Abstract:
A dual field of view lens system comprising a first lens group aligned along an optical axis and a third lens group aligned along the optical axis and positioned a predetermined distance from the first lens group. A second lens group, having an optical stop attached thereto, is aligned along the optical axis and disposed between the first and third lens groups such that the second lens group and the optical stop are moveable along the optical axis between a first position and a second position. Positioning the second lens group and the optical stop in the first position provides a wide field of view and moving the second lens group and the optical stop to a second position provides a narrow field of view.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to optical lens systems and in particular to a dual field of view lens system. 
   BACKGROUND 
   In lens systems for military, marine, industrial, outer space, or other rugged use, various machine vision systems having zoom and focus capability are used. However, one problem with these systems is that typical commercial zoom and focus lenses have mechanisms that are not sufficiently robust for these uses. 
   One possible solution is to use multiple cameras and lens systems instead of using one lens system. Some disadvantages to this approach are the increased cost, volume, and mass of having multiple cameras and lens systems. 
   Another possible solution is to use standard commercial lens systems that have mechanical devices to zoom the lens. One disadvantage to this approach is that a failing commercial lens system cannot be repaired or replaced in unmanned situations and these lens systems tend to mechanically fail due to shock, extreme changes in temperature, etc. Attempting to make these types of lens systems more robust makes these lens systems tend to be large and complicated (e.g. most lens systems require three or more independent lens group motions to operate). 
   There have been single motion zoom lens systems used previously, however, these systems have been used for infrared systems and the image quality has not been suitable for visible light applications. 
   Therefore, there is a need for a low risk, single unit, zoom and focus lens with sufficient quality for use in a visible spectrum camera. While the examples shown herein are discussed as for use in the visible spectrum, it is understood that they also apply to systems using portions of the UV and NIR spectrums as well. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a dual field of view lens system. The system has a first lens group aligned along an optical axis and a third lens group aligned along the optical axis and positioned a predetermined distance from the first lens group. A second lens group is aligned along the optical axis between the first and third lens groups and is moveable along the optical axis between a first position and a second position. An optical stop is attached to the second lens group. 
   The present invention also relates to a method of making a dual field of view lens system. A first lens groups is positioned along an optical axis and a third lens group is positioned along the optical axis a predetermined distance from the first lens group. An optical stop is connected to a second lens group and the second lens group and the optical stop are positioned between the first and third lens groups such that they are moveable along the optical axis between first and second positions. 
   The present invention also relates to a method of using a dual field of view lens system. A second lens group and an optical stop are positioned in a first position between a first lens group and a third lens group to obtain a wide field of view and the second lens group and the optical stop are moved to a second position between the first lens group and the third lends group to obtain a narrow field of view. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of one example of a dual field of view lens system of the present invention with the second lens group and optical stop in a first position. 
       FIG. 2  is a side view of the dual field of view lens system of  FIG. 1  with the second lens group and optical stop in a second position. 
       FIG. 3  is a side view of a second example of a dual field of view lens system of the present invention with the second lens group and optical stop in a first position. 
       FIG. 4  is a side view of the dual field of view lens system of  FIG. 3  with the second lens group and optical stop in a second position. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , an exemplary dual field of view zoom lens system  10  is shown. In the example shown, the system  10  generally has a first lens group  20 , a second lens group  60 , a third lens group  40 , and an optical stop  70 . As is well known in the art, each lens “group” can consist of a single lens or can be made up of multiple lenses, depending on the usage and requirements of the system. 
