F-stop is the adjustment to the "Aperture" or diameter of the camera's iris.
This is one way to control the amount of light reaching the film. The larger the number,
as in. f-stop 22, the smaller the opening. The smaller the opening the greater the depth
of field.
Depth of field refers to range of distance that the focus will be sharp. If the focus is sharp between 10 and 15 ' then the depth of field is 5' . Smaller aperature settings usually result in a greater depth of field.
Shutter speed is the amount of time that the aperture is open. This is usually calibrated in fractions of seconds like 1\60 of a second. This is one way to control the amount of light reaching the film.
TTL metering is electronically controling the flash with sensors built into the camera. This can control both timing and intensity of the strobe as well as the shutter speed.
Film speed refers to the sensitivity to light for a certain film. A film speed of 400 will require less light to atain proper exposure than say film with a 100 asa rating. Faster films are more likely to become grainy when enlargements are made.
Colour absorbtion is caused by the water filtering the light spectrum. The first colours to be absorbed are the reds and oranges, the last are blues and greens. When considering color absorbtion, be sure to use the actual distance,(from the flash to the subject and then back to the film in the camera.) When shooting 10 feet from your subject the light from the flash travels 20 feet before reaching the film.When taking photographs at 50' and being 10 ' from your subject the "natural" light has travelled through 60' of water.
Parallax correction is required when shooting close-up photographs with a viewfinder
camera. The adjustment corrects for the difference between what the lense sees annd what
the viewfider shows. Most cameras require no correction when taking pictures from more
than 5' away .
Macro photography underwater translates loosely to shooting small subjects at 1\3 to 1\2
the actual size, at close range. Photographers will most likely get the best results
shooting static coral and small creatures such as nudibrach in this manner. This is
extreme close-up photography. (0-18 ") Due to parallax distortion the use of a framer
is recommended with viewfinder cameras. Be careful as most subjects don't like being
bumped while you set up your shot. When using housed cameras, what you see is what you
get. No framing required. This benefit may be offset by increased bulkiness of housed
systems. Personal preference and of course your budget would determine which is for you.
It takes a lot of patience and stealth tactics to capture timid, faster moving marine
life.
Wide angle photography has the benifit of enabling the photographer to get closer to their
work, there by reducing the water column between them and their subject. A typical
"standard" lense is usually between 28 and 35 mm. Wide angle lenses vary from 15
to 20 mm. The field of view for a 20mm lense (Motormarine) is 80 degrees, a 15mm will
cover over 90 degrees. A standard lense covers approx. 46 degrees.
When "upgrading" to a wide angle lense a photographer must also consider
their strobe. Most underwater flashes have a coverage of 90 degrees. This means that if
you go for the 15 or 16mm lense then you must also add a second flash for even lighting.
(Nikonis makes a strobe that will cover the wider lenses, though it is fairly pricey) The
main uses for this type of photography is in capturing large marine life, shipwrecks, and
divers. These types of lenses do not work well for smaller reef life as the subject gets
lost in the background. These photographs are best shot with a Standard or Macro setup.
Portraits of scuba divers can take many forms.Whether you are shooting subjects on
shipwrecks, on reefs or silouettes in the sun, the key is framing your subject and
composing the shot to enhance the surroundings. Many photographers like to use wide angle
lenses to reduce the water column between them and their subject. A 20mm wide angle lense
will capture a average size diver from about 4 feet. A predive breifing is important when
working with "models" . It should cover basic hand signals and how you would
like to execute the dive.
Portraits are not neccessarily "static" or posed. An excellent technique is to stalk divers the same way you would approach animal life. Divers who are more intent on the surroundings, rather than the photographer will yeild natural shots of divers diving rather than "smiling" with regulators.