Camera Lenses Get a Boost
Dr. Tsuyoshi Asaeda
Diffractive optical elements play a significant role in monochromatic imaging applications, such as in the pickup lenses in compact disc and DVD players. Canon Inc. of Tokyo, however, has displayed elements that enable smaller and lighter camera lenses. The laminated diffractive optical elements appear in the company's new 400-mm f/4 telephoto and 16- to 35-mm f/2.8 short-zoom lenses.
Canon Inc. has incorporated multilayer laminated diffractive optical elements into camera lenses. Using a pair of single-layer elements with opposing concentric circular diffraction gratings eliminates flare from superfluous diffracted light.
The company has laminated circular blazed gratings onto meniscus glass lenses. In the camera applications, two laminated elements with opposing gratings face each other across a micron-size air gap.
The gratings are constructed of UV-cured epoxy resin, and the number of gratings is more than 100 rules across the lens diameter, with pitches varying from a few millimeters to several dozen microns. The two laminated elements, one with a concave and the other with a convex profile, together achieve almost 100 percent diffraction efficiency for visible wavelengths.
A diffractive optical element behaves like a high-dispersion aspheric lens. It displays two optical effects when applied to imaging: chromatic aberration correction and aspheric compensation. Contrary to conventional refractive optics, the elements have a shorter focal length for red light than for blue light because of their negative Abbe constant.
The design yields significant savings in size and weight. For example, a conventional 400-mm f/4 telephoto lens made with refractive optical elements is 317 mm long and weighs 3000 g. The lens made with laminated diffractive optical elements is 84 mm shorter and 920 g lighter.
Takehiko Nakai, a researcher at Canon's Lens Products Development Center, reported the development in November at the Optical Society of Japan's meeting. The paper received the grand prize from the Optics Design Group.
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