   First and third lens groups  20 ,  40  are aligned along an optical axis A and their positions relative to each other and to focal plane  50  are fixed. Second lens group  60  is aligned along optical axis A between first lens group  20  and third lens group  40  and is movable linearly between a first position (shown in  FIG. 1 ) and a second position (shown in  FIG. 2 ), for example through a linear drive screw, a worm-gear drive with a steeper motor, or any other well know device for providing linear motion to a lens group. Optical stop  70  is connected to second lens group  60  such that optical stop  70  moves along with second lens group  60 . 
   In the example shown, first lens group  20 , which is sometimes referred to as the “imaging” lens group, has singlet lens  22 , first doublet lens  24 , and second doublet lens  30 , which have the properties listed in Table 1 below. Singlet lens  22  is bi-concave, first doublet lens  24  is a compound meniscus lens, and second doublet lens  30  is a compound bi-convex lens. As can be seen from Table 1, the surfaces of singlet lens  22  and first doublet lens  24  are spherical. In addition, surface  38  of second doublet lens  30  is spherical and surface  36  is a conic. By making surface  36  a conic, finer control of spherical aberration or coma at the narrow field of view setting (when surface  36  is filled) is accomplished. Alternatively, other combinations of spherical and conic (or higher order aspheric) surfaces may be used, depending on the requirements of system  10  and manufacturing constraints such as cost. 
   The example shown, system  10  operates on the principal that a lens with two finite conjugated planes at different finite focal lengths can be moved to a second location while still mapping those same two conjugate planes, but reversing magnification between them. In the examples discussed herein, second lens group  60  acts as the moving lens. The two conjugate planes are the image of first lens group  20  and the object of third lens group  40 . Reversing the magnification results in the two different fields of view. The two conjugate planes may be real or virtual, and forming aerial images is not required. 
   By using the lenses described above for first lens group  20 , first lens group  20  has positive power. However, other lenses or combinations of lenses could be used such that first lens group  20  would be of negative power, as long as first lens group  20  forms an image (real of virtual) of the object being imaged at a reasonable distance from second lens group  60 . Similarly, as discussed below, second lens group  60  or third lens group  40  could be positive or negative. The positive-negative-positive configuration of exemplary system  10  discussed herein tends to result in very compact forms. 
   Third lens group  40 , which is sometimes referred to as the “focus” lens group, has a singlet lens  42  and a doublet lens  44 , which have the properties listed in Table 1 below. Singlet lens  42  is a bi-convex lens and doublet lens  44  is a compound meniscus lens. As can be seen from Table 1, the surfaces of doublet lens  44  are spherical. In addition, surface  41  of singlet lens  42  is spherical and surface  43  is a conic. By making surface  43  a conic, finer control of spherical aberration or coma at the narrow field of view setting (when surface  43  is filled) is accomplished. Alternatively, other combinations of spherical and conic (or higher order aspheric) surfaces may be used, depending on the requirements of system  10  and manufacturing constraints such as cost. 
   By using the lenses described above for third lens group  40 , third lens group  40  has positive power. However, other lenses or combinations of lenses could be used such that third lens group  40  would be of negative power, as long as third lens group  40  forms an image (real or virtual) of the object being imaged at a reasonable distance from second lens group  60 . 
   Second lens group  60 , which is sometime referred to as the “zoom” lens group, has a doublet lens formed by lens  62  and lens  64 , which form a compound bi-concave lens and have the properties listed in Table 1 below. As can be seen from Table 1, the surfaces of lenses  62 ,  64  are spherical. Alternatively, the surfaces of lenses  62 ,  64  could be any combination of spherical and conical surfaces depending on the usage and requirements of the system  10 . By using the lenses described above for second lens group  60 , second lens group  60  has negative power. Alternatively, other lenses or combinations of lenses could be used such that second lens group  60  would be of positive power. 
   In the example shown, optical stop  70  has a fixed size aperture  72 . By having a fixed size aperture  72  and connecting optical stop  70  to second lens group  60 , optical stop  70  shapes the light paths  100  between first lens group  20  and third lens group  60 , as described in more detail below. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Radius of 
               Thickness 
               Thickness 
                 
