The inability of a lens to produce a true image, particularly at the edge of a photograph. Usually, the more expensive the lens, the better its optical quality and the fewer aberrations.
Angle of View
The area of a scene that a lens can cover. The focal length of the lens determines the angle of view. A wide-angle (short-focal-length) lens includes more of a scene than a standard (normal-focal-length) lens or telephoto (long-focal-length) lens. Angle of view is basically the angle at which light rays can pass through the lens to produce an image on the film.
The aperture is the opening formed by the blades of the iris or diaphragm in the lens, through which light passes to expose the film. Aperture size is usually given in f-numbers, the larger the number, the smaller the opening. Aperture size together with shutter speed determine the amount of light falling on the film (exposure). The aperture is sometimes called the “stop".
A lens with a curved, non-spherical surface. Used to reduce aberrations and achieve a more compact lens size. With a spherical lens, rays travelling from the lens periphery create the image before the ideal focal point and give a blurred image centre. With an aspherical lens, even the rays travelling from the lens periphery converge at the ideal focal point, thus producing a sharp image.
The inability of a lens to bring all light wavelengths (particularly red & blue) into the same plane of focus, thus causing overall blur. Usually found in regular large-aperture telephoto and super-telephoto lenses. Not improved by reducing aperture size. Can be corrected with low dispersion (ED, LD SD) glass.
A method of expressing the colour content and quality of light and measured in Kelvin (K). “Photographic daylight" has a colour temperature of about 5500K. Photographic tungsten lights have colour temperatures of 3200K to 3400K depending on their construction.
Depth of Field
The distance between the nearest and furthest objects in a photograph that are considered to be acceptably sharp. Dependant on aperture, focal length and focused distance. The smaller the aperture, the wider the lens and the further the focused distance, giving a greater depth of field and vice versa.
Designed to provide light where the lighting on the scene is insufficient. Electronic flash requires high voltage, usually obtained through batteries and a voltage-multiplying circuit which discharge a brief, intensive burst. Generally considered to have the same photographic effect as daylight. Modern flash units have multiple TTL exposure control functions and auto focus control.
F-numbers or F-stops
Numbers on the lens aperture ring and the camera's LCD (where applicable) that indicate the size of lens aperture. The lower the number the larger the aperture. As the scale rises, each number is multiplied by a factor of 1.4. Standard numbers are 1.0,1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. , each change resulting in a doubling or halving of the amount of light transmitted by the lens to the film.
ISO stands for International Standards Organization and numbers such as ISO 100 or ISO 400 etc. give the sensitivity of film to light. The higher the number, the more sensitive or faster the film. Basically, the slower the film (low ISO No. ) the sharper and clearer the photograph. Grainy effects can be achieved with fast films (high ISO No. ).
Flash sync speed
Exposure time with a focal-plane shutter is measured from the moment the first curtain is released until the moment the second curtain is released. The instant the first curtain closes, the electrical contacts for X sync close and instantly fire the flash.
The distance from the film to the optical centre of the lens when the lens is focused on infinity. Focal length on most adjustable cameras is marked in millimetres on the lens mount. On 35mm-format cameras, lenses with a focal length of 50mm are called normal or standard lenses. Lenses of 35mm or less are called wide angle lenses and lenses of 85mm or more are called telephoto lenses. Lenses which allow varying focal lengths without changing focus are called zoom lenses.
One or more pieces of optical glass or similar material designed to collect and transfer rays of light to form a sharp image on film, paper or a projection screen. In practical photography, compound lenses made of a number of elements of different types of glass are used. This enables the manufacturer to correct most of the faults (aberrations) found in simple lenses and provide images that are sharp across the whole picture.
The largest aperture(smallest F-stop) at which a lens can be set. Fast lenses transmit more light and have larger openings than slow lenses. Determined by the maximum aperture in relation to focal length. Lens speed is relative: a 400 mm lens with a maximum aperture of F/3.5 is considered extremely fast, while a 28mm F/3.5 lens is considered to be quite slow.
Perspective is a two-dimensional representation of a three-dimensional scene. In photography this can be achieved by viewing 3-D objects from an angle rather than head-on. A photograph is also given perspective if there are objects in the foreground, middle distance and background, giving the whole scene “depth".
Single-Lens-Reflex (SLR) Camera
Light entering the camera through the lens is reflected up by a mirror behind the lens onto a ground glass screen above. This screen is viewed through the viewfinder and a glass pentaprism which turns the image the correct way up. Other camera functions such as light metering and flash control also operate through the camera lens.
A lens which can be adjusted to a wide range of focal lengths without a change in focus, thus an alternative for a number of individual lenses of various focal lengths. A difficult type of lens to design and manufacture, but very useful for the photographer who likes to travel light.
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