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Let's talk about
some basic photographic rules
 The camera
It's the device that allows us to acquire photos, by
single or continuous mode.
The optic
It's the device that allows the projection of a luminuous
image on a surface. Basically, its goal reside to the
opportunity to let the light reflecting from the subject, to
converge all to the same point on the surface, where the subject
image will be acquired (specifically, on film support or digital
sensor). Typical feature of the optic is the focal lenght,
displayed in mm. This value's distinguishing the optic as super
wide-angle, wide-angle, normal, medium, tele and super tele.
In
the chart below, you can see how to each focal lenght
corresponds a certain value of field angle and consequently, a
different classification.
The film support
It's the device assigned to save the images when using an
analogic camera
The digital sensor
It's an electronic device with the same assignment of
film support, but with the great advantage that make it usable
infinitely.
The
shutter
It's the mechanical or electronic device that control the time
with who the film support or digital sensor, will be exposed to
the light.
The
exposure time
Also known as shutting time, or shutting speed, it's that time
for who, the
camera shutter, keep itself opened to make the light reaching
the film support or the digital sensor.
The
diaphragm
it's the optical element that determinates the aperture
diameter where the light's passing and its phisical dimension is
written on the lens barrel by using umbers. Such values are
showing the ratio between the in use focal lenght and diaphragm
value. If, for instance, the diaphragm value is 8, it means that
if we divide the in use focal lenght of the optic with the value
of the in use diameter hole, we'll obtain, as result, the factor
8! Much smaller will be this value, much bigger will be the
amount of light passing through. An optic with a minimum
diaphragm value of 2,8, shows that, when the diaphragm's totally
opened and so of the same value of the front lens diameter, the
ratio between the focal lenght and the front lens diameter it's
exactly 2,8! Usually, preceding this number, there is the "f"
letter, to show the nature of a focal ratio. Example: focal
lenght optic of 105mm; front lens diameter of 37,5mm. With the
simple operation 105/37,5 = 2,8 it's possible to easily realize
the value of the ratio! It is, infact, 2,8!
The ASA
Are the measurement scale of the "film speed impressing",
originally define from the American Standard Association and
today substituted by
ISO
scale (International Standard Oraganization). So, a correct
image will be created by properly managing the following
parameters:
ISO,
exposure time and
diaphragm. How do we get these values? In this case another
important camera device will
help us:
The
exposure meter
This
"electronic eye" is capable to "read" the light scene, to
preestimate the camera set up parameters and to help us to find
out which are the best setting about the light metering. Usually
the exposure meter shows a graduate scale with a indicator that
we have to manage to move it on the "0" value, in order to set
the best compromise.
Underwater
photography isn't too different fron the terrestrial one and
shares the same parameters. The biggest problem is the lack of
light in the water, due to its density that absorbs a large
amount of luminuous radiations as much as the depth increase. In
such case, the use of an artificial light source, like a flash
or an electric torch, is mandatory, in order to keep the real
chromatism. As you can observe from the chart on the up, the
first color to disappear will be the red, at already 5mt of
depth, followed from orange and yellow, until to reach, at the
depth of 30, a grey tonal environment.
Underwater flashes are
properly sized and potentiated respect to the same terrestrial
devices and that's why water, while absorbing, is capable to
reduce their light power at 1/3 than if employed in dry
conditions. Power rate of a flash is showed from the
GN (Guide Number).
Much higher will be this number, much elevated will be the power
of the flash. Thanks to the GN the photographer will be able to
determinate the right aperture value of the diaphragm to catch a
subject standing at a determinate distance and to choose the
right ISO sensitivity. The GN is usually suitable by using as
references the distance of 1 mt and the ISO of 100. Here an
example: if we get a flash with GN of 8, we must to set the same
value for the diaphragm of the optic and off course if the
subject is located at the distance of 1 mt and that the film
support or the digital sensor, will have an ISO value of 100.
