Refractive and Diffractive Photo Lenses

Refractive and Diffractive Photo Lenses - REFRACTIVE...

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Unformatted text preview: REFRACTIVE / DIFFRACTIVE PHOTOGRAPHIC LENSES ECE 4500 Optical Engineering Thomas K. Gaylord Georgia Institute of Technology PHOTOGRAPHIC LENS DEVLOPMENT Many, optical scientists and engineers have devoted their entire careers to the development of improved photographic objective lenses. Numerous historically significant lens designs required many years to develop. A few examples of famous photographic lenses are shown below [1]. ln essentially all cases there is a patent on the design. [1] R. Kingslake, A History of Photographic Lenses. San Diego: . Academic Press, 1989. fill fill The basic Emostar-Sonnar type. The Gundlach Ultrastigmat, ‘ | The Wild Aviotar lens. ‘ The Metrogon lens. ‘ The Makro Plasmat. The Schneider Xenotar. The Leitz 35 mm f/ 1.4 Summilux. The first Zeiss Biogon. l . l The Leitz Elcan. The Busch Bis-Telar f/ 8 telephoto. @% An Angénieux Retrofocus lens, 9.5 mm at f/ 2.2 (1950). The Elgeetf/ 1.2 Golden Navitar. The Zeiss Tele-Tubus. REFRACTIVE / DIFFRACTIVE LENSES In spite of centuries of development and the introduction of computer hardware and software technology, the quality of the" images formed by photographic objectives is still limited by optical aberrations inherent in the designed lenses. This is not a problem associated with imperfectly fabricated lenses. Optical aberrations are present even in lenses that are constructed exactly according to the design. There are seven optical aberrations: spherical aberration, coma, astigmatism, curvature of field, distortion, transverse chromatic aberration, and lateral chromatic aberration. The relative importance of the aberrations depends on the field of view, the focal length, the magnification, and other parameters. Due to the range of wavelengths from 400nm to 700nm involved in white-light photographic imaging, it has been a major challenge to get all of these wavelengths to focus at the same place (the film plane or detector plane) with the same magnification and without aberrations. Historically, it has been a significant achievement to correct the aberrations for two colors, blue and red. Such lenses are referred to as achromatic. The standard blue wavelength used is 7t. = 486.13 nm (hydrogen F line). The standard red wavelength used is 7» = 656.27 (hydrogen C line). Further refinement has produced lenses that are corrected at three colors blue, yellow, and red. These lenses are referred to as apochromatic. In addition to the blue and red wavelengths above, the standard yellow wavelength used is k = 587.56 (helium d line). Both achromatic and apochromatic lenses still exhibit chromatic aberration and this leads to a level of degradation in the image. Designing and fabricating lenses that are corrected across the entire visible range has not been possible before the introduction of diffractive elements into the lens. Diffractive gratings have the attribute of dispersing the short wavelengths over small angles and long wavelengths over large angles. This dispersion is opposite that of a refraCtive element such as a glass prism or a glass refractive lens. This leads to the possibility of combining refractive and diffractive elements to correct chromatic aberration across the entire visible spectrum. This combining of refractive and diffractive lenses is illustrated in Fig. 1. Refractive o tical element (convex lens _‘ Chromatic aberrations 6“ Refractive optical element and multI-lager dlf f ractive optical element combined " If Image formation in the blue, green, and red wavelength order Multi-lager diffractive optical element _ if.” _ ' Chromatic aberrations ,_ ~ Chromatic reversed from that of . g " aberrations ' a ref ractlve optical element ‘ a canceled out "' lma e formation in the red, green, an blue wavelength order """" "" Red Green ................ .. Fig. 1 Refractive optical element, convex lens (upper left), diffractive optical element, grating lens (lower left), and combination refractive/diffractive lens (right). The opposite chromatic aberrations of the separate refractive and diffractive lenses are canceled in the combination. The possibility of using refractive/diffractive lenses to achieve higher levels of image quality is an exciting prospect. However, other, equally important requirements of modern photography are creating a major need for the introduction of refractive/diffractive lenses. One factor favoring the introduction of refractive/diffractive lenses is the need for a nearly constant center of gravity in focusing the lens. No photographer wants the lens/camera combination to be tilting or otherwise moving during the focusing process. That, in turn, would require re—aiming the camera. This type of operation has been a fact of life in photography until recent times. Constant center-of- gravity focusing is achieved in modern computer-designed lenses by incorporating zoom elements, a lens group that moves within the lens. This group of elements moves within the lens rather than the entire lens moving. With the latest design technology, zoom lenses can be efficiently realized by incorporating diffractive elements. A second factor favoring the introduction of refractive/diffractive lenses is the need for compact size. No photographer wants to carry a large, bulky lens/camera combination. Telephoto lenses are ' particularly long and awkward to handle. Everyone has undoubtedly seen telephoto lenses at sporting events. These lenses are truly impressive in size. However, every photographer would welcome more compact light-weight lenses. Refractive/diffractive lenses offer an outstanding opportunity for achieving small compact sizes. A third factor favoring the introduction of refractive/diffractive lenses is the need for single lenses that can be continuously adjusted to provide a range of focal lengths. Such lenses can be telephoto lenses (long focal lengths) or wide-angle lenses (short focal lengths) all in one single lens assembly. Variation in the focal length can be achieved by again incorporating zoom elements, groups of lens elements that move together. Again, zoom lenses have the potential of being efficiently realized by incorporating diffractive elements in the lens design. - The optical engineering evolution of a refractive/diffractive lens is shown in Fig. 2. This particular lens is a 400mm focal length telephoto lens. The modern conventional design (top) incorporates a fluorite (calcium fluoride, Can) element which has very low disperion. It also incorporates ultra-low-dispersion ('UD) glass elements. A redesigned version of the lens for more compact size (upper middle) is also shown. Colors are focused in the order of green, red and then blue. This is not in the order of wavelengths. A further redesigned lens (lower‘middle) moves the positions of the fluorite and ultra-low dispersion elements. The lens still exhibits chromatic aberration, but with the focusing now in order of wavelength: red, green, then blue. The final design (bottom) incorporates a multi-layer diffractive optical element. The’compact size is maintained while simultaneously correcting the chromatic aberration. Thus the concept of a compact, high image quality refractive/diffractive is demonstrated. [1] 100mm f 2'4 lens with a conventional design Protective glass a U0 element 0 Fluorite Chromatic aberrations more pronounced [ma e formation in t e green, red, and blue wavelength order ....... _—.~—- Bed —————————- Green [3] Conventional elements replaced by fluorite and U0 to correct the wavelength order image f ormation in regular intervals in the blue, green, and red wavelength order [4] Front, lens group replaced by multi—lager dif f ractive op ical element Chromatic aberrations canceled out fl Mufti-lager dif f ractive optical element Fig. 2 Steps in the development of a 400mm focal length lens. Conventional design (top) incorporating a fluorite element and . ultralow—dispersion elements. Redesigned lens for more compact size (upper middle). Colors are not focused in order of wavelength. Further redesigned lens (lower middle) still exhibiting chromatic aberration, but with focusing in order of wavelength. Final design (bottom) with diffractive elements added to maintain compact size and correct chromatic aberration. Diffractive optical elements, however, also introduce unwanted problems into the imaging process. A_single-layer diffractive optical element, diffracts only a limited range of wavelengths into the desired image. This is shown in Fig. 3. Even though the diffraction efficiency Single-lager diffractive optical element Nulti-lager diffractive optical element Diffraction grating Sp erfluous - _l fracted ‘ the incident ight IS ' ~ light IS used produced , for the Image , ===z> Diffracted light used for the image m Diffracted light causing flare Fig. 3 Single-layer diffractive optical element (left) and two-layer diffractive optical element (right). In