minor increase to saturation to bring out the color
contrast between the flash-lit ocotillo and the
star-filled sky above.
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Ian
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