Light rays entering the lens from the edges of the picture area are partially blocked by the lens frames in front of and behind the diaphragm, preventing all the rays from passing through the effective aperture (diaphragm diameter) and causing light fall-off in the peripheral areas of the image. This type of vignetting can be eliminated by stopping down the lens.

According to the dictionary, Light is defined variously as: 1. something that makes things visible or affords illumination; an illuminating agent or source, as the sun, a lamp, or a beacon: 2. electromagnetic radiation to which the organs of sight react, ranging in wavelength from about 4000 to 7700 angstrom units and is propagated at a speed of about 186,300 miles per second, and including a similar form of radiant energy that does not affect the retina, as ultraviolet or infrared rays; 3. a gleam or sparkle, as in the eyes; 4. a particular light or illumination in which an object seen takes on a certain appearance; 5. a person who is an illuminating or shining example; luminary; 6. mental or spiritual illumination or enlightenment; 7. the aspect in which a thing appears or is regarded. The definition most indispensable to the understanding of light as used in photography is 2 above. Types of electromagnetic radiation vary according to wavelength. Starting from the shortest wavelengths, electromagnetic radiation can be classified into (special form enter) rays, X rays, ultraviolet light rays, visible light rays, infrared light rays, far-infrared light rays, microwave radiation, ultrashort wave radiation (VHF), short-wave radiation, medium wave radiation (MF) and long wave radiation. In photography, the most utilized wavelengths are in the visible light region (400nm-700nm). Since light is a type of electromagnetic radiation, it can be thought of as a type of wave in the category of “light waves.” A light wave can be regarded as an electromagnetic wave in which an electric field and magnetic field vibrate at right angles to each other in a plane perpendicular to the direction of propagation. The two elements of a light wave which can actually be detected by the human eye are the wavelength and amplitude. Differences in wavelength are sensed as differences in color (within the visible light range) and differences in amplitude are sensed as differences in brightness (light intensity). The third element which cannot be detected by the human eye is the direction of vibration within the plane perpendicular to the light wave’s direction of propagation.

The distance from the front edge of the lens barrel to the subject. An important factor especially when shooting close-ups and enlargements.

A single standard zoom lens does the work of several lenses with fixed focal length. Take advantage of the lens’ speedy operation to capture the full breadth of a scene with a wide-angle setting, or zoom in instantly to shoot a close-up of some particularly attractive highlight in telephoto mode. Telephoto zoom lenses bring an added dimension to EOS system performance. Follow moving subjects such as athletes or animals in action, and experience the telephoto lens’ characteristic shallow depth of field and “compressed” effect as you enjoy this added dimension of expressive power.