There are two types of polarizing filters: linear and circular. Linear polarizing filters are made of mechanically stretched plastics with oriented rod-shaped molecules which are stained with a dye. For lenses there are glass-mounted varieties. For lighting purposes large-sized foils are also available. The filters are used to convert unpolarized light into polarized light. The exposure factor amounts to about 2-3. Circular polarization filters are used together with cameras systems whose exposure measurement works together with a mirror. This can result in an inaccurate measurement, because the measuring light can be polarized with the deflection. Moreover, circular polarizing filters are necessary for autofocus cameras, because linear polarizing filters negatively affect the autofocus function, as they block out light from certain angles which is intended to reach the AF sensor. The effect of a polarizing filter is best demonstrated with a shining water surface. The polarizing filter causes a strong decrease in reflections at a certain position. Moreover, polarizing filters strengthen the saturation of colors of shining objects. For example, the blue color of the sky becomes very intense, because annoying scattered light from certain angles is eliminated. This is particularly true for slide photography.
With optical systems the beams of light which come from infinity are broken to a certain focus. The imaginary plane where this happens is called the principle plane. The principle plane must not necessarily lie within the lens or the optical system.
Ultrasonic motors are divided into two categories depending on the principle that generates the energy to move the drive: traveling wave motors and standing wave motors. Traveling wave motors include the ring type ultrasonic motor used in the recently launched 70-300mm F/4-5.6 VC USD, as well as other lenses, but this lens employs a newer technology, the PZD ( Piezo Drive), which functions on the standing wave principle.
A standing wave ultrasonic motor utilizes high-frequency voltage to extend and turn the piezoelectric (piezoceramic) element, thus moving the entire element in a standing wave movement. A metal tip on the piezoceramic element is elliptically rotated by the rotary movement of the element and in turn drives the rotor by means of friction. Standing wave ultrasonic motors have the distinct advantage of being smaller than their traveling wave counterparts, and therefore allow a more compact SLR lens size.
The Tamron Piezo Drive distinguishes with its precise, fast and silent autofocus.