How to use the flash underwater
In the water we
dive, there are many kind of particles of various nature and
dimension. When hit from the flash light, these particles starts
to bright and their presence is perceived from the film support
or the digital sensor. The generated effect is disagreeable
cause image will appear as if captured in a foggy day. To avoid
or limit such effect, we need to incline the flash towards the
optic axis. To explain better: try to imagine the particle
suspension, posed between you and the subject, as the moon! You
may observe that when moon is moving respect to us and the sun,
it'll be ever more or less visible, due to its phases. So We
must to reduce as well the brighten visible particle surface! To
do so we have to incline and pull away the external flash, aside
from the camera. The image below shows schematically how to
manage about:
On the market are existing
many flashes working in TTL mode (Through The Lens), able to
"read" the light on the scene, simply through the camera lens
and adjusting the emission of the flash light, in order to
ensure the correct exposure. With the advent of the digital SLR,
the manufacturers modified some operating protocol of such
flashes, introducing the new "iTTL" acronym, to differentiate
them from the old "TTL". The two system are equals. The only
inconvenience is about the old TTL underwater flashes, needing
an adapter containing a Heinrich circuit variant. This system is
still under test, though some underwater equipment it includes
in their cases. Most of these adapters allows manual mode
employment of flashes, by leaking the TTL detection.
Choosing the dome port
When in the water, there is not anymore the opportunity to
change the employed optic, so, once we have choose our diving
modality, we must decided which optic to use among the others in
our equipment.
There are two main guide lines about "fotosub": the
environmental photo (equivalent to the terrestrial landscape
photo) and the macro photo (or close-up photo); the last one
it's technically equal to the terrestrial macro.
The
environmental photo
It provide the employment of a wide-angle (or super wide-angle)
optic. Our choice will go on that optic capable to frame the
most larger angle of view. Best choice for our goal is the
fish-eye lens, capable to show a 180° angle of view, or the 20mm
lens with its above 90°. To such lens we need to couple the
super dome. Using a normal flat dome we'll obtain a noticeable
vignetting effect at the edges of the image, just because optic
will frame the inside dome part too; beside to that, refraction
will start to spread considerable cromatic aberration. The
refraction is that phisical phenomenon that cause the deviation
of the ray of lights, passing through materials or substances of
various nature, like water, glass and air. Less orthogonal the
ray lights will be (passing through the material surface) and
more evident the refraction will appears. That's why the super
dome are also named "correctors". Despite of this, refraction
will be always present. When cromatic aberration is negligible,
there is still present another oriblem: the "virtual image".
What is virtual image? It's that phenomenon once more
ascribable to the refraction, which deviating ray lights, will
produce, in front to the dome, an identical but closer image of
the subject we are capturing. So, our optic has to be able to
focus that virtual image to a much lower distance than the real
one. If the optic we use has got a minimum focus distance
superior to that one where the virtual image appears, it'll be
not able to properly focus. Our lens will be, so, a myopic one!
What to do? Let's put glasses on our optic! Over the front lens
of our optic, we can mount a dioptric one. This lens allows us
to reduce the minimum focus distance, making our optic "to see"
the closer image. Solving the main problem, unfortunately we'll
get another one: The optic will lose the ability to focus at the
infinite: that's why, reducing the minimum focus distance, we
will also reduce the maximum one! Dioptric lenses more in use,
shows values among +3 and +4 diopters. In the picture below is
possible to see how the virtual image appear:
In the picture below, some of the most commonly aspherical dome
employed:
 
The macro photo
(Or close-up) it's performable by employing dedicated
lenses, just named "macro", or using normal lenses too, but
anyway able to focus at close distance (about 40cm). Best lenses
about macro, are among 50 and 100mm of focal lenght and in this
case, the flat dome, will be plate, because, thanks to the
reduced angle of view of such optics, will be also possible to
avoid cromatic aberration.
This section's under developing work, to be expanded to ensure
the best informations.
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