Atelier Bonryu(E)
zone plate photography
Laboratory: Zone Plate Photography
Applications of Zone Plate Photography
- Diffractive Optics Lens of a Camera -
One of the important features of a zone plate is a “large chromatic aberration”. Though this feature is undesirable and should be avoided from the viewpoint of a photographer, this feature can be utilized to reduce a chromatic aberration of a glass lens for developing high performance lenses of a camera. Some of such lenses are already available commercially.
Chromatic aberration: A phenomenon that an image projected by an optical system blurs or is distorted is called an aberration. A chromatic aberration of a glass lens is caused by a dispersion of light waves due to the variation of the refractivity of glass with respect to the wavelength. Because of this phenomenon the focal length of the glass lens changes according to the wavelength (color) of a light. For a light with a longer wavelength (red light) the refractivity of glass becomes smaller and the focal length of a convex lens made from such glass becomes shorter. Contrarily for a light with a shorter wavelength (blue light) the refractivity is larger and the focal length of a convex lens becomes longer. As the dependences of the focal length on the wavelength of a light of a glass lens and of a zone plate are in the inverse direction, by combining them appropriately a chromatic aberration of a lens can be reduced effectively.
Achromatic lens: As a lens of a camera of which the chromatic aberration is minimized is called an achromatic lens. Strictly speaking a lens corrected for two wavelengths to be brought into focus on the same image plane is called an achromatic lens and a lens corrected for three wavelengths is called an apochromatic lens. But here we call a lens corrected to minimize the chromatic aberration as an achromatic lens. There are more than 200 kinds of optical glasses to be used as material of lenses. The right figure shows the relation of the refractivity versus the wavelength for only small number of representative optical glasses. This figure clearly shows that the refractivity of any glass decreases with increasing wavelength of a light. This means that the focal length of a convex lens made from any glass becomes longer with increasing wavelength of a light. Therefore, it is impossible to make an achromatic lens by a combination of convex lenses of different kinds of glasses, and usually an achromatic lens is made from a combination of a convex lens of crown glass and a concave lens of flint glass.
Dependence of the refractivity of optical glass on wavelength of a light
Generally the refractivity of optical glass becomes small with increasing the wavelength of a light. Symbols denote names of optical glasses and the alphabetic parts are the kinds of glasses. K: Crown glass (Krone Glas in German), F: Flint glass.
Application of a zone plate to an achromatic lens: As described above the focal length of a zone plate is shorter for longer wavelength. Because this dependence is quite contrary to that of an optical glass it is advantageous to make an achromatic lens by combining a zone plate and a glass lens. However, the zone plate we described until now is not appropriate as it is for this purpose because there are a light blocked off at opaque zones, a directly proceeding background light, a light proceeding to sub-focal points, and so on, which cause a flare and reduce the light intensity at the focal point as small as 10 %. It is known that these problems can be solved by employing a relief-type zone plate and a layered- structure of more than two plates. As a matter of course the number of zones should be far larger than that we described up to now and high-precision processing technology is necessary. As a result of advancement of the design technique and the processing technology such a kind of the diffractive optics elements is well-developed and some of them are available commercially or announced to release. Examples are a telephoto lens (EF400 mm F4 DO IS USM) and a telephoto zoom lens (EF70-300 mm F4.5-5.6 DO IS USM) by Canon, a convertor lens for a point-and-shoot camera (TC-E3P) by Nikon, and a telephoto zoom lens (Zuiko Digital ED50-200 mm F2.8-3.5 II SWD) by Olympus. These lenses are called differently by different makers, as DO (multi-layer Diffractive Optics) by Canon, PF(Phase Fresnel lens) by Nikon, and DL(Diffractive optics Lens).
