| For the purpose of discussion, let’s
suppose that there are three main factors determining how
much backscatter will be in your photographs. First to be
considered is how much particulate matter (crap) is suspended
in the water. The "visibility" factor varies from
place to place, from season to season, from day to day, even
from hour to hour, so that a little planning can go a long
way towards eliminating, or at least reducing the handicap
of dirty water. Plankton blooms and silt from rain runoff
are generally seasonal. If you can choose where you travel,
you might inquire as to the months which will offer the highest
probability of clear water. Once you have arrived on the dive
site and an assessment of conditions determines that the visibility
is poor, you could ask if conditions might be better at another
site.
Now that you’ve done the best that you can to get
the odds in your favor, you swim down through the "nutrient-rich"
(dirty) water toward the reef which lies ahead in a haze.
You may be able to find a sort of oasis of clearer, if not
downright clear water somewhere around the reef. A tunnel
would be an excellent prospect, especially if you’re
the first to find it. Look in nooks and crannies, especially
on the side of the reef that’s on the lee of the current.
Close to this side of the reef there may be some relatively
still water and much of the particulate matter may have settled
out.
Now that you’ve done your best to find the clearest
water you can, the next step is to eliminate as much of it
as possible. You’ll have fewer suspended particles to
light up between camera and subject if there’s less
water between camera and subject. This means macro or shorter
focal length lenses. If you’re using a standard Nikonos
camera, select you shorter focal length lens (a twenty-eight
instead of a thirty-five) or an extension tube on the boat.
If you’re using a Nikonos SLR or a housed SLR, a fifty
or sixty macro will give you a great deal of flexibility to
choose between fish portraits and macro, so that if conditions
don’t favor longer shots, you can switch to macro during
the dive.
Lighting technique is your third line of defense against
backscatter. Since the largest-appearing particles are the
most disagreeable, and since the closer a particle is to the
lens, the larger it appears on film, the best lighting techniques
are those that don’t direct light between the lens and
the subject. It’s natural to want to light the subject
with the middle of the strobe beam since there’s less
chance of missing with the light. In gin-clear water, this
works fine, but in dirty water bigtime backscatter is the
result. Try to light the subject with the inside edge of the
beam. Learning to do this takes a little study and practice,
but the results will be worth the effort. You can make it
easier to judge the edge of the beam if you secure a piece
of ten mil plastic about ninety to about 120 degrees around
the inside front edge of the strobe, forming a deflector.
The deflector needs to protrude about an inch or so.
Backscatter will become less noticeable as you move the strobe
light off the axis of the lens. It’s generally best
to avoid front lighting in anything less than very clear water.
At around forty-five degrees, scatter becomes less obvious,
and as you move further around toward side lighting, you see
less and less scatter. Of course you get harsher and harsher
shadows. However, this type of dramatic lighting may be very
pleasing. If it doesn’t please you, using two strobes,
each at forty-five degrees, and using the inside edges of
the beams to light the subject results in even, shadowless
lighting with a minimum of backscatter.
An observation: In underwater photography, as in life itself,
we seem to create most of our own backscatter. Be certain
that you’ve achieved total buoyancy control before you
settle to the reef. It’s important to arrive at camera
position as if you were gently descending by parachute and
not mask over teakettle as if you were shot out of a cannon.
Any kicking of fins will bring up murk and may be damaging
to pictures and the reef. If there’s a reef above you,
be aware that your bubbles will dislodge debris, and what
doesn’t become reef dandruff may become backscatter.
If you’re using extension tubes, do so with great care.
When adjusting the framer around the subject while maintaining
a stable body position in the water, it’s easy to inadvertently
grind up the reef with a framer, resulting in a cloud of scatter
around the subject.
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