the two-layer design, all visible wavelengths are focused into the image. can be nearly 100%, shorter and longer wavelengths are diffracted outside of the limited wavelength range. The resulting image appears to have “glare” associated with it. Thus single-layer diffractive optical elements are not suitable for white-light imaging purposes. Using a two-layer diffractive optical element with a spacer (order of a micron) between the diffractive layers allows a greater range of visible wavelengths to be focused into the image. This is based on the compensating effect of the second grating. In the limit of a grating of the same period, the grating pair would produce all colors (wavelengths) traveling in the same direction. This case is shown in Fig. 4. However, it is necessary to have both the designed amount of angular deviation and the designed amount of dispersion remaining in the diffracted beam. Incident Wave Fig. 4 Complete compensation of diffractive dispersion through the use of a pair of gratings. However, the two-layer diffractive optical lens, even though it corrects the diffraction direction across a band of wavelengths, it does so for only a single angle of incidence. Adding a third diffractive element allows correction over a working range of angles of incidence from small (telephoto) to large (wide-angle). Two-layer and three- layerdiffractive optical elements are shown in Fig. 5. The correction for a range of angles of incidence is shown in Fig. 6. ‘E‘WwLayar Di} WWW Lam Diffraction Grating 2 Diffraction Diffraction Diffraction Grating Grating 1 g Grating 3 Glass Lenses Glass Lenses Fig. 5 Two-layer and three-layer diffractive optical elements incorporated between glass refractive elements. ‘E‘wwLayer flit} Lens Incident Light at Wide Unwanted f lare-causing Dif f racted Light \3 . . H ’ . Image-forming diffracted Light I Image—forming IHOident Light ' " » diffracted Light at iele ; - ' Changes in the Incident FIngle Cause No Unwanted Light Despite Unwanted Dif f racted Light in the Shot 4 Changes in the Incident Flngle Fig. 6 Two-layer diffractive optical element with unwanted diffracted light for a range of angles of incidence. Three-layer diffractive optical element correcting this problem. INTRODUCTION OF REFRACTIVE I DIFFRACTIVE LENSES 2002 — introduction of first refractive/diffractive camera telephoto lens (Canon EF 400mm f/4 D0 is USM) 2004 — introduction of first refractive/diffractive general-purpose zoom lens (Canon EF 70-300mm f/4.5—5.6 DO IS USM) REFRACTIVE I DIFFRACTIVE GENERAL— PURPOSE ZOOM LENS The first refractive/diffractive general-purpose zoom lens was commercially introduced in April 2004 by Canon. It is the model EF 70-300mm f/4.5-5.6 DO IS USM. It consists of 18 refractive optical elements and 3 diffractive optical elements. The arrangement of lens group and lens elements for this lens is shown in Fig. 7. One of the refractive elements is a molded glass aspherical element. A photograph of this lens is shown in Fig. 8 Group 2(lmage Stabilizer Lens Group) Group 6(Focusing Lens Group) Group 1 Group 3 Group :1 Group 5 Group 7 Focal Plane H-irHr-J—w rH r- Diaphragm Three-Lager DO Lens GHo aspherical Lens Fig. 7 Arrangement of lens-groups and lens elements of the refractive/diffractive zoom lens (Canon EF 70-300mm f/4.5-5.6 DO IS USM) ' Fig. 8 Refractive/diffractive general-purpose zoom lens (Canon EF 70-300mm f/4.5-5.6 DO IS USM). A conventional refractive lens that performs the same zoom lens function is the Canon EF 75-300mm f/4.5-5.6 lS USM). This refractive lens is compared with the refractive/diffractive zoom lens (Canon EF 70-300mm f/4.5-5.6 DO IS USM) in Fig. 9. EF75-300mm f .c“ 4-5.6 IS USN END-300mm ffd.5~5.8 DO IS USN Three-Lager DO Lens GHo fisphericat Lens Fig. 9 Optical configuration of conventional refractive lens (top) and refractive/diffractive lens (bottom). REFRACTIVE vs. REFR'ACTIVEI DIFFRACTIVE ZOOM LENS COMPARISON — W [KEE— __ mum- ' m— Molded Glass As o herical Lens mar_ Diffractive O tics Stabilizer Stabilizer Ultrasonic Motor Auto Focus Auto Focus ASPHERICAL LENSES Spherical aberration, distortion, and other aberrations can be, at least partially, corrected by lenses with aspherical surfaces. Such lenses are used in the following: 1) high-definition large aperture lenses (compensating spherical aberration), 2) wide-angle lenses (correction of distortion), 3) zoom lenses (correction of multiple aberrations). The types of commercial aspherical lenses are as follows: 1) ground and polished glass