Abstract:
A device for adjusting the back foci of an objective and a camera to one another has a housing by which a graphic pattern and an optics are mounted at a predetermined distance F′ from each other. The temperature compensation for the distance F′ is constant to ±2 mm for a temperature change of Delta 20° Celsius. The device also includes an apparatus for matching the back foci of an objective and a camera to each other, in which a housing has a fastening mechanism for fastening an object-side end of the objective and a graphic pattern is arranged in the housing on the end of the device opposite the fastening mechanism. The invention also includes a method for equalizing the back foci of an objective and camera using specific steps.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   A new series of objectives under the name “Digiprime” has been offered by the Zeiss company. The objectives are for high definition digital cameras. Such cameras are offered, for example, by the Sony or Panasonic companies. 
   An essential difference from the objectives for film cameras heretofore consists of the possibility of changing the image-side intercept distance (termed “back focus” in English) of the objective by means of a rotary ring. This is necessary because the tolerance for the back focus within the camera is large in comparison with a film camera. Moreover, the objective holder in the camera is as a rule made of aluminum, and greater wear can be expected. Such an adjustment can of course be achieved in that the back focus of the camera is changed, or a back focus change takes place by means of an intermediate ring inserted between objective and camera. 
   The adjustment to the correct back focus must take place very accurately, since a faulty setting results in out of focus pictures. 
   Methods are described hereinafter which are up to now known for matching the back focus of the objective to the back focus of the camera. 
   In methods used in practice up to now, a Siemens star printed on cardboard is imaged from a distance of 1–2 m by means of the camera, which is provided with a corresponding objective. The distance of the Siemens star from the objective or the camera is measured with a tape measure. The image of the Siemens star obtained with the camera is assessed. For adjustment, the image is made sharp by turning the back focus adjusting ring. The image can be observed during this adjustment by means of the viewfinder, or else by an external control monitor. 
   SUMMARY OF THE INVENTION 
   The invention has as its object to be able to comfortably carry out the equalization of the back foci of objective and camera. 
   The object is attained by a device for adjusting the back foci of an objective and a camera to one another, wherein the device has a housing by which a graphic pattern and an optics are mounted at a predetermined distance (F′) from each other. The object is also attained by a device wherein by the temperature compensation the distance F′ is constant to ±2 mm for a temperature change of Delta 20° Celsius. 
   By the measure of providing a device by means of which a graphic pattern is provided at a predetermined distance, it is possible to be able to comfortably perform the equalization of the back foci by making use of this graphic pattern. 
   In particular, a Siemens star has been found to be suitable as the graphic pattern. 
   A device, preferably a clamping device, has furthermore been found to be advantageous for fastening the device to the objective, so that no separate holder is required for the device. 
   A temperature compensation has furthermore been found to be advantageous. The temperature compensation ensures that the graphic pattern is sharply imaged at a predetermined distance over a large temperature range, such as e.g. −10° C. to 50° C., or 20° C. to 70° C. 
   It has furthermore been found to be advantageous to illuminate the graphic pattern by means of an illuminating arrangement. 
   An advantageous development of the device is to construct the device in modules, so that the ability of the device to function can be quickly restored by changing individual modules when there is a defect. Furthermore, by interchanging individual modules it is possible to change the properties of the device. Thus, for example, the distance at which the graphic pattern is imaged, or the graphic pattern itself, can be changed. 
   Further advantageous measures are described in the further claims. An embodiment example is described in detail using the accompanying Figures, and the temperature compensation is explained in detail using the sketch in  FIG. 4 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a device for equalizing the back foci of objective and camera, in side view, 
       FIG. 2  shows a back view of the device of  FIG. 1 , 
       FIG. 3  shows a section through the device shown in  FIG. 1 . 
       FIG. 4  shows a diagram of the basic principle of a temperature compensated device for equalization of the back foci of objective and camera. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an external view of a device according to the invention. The device  1  includes a clamping device  13  with a tightening ring  14  which coaxially surrounds clamp jaws  12 . This clamping device represents one of the modules included in the device. The device can be frictionally connected to the objective of a camera by means of the clamping device. Alternatively, a positive connection such as screws or a latch connection can be provided. 
   Further modules, which can be seen in  FIG. 1 , are the image module  23  and the middle module  27  with the housing  3 . 
     FIG. 2  shows a plan view of the end situated opposite to the clamping device. 
   Arranged at this end are a on/off switch  30 , a plug connection  41  for an external current supply, and a brightness control  39  for an illumination arrangement provided within the device, and the battery compartment  37 . The current supply can also be used for charging the battery. 
     FIG. 3  shows a section through the device  1  along the optical axis  2 . The battery compartment  37  with the battery  38  is constituted in the housing  22  of the image module  23 . A control device  35  in the form of a circuit board  35 ′ is arranged behind the battery. This control device  35  is in signal connection with the on/off switch  30  and with the brightness control  39 . This control device  35  is furthermore in signal connection with LEDs for an illumination of the Siemens star  7  provided as the graphic pattern  5 . These LEDs are advantageously arranged directly on the circuit board  35 ′. Another lighting source can of course be provided as the illumination arrangement. 
   A matt plate  29  is provided between the circuit board  35 ′ and Siemens star  7 , for the uniform illumination of the Siemens star. By the use of such a graphic pattern, good illumination can be provided by means of transmitted light illumination, without reflections occurring. Another kind of illumination can however be selected. Furthermore an illumination is not required when an illumination of the graphic pattern is provided by directly incident radiation by means of daylight. Furthermore it is also possible to provide an illumination of the graphic pattern by means of luminescent material, such as is known from the field of target telescopes for providing an illuminated target marking. 
   A first holder  15  is formed as a portion of the housing  22  for mounting the Siemens star. 
   The housing  22  is fastened to the housing  3  of the middle module  27 . The holder projects into the housing  3  of the middle portion  27 . 
   Rods  21  are furthermore fastened to the housing  3  of the middle portion  27  and are a portion of a second holder  17 . These rods are made from a material other than the first holder  17  and a mount  18  in which an optics  9  is mounted. An achromat  11  is provided as the optics  9 , and images the Siemens star  7  at infinity. However it can also be provided that the graphic pattern  5  is imaged to another distance. 
   Arranged between the rods  21  and the mount  18  are distance plates  19 , by means of which the basic distance between the graphic pattern  5  and the optics  9  can be changed. 
   The functional principle of the device  1  is described hereinbelow. 
   By means of the device  1 , the illuminated Siemens star  7  is imaged to ∞ by means of the achromat  11 . The device  1  is set directly on the objective to be adjusted (not shown) by means of the clamping device  13 . 
   In principle, the distance ring of the objective is set to the distance at which the graphic pattern  5  is imaged by the optics  9 . Here the focusing ring of the camera is thus set to ∞. The Siemens star/graphic pattern can be observed in the viewfinder of the camera and can be set sharply by turning the back focus adjusting ring of the objective of the camera. 
   Further general advantages of the device are: The device  1  is directly pushed from the front onto the objective. No additional support is necessary. The clamping device  13  at the mechanical interface with the objective ensures additional holding. 
   A Siemens star  7  on cardboard and a corresponding illumination arrangement  31  are not necessary. The device  1  can also be used in the dark because of its integrated battery-operated illumination arrangement  33 . Alternatively, the device  1  can also be operated by means of an external current supply via the plug connection  41 . To protect the battery  38  from inadvertent discharge, the illumination arrangement  31  switches off after 0.5–10 min (adjustable). 
   The distance measurement to the Siemens star  7  is omitted. 
   The brightness of the Siemens star  7  in the viewfinder can easily be adjusted with a control  39 . 
   Errors in distance adjustment are not present, since it is easy to turn to the ∞ stop, with a selected imaging at infinity by the optics  9 . 
   The clamping device  13  can be unscrewed. Clamping devices are obtainable in different diameters. Due to this, the device can also be adapted to objectives of different producers and thus can be used universally. 
   An increase in accuracy can be attained, above all with objectives of longer focal lengths. If 5 μm is set as a permissible focusing error, the following values result for the required measuring accuracy of the distance to the graphic pattern: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Focal length of objective: 
               40 mm 
             