                 
                 
             
             
               Component 
               Curvature 
               (NFOV) 
               (WFOV) 
               Glass 
               Aperture 
               Conic 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               First Lens 
               −20.05362 
               3.5 
                 
               BK7 
               12.6 
               0 
             
             
               Group 20 
               14.93529 
               4.9 
                 
               AIR 
               12.6 
               0 
             
             
                 
               −32.26625 
               3.5 
                 
               BK7 
               15.4 
               0 
             
             
                 
               −9.462314 
               3.5 
                 
               F2 
               15.4 
               0 
             
             
                 
               −16.63787 
               2.8 
                 
               AIR 
               19.6 
               0 
             
             
                 
               118.8371 
               5.6 
                 
               BK7 
               21 
               0 
             
             
                 
               −13.33854 
               2.8 
                 
               F2 
               21 
               0 
             
             
                 
               −20.51888 
               55.92987 
               2.2095 
               AIR 
               22.4 
               0.218654 
             
             
               Optical Stop 70 
               INFINITY 
               0.7 
                 
               AIR 
               8.2 
             
             
               Second Lens 
               −30.514 
               2.8 
                 
               BK7 
               9.8 
               0 
             
             
               Group 60 
               12.68649 
               2.8 
                 
               F2 
               9.8 
               0 
             
             
                 
               27.0632 
               0.1403 
               53.8606 
               AIR 
               9.8 
               0 
             
             
               Third Lens 
               23.88007 
               9.8 
                 
               BK7 
               35 
               −1.485753 
             
             
               Group 40 
               −48.59682 
               1.4 
                 
               AIR 
               35 
               0 
             
             
                 
               −4281.123 
               3.5 
                 
               F2 
               28 
               0 
             
             
                 
               14.34499 
               8.4 
                 
               BK7 
               25.2 
               0 
             
             
                 
               −547.901 
               48.1314 
                 
               AIR 
               25.2 
               0 
             
             
                 
             
           
        
       
     
   
