F-NUMBERS: A series of numbers designating the apertures, or openings at which a lens is set. The higher the number, the narrower the aperture. For example, f/16 is narrower (by one stop) than f/11--it lets in half as much light. An f-number range might be f/2.8, f/4, f/5.6, f/8, f/11...To find the next aperture in a narrowing series of single full stops, multiply by 1.4. F-numbers are arrived at by dividing the diameter of the opening into the focal length of the lens, thus a 10mm diameter opening on a 110mm lens is f/11. Alternately used with f-stops.
FAST: A term used to describe a film or sensor setting with a relatively high light sensitivity, a lens with a relatively wide maximum aperture, or a shutter speed, such as 1/8000 second, that will freeze quick action.
FILE FORMAT: An arrangement of digital information that may be particular to an application or generally adopted for use by a wide range of devices. Image formats in wide use include JPEG and TIFF. Raw format is proprietary to each maker, and often each camera by that maker.
FILL-IN FLASH: Flash used outdoors, generally to balance a subject that is backlit. Can also be used to control excessive contrast, add light to shadows, or brighten colors on an overcast day.
FILTERS: Any transparent accessory added to the light path that alters the character of the passing light. With film, filters can alter contrast, color rendition, or the character of the light itself (diffusion, diffraction, etc.) In printing, variable contrast filters are used to evoke different contrast grades from variable contrast black and white paper. In computer imaging software, a set of instructions that shape or alter the image information. In digital processing, filters are algorithms or a set of actions that change the character of the original image. Filters in digital are often "plug-ins", which means they work within the architecture of the main image processing program.
FILM: A compilation of light sensitive silver salts, color couplers (in color film), and other materials suspended in an emulsion and coated on an acetate base.
FINE GRAIN: Usually found in slow speed films, a fine-grained image is one where the medium of light capture and storage, the silver halide grain, is virtually invisible in the print or slide. With high, or coarse grain films (usually very high speed films) the texture of the grain becomes part of the physical reality, or weave of the image.
FIXER: The third step in black and white print and film processing; the bath removes unexposed silver halides.
FLARE: In lenses, internal reflections and/or stray light that can cause fogging or light streak marks on film. In general, zoom lenses have more potential for flare than fixed-focal-length lenses; in either case a screw-on lens hood helps reduce the problem.
FLASH: The common term used to describe the burst of light produced by passing electrical current through gas in a glass tube.
FLASH MEMORY: A special type of RAM memory that can hold data without electrical current. It is used in memory cards, the removable "digital film" used in digicams.
FLAT: Low in contrast, usually caused by underexposure or, in film, underdevelopment. Flat light shows little or no change in brightness value throughout the entire scene.
FLATBED SCANNER: A scanner that uses a linear CCD array for digitizing prints and film. Generally, the image is placed on a glass plate and the array moves past the artwork.
FOCAL LENGTH: The distance from the lens to the film plane or sensor that focuses light at infinity. The length, expressed in millimeters, is more useful as an indication of the angle of view of a particular lens. A shorter focal length lens, such as a 28mm, offers a wider angle of view than a longer one, such as 100mm.
FOCUS: Causing light to form a point, or sharp image on the image sensor or film.
FOCUS LOCK: In autofocus camera systems, a button, lever, or push-button control that locks focus at a particular distance setting, often used when the main subject is off to the side of the frame or not covered by the autofocus brackets in the viewfinder.
FORMAT: The size of the film, thus the camera that uses such film. Large format refers to 4x5 inches and larger; medium format uses 120 or 220 (6cm wide) film. Smaller formats include 35mm and 24mm. In computer imaging, the file structure, or "language" that can be understood by the device. The film analogy to format in digital terms would be more akin to sensor size and megapixel count.
FRAME: The outer borders of a picture, or its ratio of the height to width. The individual image on a roll of film. Also, to compose a picture.
FRAME GRABBER: Usually refers to a board that can digitize and process video signals to a single frame. Mac's Grab utility is a frame grabber, but can also be used to select certain areas of the monitor for capture.
GAMMA: A value that defines contrast of a photograph or electronic image. Gamma curves are key elements of monitor calibration. Gamma can be altered in black and white printing by working with various contrast grade papers. Gamma can be changed in an electronic image by working the curves in processing programs.
GAMUT: The range of colors available in an image or printer.
GIGABYTE: One billion bytes, or 1000 megabytes.
GRADE: With black and white printing papers, the built-in contrast of that paper, or the contrast evoked in variable contrast papers when printing through variable contrast filters. The lower the grade number the lower the contrast.
GRAIN: The appearance or echoes of the silver crystals in film in the final negative or positive image. The larger the area of the grain in the film emulsion, the more sensitive the film is to light; the more sensitive it is to light the "faster" it is. Larger grains are manifest in the image as mottled or salt-and-pepper clumps of light and dark tones, usually apparent in very fast films on visual inspection, in slow films upon extreme magnification. Grain is most easily seen as non-uniform density in areas sharing the same tone (such as a gray sky.)
GRAY SCALE: The range of tones, from bright white to pitch black that can be reproduced in a film and print.
GROUND GLASS: A specially prepared glass used as the focusing screen in cameras.
GUIDE NUMBER: A number that relates the output of electronic flash when used with a particular speed film or ISO setting on a digital sensor. The higher the guide number, the more the light output. Guide numbers, or GN, serve as a way to calculate aperture when shooting flash in manual exposure mode. Dividing distance into guide number gives the aperture: For example, a flash with a guide number of 56 (with ISO 100 film) would give a correct exposure at 10 feet with an aperture of f/5.6. With the state of today's automatic exposure flashes, guide numbers today are mostly useful for comparing the relative power of one flash to another and gauging the coverage a flash will afford. In most cases GN relates to the coverage when using ISO 100 film. It should be noted that some flash manufacturers will fudge the numbers by stating the GN at ISO 400.