             
                 
               Distance from Siemens star: 
               1 m or 2 m 
             
             
                 
               Permissible error of distance 
               0.2% or 0.6% 
             
             
                 
               measurement with tape measure: 
             
             
                 
                 
             
           
        
       
     
   
   The required accuracy cannot be reliably attained with a commercial tape measure. 
   The basic principle of a temperature-controlled device  1  for equalizing the back focus of objective and camera is described hereinbelow using  FIG. 4 . 
   The focal length F′, which corresponds to the distance between the main plane  55  of the achromat  11  and the Siemens star  7 , is to be kept constant over a wide temperature range, in order to ensure the imaging of the Siemens star  7  at infinity in every case. 
   For this purpose, the rods  21  (only two rods are shown in  FIG. 4 ) consist of a material with small thermal expansion coefficients. These rods act against the short mounts  51 ,  53 , which consist of a material with large thermal expansion coefficients. If the differences of the thermal expansion coefficients are too similar, this would lead to a large structure. 
   The geometrical and optical changes of the achromat with the course of temperature, that is, when the temperature changes, are also to be considered in the calculation. The aim is a total compensation of all expansions and of the optical property of the achromat. 
                                     Parameters                                L1   Effective length of mount 1, of material 1 with thermal expansion           coefficient α1       L2   Effective length of mount 2, of material 2 with thermal expansion           coefficient α2       L3   length of the rod, of material S with thermal expansion coefficient           of the rod αS       F′   focal length of the achromat       E′   principal point back focus of the achromat       R   salient radius of the achromat       D   support diameter of the achromat       A   distance of the vertex point of the achromat to the support point                    
Basic Formulae
 
