Aperture: The lens opening, expressed in what is called an "f" number (or f/stop). The f number is the number of times the diameter of the aperture divides into the focal length of the lens. i.e.: f/2, f/2,8/, f/4, f/5,6, f/8, f/11, f/16, f/22, f/32. In this example, as you move from f/2 to f/2,8, the amount of light, thus the diameter of the diaphragm opening (iris), is reduced. In our example, each consecutive number halves the amount of light that penetrates into the camera, e.g. f/2,8 cuts in half the amount of light allowed by an aperture of f/2. The same applies between f/4 and f/2,8 or between f/16 and f/11. The main thing to remember: The higher the number, the smaller the amount of light that hits the film.
Exposure meter (light meter): A device which measures the amount of light (ambient or flash). Most cameras have a built-in light exposure meter that is used to determine the correct exposure. In underwater photography, separate (external) light meters are commonly used since built-in meters are not always easy to operate when wearing a mask.
Extension tube: A metal cylinder placed between the camera body and the lens. This device shortens the focusing distance to a few inches. Extension tubes are designated according to the reproduction ratio they afford (image/subject ratio). i.e.: 1:3, 1:2, 1:1, 2:1
Film speed: The relative light sensitivity of the photographic emulsion (film). Expressed in ISO (International Standards Organisation) (ASA on older cameras), the film speed, or sensitivity to light, increases with the number. For example, an ISO 100 film requires less light than an ISO 50 film. Each time you double the film speed, half as much light is needed for a correct exposure. Faster films need less light but they produce grainier pictures. Slower films have a finer grain and they produce more contrast. Pictures taken on slower films are sharper in appearance. The most popular film speeds are: ISO 25, 50, 64, 100, 125, 200, 400, 800, 1600.
Fixed-focus: A camera where focus is not adjustable. These cameras are set for a typical focusing distance of more than three feet. Usually, everything from three feet to infinity appears in focus.
Focal length: The distance, in millimetres, between the centre of a lens and the plan of focus (film plane). This distance is measured with the lens focused at infinity.
Macrophotography or close-up photography: This term designates pictures taken at very close range where the image/subject ratio is larger than 1:6. Macrophotography pictures are usually taken at camera-subject distances of less than 18 inches (46cm). With a Nikonos or a Sea & Sea camera, such pictures are achieved using add-on lenses (diopters) or extension tubes whereas housed cameras rely on special lenses designated as micro lenses. The focal length of micro lenses varies between 50-60mm, 90-105mm and 200mm.
Normal lens: This term designates lenses that have an angle of coverage of approximately 46 degrees, which corresponds to the perspective given by the human eye. In 35mm photography, lenses of focal length between 48 and 60 degrees fall in this category. However, due to the refraction phenomenon, a 35mm lens used underwater behind a flat port has an angle of 46 degrees and is dubbed "normal".
Plane of focus: The point in a camera where all the light rays converge, forming a sharp image. In a camera, this corresponds to the film plane.
Port: The optical surface by which a housed camera or a lens sees an underwater image. Ports are manufactured out of glass or plastic, the latter being the most popular material. There are two kinds of ports: Flat ports, used for lenses with focal lengths longer than 35mm and dome ports (hemispherical) used with shorter focal length lenses.
Shutter speed: The length of time during which the camera shutter remains open. These speeds are expressed in seconds or fractions of a second. i.e.: 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. Each speed increment halves the amount of light.
Shutter speed-f/stop relation: There is a relationship between shutter speeds and f/stops. Consider the following shutter speed-f/stop combinations: 1/60 at f/16, 1/125 at f/11, 1/250 at f/8, 1/500 at f/5,6, 1/1000 at f/4, 1/2000 at f/2,8, 1/4000 at f/2. Any of these combinations represents the same amount of light. Thus, if your light meter gives a reading of 1/60 at f/16, you can use any of the above mentioned combination to achieve the same exposure. As you can see, each time you open the diaphragm by one f/stop, to maintain the same amount of light, you have to increase the shutter speed (you actually double it). For any given amount of light, this relationship remains constant.
SLR: Single Lens Reflex. This term designates a type of camera which incorporates a viewing system where the subject is seen through the lens. This image is reflected on a mirror and passes through a prism that restores the normal view. Interchangeable lens 35mm cameras are mostly of this type. Only one amphibious SLR camera is available on the market today: the Nikonos RS sold by Nikon.
TTL (Through the lens): This expression designates light metering systems installed inside the camera itself. Even though it applies to both ambient and artificial (flash) light, TTL is used primarily to describe an automatic, built-in flash exposure control system used in modern cameras.
TTL flash exposure: When TTL flash exposure control is used, a light sensor, positioned inside the camera, measures the amount of light reflected by the film during the actual exposure. Depending upon the film speed and the aperture, the TTL "brain" switches off (quenches) the strobe whenever a sufficient amount of light has produced proper exposure.
Ultra wide angle lens: This term designates lenses with an angle of coverage greater than 90 degrees.
Wide angle lens: This term designates lenses that have an angle of coverage between 62 and 84 degrees. In 35mm photography, this applies to lenses with focal lengths between 24 and 35mm. Remember that the angle of coverage varies depending on the type of lens and the port used underwater. For example, the Nikonos underwater 20mm lens falls into the wide angle category even if a topside 20mm belongs to the ultra-wide angle category.
Return to Photo 101: History of Underwater Photography
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Return to Photo 101: Macro Photography
Return to Photo 101: Ambient Light