| First, please allow me to "cop
out" by saying that mode selection might depend on the
experience of the photographer, how fast things are happening
during the dive, how fast a subject might be moving, how much
light is around, whether you can change the mode during the
dive, what controls you can access on the housing, and a number
of other factors. Automatic exposure modes provided include
shutter priority, wherein you select the shutter speed and
the "brain" in the camera selects an aperture at
which a "normal" exposure is achieved; aperture
priority, wherein you select the aperture and the camera selects
the shutter speed; and the different program modes, wherein
in various physical and metaphysical data is processed by
the camera’s computer and an ultimate "optimum"
shutter speed/aperture combination is divined. There are two
major reasons that automatic exposure modes came to be offered
in the first place. First, this made it possible for the average
person on the street (or on the reef) to achieve much better
exposure results, since the foundation of knowledge on which
to base exposure decisions was now present in the camera and
didn’t have to be learned. Secondly, there are just
times when so much is going on and things are happening so
fast that even the accomplished photographer would appreciate
being able to concentrate on the action and matters of composition
and accept calculations on exposure provided by the camera.
The computer in the camera will just about always make a quicker
decision than any human, and in many situations, that decision
will be as food or better than even the experienced photographer
can make under the circumstances.
Virtually all of the time you’re shooting macro you’ll
be looking to maximize your depth of field and will therefore
be using very small apertures—probably f22 or so. Areas
not lit by the strobe will generally be black since there
will probably not be enough ambient light to affect the film
at f22, so you might as well use a faster sync speed. No useful
purpose would be served by shooting in any mode other than
manual, although you can use aperture priority, since your
particular camera will float the shutter speed between 1/60
and 1/250 in this mode and would select 1/60 in this situation.
Program modes or shutter priority will probably result in
a wide open aperture and no depth of field, since the brain
in the camera makes decisions but has no judgment.
When a blue water background is desired, and the strobe is
used mainly to reintroduce color to the close subject, it
is useful to think in terms of two separate and independent
exposures in a single frame. First, you have a strobe exposure
of the close subject, which will be determined either by aperture
and strobe distance and output, or by TTL. You also have an
ambient light exposure determined by a shutter speed/aperture
combination. Most of the time, the ambient exposure is background
water. There may be a diver, boat, shark, sea serpent, or
whatever in the background water, and they will be exposed
at the same relative level as the water around them—but
when you talk about ambient light exposure, you’re generally
talking about the color of the water in the shot. You may
select that combination, using the light meter in the camera
to guide you, and expose in the manual mode. Most experienced
photographers would choose to shoot this way if there were
time to check the meter, make the adjustments, and still make
the shot.
You could also choose to let the camera direct the ambient
light exposure. If you’re using a strobe, you can’t
select a shutter speed faster than the maximum sync speed
for your camera (with at least one tricky exception) . You
could select this speed, or a slower one, and allow the camera
to select an aperture. You’d want to monitor the information
in the viewfinder to be sure the aperture selected will provide
enough depth of field for the shot. Alternatively, you could
go with aperture priority, allowing the camera to select a
shutter speed. You’d want to monitor the info to be
sure that your shutter speed isn’t so slow that a blurred
available light image will result. You could also select program
mode and let the camera "optimize" the exposure.
It might be useful to think of the light meter in the camera
as a sort of "blue-ometer," since its’ used
to measure the blue off in the distant background most of
the time. It might also be very helpful to use some of the
leftover frames you might have at the end of a dive to run
some blue-ometer tests. Set the camera to manual. Twist either
or both of the shutter speed and aperture control knobs until
you get a zero reading on the meter while pointing the camera
into the background water. Make an exposure. Repeat at settings
that give meter readings in one-third-stop increments to plus
and minus two stops. Take a look at the resulting frames,
and you’ll find a range of blues from almost white to
almost black. When making an ambient light exposure decision
in the future, you can pretty much decide how dark or light
blue you want the water to be and go from there. You may want
to separate a diver in a wet suit from the surrounding water
and you may want to go a little lighter than "normal"
on the background water. Be careful on the plus side to make
sure your subject is not going to be overexposed by the ambient
light. Far more often than not your foreground won’t
have as much ambient light on it as the background due to
frequent upward camera angles, subjects in the shadow of all
or part of the reef, the sun being in the shot, etc., but
the possibility of overexposure of the subject by ambient
light must be considered when electing to overexpose the water
column. You may elect to underexpose the water to reduce ambient
blue light on the subject, giving more pop to your strobe-lit
colors, to isolate a subject from the background, or simply
because you like the rich blue color of the water when it’s
a third or two underexposed.
If you don’t want to deal with these considerations
and just want to shoot carefree and automatic, or you’re
in the middle of a passel of zipping, ripping sea lions or
a shark feeding frenzy and are constantly changing camera
angle and direction, you can use a program mode as a default
setting. You’ll get your blue water and TTL fill light
pretty near right most of the time. IF your pictures come
out incorrectly exposed, it wasn’t the fault of the
cameraman, it was de fault of the camera—mon!
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