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Tungsten-Halogen Lamps
Principle of a Lens

Photography (Basic) - Introduction to photography and other graphic techniques
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Figure 1-14.-The position of the discharge tube in relation to the reflector. different  distances  from  the  subject.  Generally,  they  are designed  to  provide  maximum  efficiency  at  distances  of from 6 to 12 feet from the subject. Professional type of electronic  flash  units  may  have  a  dual  reflector system-one position for a normal angle and the other for a wide angle (for a wide-angle lens); others may have a zoom system to provide optimum light distribution for any lens within a wide range of focal lengths. Depending on the position of the discharge tube in relation to the reflector, the unit can be used as a spotlight or floodlight (fig. 1-14). PLANE REFLECTORS. –When you want to provide fill-in light for shadow areas, it is often desirable to substitute a plane reflector (sometimes called a reflector board) to redirect the light from a direct light source (fig. 1-15). The plane reflector is placed so it receives  light  from  the  primary  light  source  and  reflects the light into the shadows. The efficiency of such a reflector depends on its surface and tone, as well as size and  distance  from  the  subject  being  photographed.  The subject area covered by a plane reflector depends on the size of the reflector. When the surface of the reflector is matte or textured, it reflects diffused light and some of the reflected light is dispersed over a wide angle. OPTICAL   PRINCIPLES Cameras have optical systems, or lenses, made up of several separate pieces of glass, called  elements. There are two reasons for having several elements. First, it allows the designer to make many different types of lenses to suit different purposes. Second, the quality of the image formed by the lens can be controlled by choosing different lens elements. The most important choices the lens designer makes are the shape and position of each lens element. These govern properties like focal length, angle of view, physical weight, and size. Lenses are probably the least understood but the most discussed component of the photographic process. Photographers  (generally  amateurs)  speak  of  a  lens formula as if they knew what it was about. Even if the designer’s formula were made available, it would not provide information about the lens photographic quality. A perfect lens cannot be made. A lens is a compromise of inherent errors called aberrations, but do not let this worry you. Lens aberrations are defects in the formation of an optical image. Today’s lenses are so highly corrected for lens aberrations that, except for a few ultra wide-angle  (fisheye)  lenses,  you  would  be  hard  pressed to find a lens that produces subjectively identifiable 1-10







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