aspherical lens, 2) molded glass aspherical lens (GMo), 3) molded plastic aspherical lens, 4) ultraviolet-light-hardened resin layer on spherical lens. ULTRASONIC MOTOR Automatic focusing is achieved with an ultrasonic motor (USM) drive. In an ultrasonic motor, rotational force is generated from ultrasonic vibrational energy. The ultrasonic motor consists of an elastic stator and a rotating rotor. The stator has piezoelectric ceramic elements incorporated in it. When the piezoelectric elements are activated by an AC voltage, they vibrate (about a micron in amplitude) and generate ultrasound. The ultrasonic energy propagates through a pressure contact between the stator and rotor and causes the rotor to rotate. Various version of the ultrasonic motor are shown in Fig. 10. Fig. 10 Various versions of the ultrasonic motor (Canon). IMAGE STABILIZER Camera shake causes images to be blurred especially in long focal length lenses. The image stabilizer (IS) system developed by Canon utilizes a vibration-detecting gyro in the lens. A control algorithm operating on the detected image determines the needed compensating signals for the actuator. The actuator, in turn, moves a group of lens elements in the lens transverse to the optical axis of the lens to stabilize the image at the film plane. Image stabilization compensates for shake in two dimensions. A second image stabilization mode compensates for shake in one dimension for panning shots of linearly moving objects. Image stabilization enables recording images about three shutter speed steps slower than without image stabilization. REFRACTIVE / DIFFRACTIVE TELEPHOTO LENS A 400mm focal length refractive/diffractive lens was introduced by Canon in 2000. It is the EF 400mm fl4 DO IS USM, where DO indicates diffractive optics, IS indicates image stabilizer, and USM indicates ultrasonic motor. It is shown in the figure below to- gether with a conventional refractive 400mm telephoto lens. In addition to superior image quality, the refractive/diffractive lens is 26% shorter and is 36% lighter. Fig. 11 Refractive/diffractive 400mm telephoto lens (top) and conventional refractive 400mm telephoto lens (bottom). ' Diffractive Optics, Auto-Focusing, Image Stabilization I Demonstration Please follow these steps: 1. Switch the settings on the lens as follows: AF/MF to “AF,” Stabilizer to “OFF,” and Stabilizer Mode to “1.” 2. Put the camera strap over your neck as a safety precaution. 3. Grasp the right side of the camera body with your right hand. Grasp the lens behind the focusing ring (the rubber ring closer to the camera back) with your left hand. Pick up the camera/lens combination. Look through the viewfinder. 4. Put your right forefinger on the shutter release button. Point the camera at a distant object (such as at infinity). While watching though the viewfinder, press the shutter release button half way down. Listen for the auto-focusing ultrasonic motor to operate. Watch for the focusing to occur. One or more red LED’s will blink in the seven auto- focusing windows (small rectangles in viewfinder) when focus is achieved. 5. Put your left thumb on the Stabilizer switch, but do not switch it yet. 6. Standing freely (without leaning on anything or othenNise stabilizing yourself), point the camera at the nearby test object. Press the shutter release button half way down. Listen for the auto-focusing ultrasonic motor. Watch for the focusing to occur. Release the shutter release button. The focus should remain locked at the test object distance. 7. Carefully watch the test object through the camera for a period of about 10 seconds. Mentally determine the average amount of camera movement that is typical for you over this time span. 8. With your left thumb, switch the Stabilizer switch to the “ON” position. While watching the test object through the camera, after a period of a few seconds, press the shutter release button half way down. Listen for the Image Stabilizer mechanism to operate. Watch for the view of the image to become more stabile. 9. Place the camera/lens combination face down on the table (setting it on the front of the lens). Remove the strap from your neck. ...
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This note was uploaded on 04/29/2008 for the course ECE 4500 taught by Professor Gaylord during the Spring '08 term at Georgia Tech.

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Refractive and Diffractive Photo Lenses - REFRACTIVE...

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