   For all temperatures, the following is to hold:
 
 F′−E′+A+L 2 −LS+L 1=0
 
   Thus:
 
Δ F′−ΔE′+ΔA+ΔL 2 −ΔLS+ΔL 1=0
 
A is given by:
 
           A   =     R   -       (       R   2     -       D   2     4       )               
Simplified Assumptions
 
   The material of the two mounts  51 ,  53  for achromat and Siemens star consist of the same material F with thermal expansion coefficient αF=α1=α2. This assumption certainly holds for most applications of this kind, since a standard material is usually used. 
   Thus there holds for the effective total length of both mounts:
 
 LF=L 1 +L 2
 
Predetermined Values
 
   For F′, E′ and A, there are calculated values for given temperatures. The changes ΔF′, ΔE′ and ΔA can be calculated from these. A system-specific value can be derived from these values:
 
 S=F′−E′+A  
 
and:
 
Δ S=ΔF′−ΔE′+ΔA  
 
Calculation
 
   The following simplified formulae result from the simplified assumptions and the predetermined values:
 
Δ S+ΔLF−ΔLS =0
 
Δ S+LF*ΔT*αF−LS*ΔT*αS =0
 
Rearranged formula:
 
 LS*αS−LF*ΔF=ΔS/ΔT  
 
 LF=LS−S  
 
From this there follows:
 
 LS*αS−LS*αF+S*αF=ΔS/ΔT  
 
Thus there holds for the length of the rods:
 
 LS =( ΔS/ΔT−S·αF )/( αS−αF )
 
Particularly suitable materials:
     As materials for this structure there may be considered:   For the mounts (as large a thermal expansion coefficient as possible):   Steel (10.3E-6/° K.), brass (21.1E-6/° K.), aluminum (24E-6/° K.), plastics (with large
 
For the rods (smallest possible temperature coefficient):
   Zerodur glass ceramic (0), Invar (1.5–2.5-E-6/° K. according to heat treatment), plastics (there are a few a highly filled plastics with about 2.5E-6/° Kelvin), ceramics (with small α).   

   Aluminum for the mounts and Invar for the rods has been found to be a good combination, since the components are easily and conveniently produced and the thermal expansion coefficients are situated sufficiently far apart. 
   Exemplary calculations have shown that the temperature course of the optics (geometric and optical) has only a small effect on the precision of the device. On the other hand, a temperature course can have a marked effect on the distance of the optics and the graphic pattern. For this reason a temperature compensation of the device can be provided by the selection of suitable materials for the rods and the mounts and a specific coordination of length. 
   For example, a device constructed in this manner with an achromat with a focal length of 200 mm to the Siemens star could be thermally stabilized so that with ΔT=20° K., a position change of the Siemens star of 0.01 mm could be attained. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Reference List 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
                1 
               device 
             
             
                 
                2 
               optical axis 
             
             
                 
                3 
               housing 
             
             
                 
                5 
               graphic pattern 
             
             
                 
                7 
               Siemens star 
             
             
                 
                9 
               optics 
             
             
                 
               11 
               achromat 
             
             
                 
               12 
               clamp jaws 
             
             
                 
               13 
               clamping device 
             
             
                 
               14 
               stress ring 
             
             
                 
               15 
               holder 
             
             
                 
               17 
               second holder 
             
             
                 
               18 
               mount 
             
             
                 
               19 
               distance plate 
             
             
                 
               21 
               rods 
             
             
                 
               22 
               housing of the image module 
             
             
                 
               23 
               image module 
             
             
                 
               27 
               middle module 
             
             
                 
               29 
               matt disk 
             
             
                 
               30 
               on/off switch 
             
             
                 
               33 
               battery-operated illumination (LED) 
             
             
                 
               35 
               control device 
             
             
                 
               35’ 
               circuit board 
             
             
                 
               36 
               plate 
             
             
                 
               37 
               battery compartment 
             
             
                 
               38 
               battery 
             
             
                 
               39 
               brightness control 
             
             
                 
               41 
               plug connection 
             
             
                 
               51 
               mount 
             
             
                 
               53 
               mount 
             
             
                 
               55 
               principal plane