   Referring specifically to  FIG. 1 , second lens group  60  is located in a first position, proximate first lens group  20 . When in this position, second lens group  60  is farther from the image being formed by first lens group  20  (which would be to the right of system  10 ) and diverges the light paths  100  exiting first lens group  20 , forming a virtual image (in the vicinity of first lens group  20 ) for third lens group  40  to relay to the image plane  50 . This results in a shorter focal length or wide field of view (low magnification), which uses the central portion of first lens group  20  is used and the majority (central and outer portions) of third lens group  40 . Further, in this first position, the system f/# is controlled by optical stop  70  and the size of third lens group  40 , which may be used to vignette the light to improve image performance in the extreme field. The image can be focused by moving second lens group  60  and optical stop  70  about the first position to focus the image. The focus motion in this example is very small (e.g. approximately 0.2 mm) compared to the 53 mm second lens group  60  travels to change the field of view and the same linear motion device can control the field of view and the focus. 
   Referring specifically to  FIG. 2 , second lens group  60  is located in a second position, proximate third lens group  40 . When in this position, second lens group  60  is closer to the image being formed by first lens group  20  (which would be to the right of system  10 ). The virtual image formed by second lens group  60  is in the same location (vicinity of first lens group  20 ), but has a lower magnification. This results in a longer focal length or narrow field of view (high magnification) for the same image plane  50  size. Further, in this position, the system f/# is controlled by stop  70  and the size of first lens group  20 , which may be used to vignette the light to improve image performance in the extreme field. Similarly, the image can be focused by moving second lens group  60  and optical stop  70  about the second position to focus the image. Therefore, changing the field of view of system  10  requires only a single linear movement of second lens group  60  and optical stop  70  from the first position of the second position. 
   In order to optimize system  10  for use with wide and narrow field of view, the curvatures of the lenses in the first, second, and third lens groups  20 ,  60 ,  40  are optimized for use with the entire lens (outer and central portions). This optimizes the performance of the system when an entire lens is used and minimizes the distortion encountered when only the central portion of a lens is used. The use of conic surfaces on some of the lenses, as described above, also enhances this improvement as conic surfaces offer significant variation in curvature between the inner and outer portions of the lenses. Another benefit of moving the optical stop  70  with second lens group  60  is that the brightness at both positions is similar. Optimization of the curvature of the lenses in this matter provides better image quality, resolution, and clarity and allows system  10  to be used in applications in the visible spectrum. 
   Use of the example described above provides a system  10  with the following characteristics: 14 mm effective focal length, 30 degree field of view, f/3.5 in the first position; 70 mm effective focal length, 6 degrees field of view, f/8 in the second position. An entrance pupil diameter of 8.7 mm with the second lens group  60  and optical stop  70  in the second position (narrow field of view) and 4 mm with the second lens group  60  and optical stop  70  in the first position (wide field of view). The exemplary system was optimized for visible wavelengths (0.49, 0.59, and 0.66 μm) and has a 7.3 mm square image. 
   Referring to  FIGS. 3 and 4 , a second exemplary dual field of view zoom lens system  10 ′ is shown. In this second example, the system  10 ′ has a first lens group  20 , a second lens group  60 , and an optical stop  70 , as described above, and third lens group  40 ′, which is a telephoto lens group. 
   First and third lens groups  20 ,  40 ′ are aligned along an optical axis A and their positions relative to each other and to focal plate  50 ′ are fixed. Second lens group  60  is aligned along optical axis A between first lens group  20  and third lens group  40 ′ and is movable linearly between a first position (shown in  FIG. 3 ) and a second position (shown in  FIG. 4 ), for example through a linear drive screw, a worm-gear drive with a steeper motor, or any other well know device for providing linear motion to a lens group. Optical stop  70  is connected to second lens group  60  such that optical stop  70  moves along with second lens group  60 . 
   In this example, first lens group  20  also has singlet lens  22 , first doublet lens  24 , and second doublet lens  30 , which have the properties listed in Table 2 below. Singlet lens  22  is bi-concave, first doublet lens  24  is a compound meniscus lens, and second doublet lens  30  is a compound bi-convex lens. As can be seen from Table 2, the surfaces of first doublet lens  24  and second doublet lens  30  are spherical. In addition, surface  23  of singlet lens  22  is spherical and surface  21  is a conic. By making surface  21  a conic, finer control of spherical aberration or coma at the narrow field of view setting (when surface  21  is filled) is accomplished. Alternatively, other combinations of spherical and conic (or higher order spheric) surfaces may be used, depending on the requirements of system  10 ′ and manufacturing constraints such as cost. 
   Third lens group  40 ′ is a telephoto lens group, which makes system  10 ′ more compact and provides a longer focal length and larger image plane  50 ′. Third lens group  40 ′ comprises a base lens group  40 A, a telephoto lens group  40 B, and a field-flattening lens group  40 C. Base lens group  40 A has a singlet lens  42  and a doublet lens  44 , which have the properties listed in Table 2 below. Singlet lens  42  is a bi-convex lens and doublet lens  44  is a compound meniscus lens. Telephoto lens group  40 B has a pair of singlet lenses  52 ,  54 , which are both meniscus lenses and have the properties listed in Table 2 below. Field-flattening lens group  40 C has a pair of singlet lenses  56 ,  58 , which have the properties listed in Table 2 below. Singlet lens  56  is a meniscus lens and singlet lens  58  is a bi-convex lens. 
   As can be seen from Table 2, the surfaces of the lenses in telephoto lens group  40 B, field-flattening lens group  40 C, and doublet lens  44  are spherical. In addition, surface  41  of singlet lens  42  is spherical and surface  43  is a conic. By making surface  43  a conic, finer control of spherical aberration or coma at the narrow field of view setting (when surface  43  is filled) is accomplished. Alternatively, other combinations of spherical and conic (or higher order aspheric) surfaces may be used, depending on the requirements of system  10  and manufacturing constraints such as cost. 
   Second lens group  60  has a doublet lens formed by lens  62  and lens  64 , which form a compound bi-concave lens and have the properties listed in Table 2 below. As can be seen from Table 2, the surfaces of lenses  62 ,  64  are spherical. Alternatively, the surfaces of lenses  62 ,  64  could be any combination of spherical and conical surfaces depending on the usage and requirements of the system  10 . By using the lenses described above for second lens group  60 , second lens group  60  has negative power. Alternatively, other lenses or combinations of lenses could be used such that second lens group  60  would be of positive power. 
   In the example shown, optical stop  70  has a fixed size aperture  72 . By having a fixed size aperture  72  and connecting optical stop  70  to second lens group  60 , optical stop  70  shapes the light paths  100  between first lens group  20  and third lens group  60 , as described above. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               Radius of 
               Thickness 
               Thickness 
                 
                 
                 
             
             
               Component 
               Curvature 
               (NFOV) 
               (WFOV) 
               Glass 
               Aperture 
               Conic 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               First Lens 
               −79.37198 
               3 
                 
