Basic Underwater Photography Terms
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