               N-SK5 
               22 
               −2.221716 
             
             
               Group 20 
               41.99095 
               3.9 
                 
                 
               24 
             
             
                 
               −40.49357 
               3.5 
                 
               N-BK7 
               24 
             
             
                 
               −21.87852 
               1.5 
                 
                 
               24 
             
             
                 
               −19.81349 
               2.5 
                 
               P-LASF47 
               24 
             
             
                 
               −25.76513 
               0.2 
                 
                 
               26 
             
             
                 
               46.28619 
               7 
                 
               N-BK10 
               28 
             
             
                 
               −26.26046 
               3 
               1.572847 
               BASF12 
               28 
             
             
               Optical Stop 70 
               INFINITY 
               0.1955687 
               68.427153 
                 
               11.71622 
             
             
                 
               INFINITY 
               0.1000107 
                 
                 
               11.92014 
             
             
               Second Lens 
               INFINITY 
               3.1 
                 
                 
               16 
             
             
               Group 60 
               −80.6955 
               2.5 
                 
               N-BK7 
               16 
             
             
                 
               16.72299 
               2 
                 
                 
               16 
             
             
                 
               17.47797 
               2.5 
                 
               N-SF1 
               16 
             
             
                 
               25.27222 
               1 
                 
                 
               16 
             
             
               Base Lens 
               27.92126 
               5 
                 
               N-PSK53 
               28 
               −1 
             
             
               Group 40A 
               −105.1543 
               1 
                 
                 
               28 
             
             
                 
               −605.7182 
               2.5 
                 
               N-SF19 
               28 
             
             
                 
               24.86922 
               1 
                 
                 
               28 
             
             
                 
               26.16411 
               5 
                 
               N-SK10 
               28 
             
             
                 
               550.1011 
               43.08781 
                 
                 
               28 
             
             
               Telephoto Lens 
               −17.66212 
               2.5 
                 
               K-10 
               9.255505 
             
             
               Group 40B 
               −33.36587 
               2.069972 
                 
                 
               9.652377 
             
             
                 
               −11.91922 
               2.5 
                 
               N-LASF44 
               9.748378 
             
             
                 
               −58.44245 
               9.047573 
                 
                 
               11.14651 
             
             
               Field-Flattening 
               −77.25824 
               3.017193 
                 
               N-FK51A 
               17.63054 
             
             
               Lens Group 40C 
               −189.4453 
               0.9030828 
                 
                 
               18.29849 
             
             
                 
               55.90267 
               4 
                 
               N-LAK7 
               20.48947 
             
             
                 
               −23.22686 
               0 
                 
                 
               19.93017 
             
             
                 
               INFINITY 
               4.847658 
                 
                 
               19.44396 
             
             
                 
             
           
        
       
     
   
   The operation of system  10 ′ is the same as that described above for system  10  except that system  10 ′ has a longer focal length and a larger image plane  50 ′. In order to optimize system  10 ′ for use with wide and narrow field of view, the curvatures of the lenses in the first, second, and third lens groups  20 ,  60 ,  40 ′ are optimized for use with the entire lens (outer and central portions). This optimizes the performance of the system when an entire lens is used and minimizes the distortion encountered when only the central portion of a lens is used. The use of conic surfaces on some of the lenses, as described above, also enhances this improvement as conic surfaces offer significant variation in curvature between the inner and outer portions of the lenses. Optimization of the curvature of the lenses in this matter provides better image quality, resolution, and clarity and allows system  10 ′ to be used in applications in the visible spectrum. 
   Use of the second example described above provides a system  10 ′ with the following characteristics: 50 mm effective focal length, 14.5 degree field of view, f/5.6 in the first position; 220 mm effective focal length, 3.5 degrees field of view, f/11 in the second position. This exemplary system  10 ′ was optimized for visible wavelengths (0.49, 0.59, and 0.66 μm) and has a 15 mm square image. 
   The foregoing description of examples of the invention have been presented for purposes of illustration and description, and are not intended to be exhaustive or to limit the invention to the precise forms disclosed. The descriptions were selected to best explain the principles of the invention and their practical application to enable other skills in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention not be limited by the specification, but be defined by the claims set forth below.