Abstract:
A photographic lens and test chart provide for a method of inspecting and adjusting the focus of the lens. The test chart is obliquely inclined relative to a plane perpendicular to the viewing axis of the photographic lens. Test photographs are taken of the test chart under low magnification and high magnifications the test photographs enabling the photographic lens to be adjusted for proper focus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a focus inspection device and a focusing method using the focus inspection device, especially to those applied to a printing lens that is used for projecting an optical image of an original, like a picture frame on a photographic film or a photo-print onto a photosensitive recording material, like photographic paper. 
     2. Background Arts 
     As a method of adjusting the focus of the printing lens in photographic printers, it has been widely conducted making a test print from a test original that is photographed on a photographic film and contains an ordinary image, like a grayscale image of a landscape, and inspecting the test print with naked eyes. In that case, it is usual to make a plurality of test prints from the same original while shifting the focus of the printing lens step by step, and compare the test prints to each other for determining the optimum focal position. Which is called a round exposure method, and is disclosed for example in JPA Nos. 64-17013 and 01-200344. 
     According to this conventional method, however, the inspector cannot exactly determine the optimum focal position because it is not easy to judge the sharpness of the test print containing the ordinary image. Particularly when the test prints are made at a low print magnification, that is, when the printing lens is set at a low magnification and thus has a large depth of field, it is still more difficult to judge from the test print as to whether a reference focal plane, in which the test original and other originals to print are positioned, coincides with the center of the depth of field, i.e. the focal plane of the printing lens in the object side. If the reference focal plane does not coincides with the center of the depth of field of the printing lens, and the original is not held flat in the reference focal plane, the printed image can be blurred. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to provide a focus inspection device for use in focusing an image forming lens, that facilitates to inspecting focal conditions of the image forming lens. 
     Another object of the present invention is to provide a focus inspection device that permits quantitative determination of a deviation of a focal plane of an image forming lens from a reference focal plane. 
     A further object of the present invention is to provide a focusing method for a printing lens, using such a focus inspection device. 
     According to the present invention, a focus inspection device for a lens that is determined to form an optical image of an original on a predetermined image forming surface when the original is placed in a predetermined reference focal plane that is perpendicular to an optical axis of the lens, comprises a focusing chart having a test pattern thereon; and a holding device for holding the focusing chart in a position inclined to the reference focal plane, wherein a deviation of an object focal plane of the lens from the reference focal plane is detected in view of sharpness of the test pattern on an image formed from the focusing chart through the lens. 
     The holding device preferably holds the focusing chart such that a center of the test pattern is aligned with the reference focal plane. 
     According to a preferred embodiment, the focusing chart further comprises a scale provided adjacent the test pattern along the inclined direction of the focusing chart, the scale serving as a measure of relative height of the inclined focusing chart to the reference focal plane. 
     The sharpness of the test pattern may be evaluated as density distribution on the test print, to detect the deviation value of the object focal plane of the printing lens as a deviation value of the lowest density area from the center of the test pattern on the test print. 
     The test pattern preferably comprises a plurality of lines extending in parallel to a transverse direction to the inclined direction of the focusing chart, the lines being equally spaced from each other in the inclined direction. 
     According to a more preferred embodiment, the test pattern comprises a plurality of rows of lines extending in parallel to a transverse direction to the inclined direction of the focusing chart, the lines being equally spaced from each other in the inclined direction within each row, and the spacing between the lines increases sequentially from one row to another. 
     Since the narrower spacing between the lines on the focusing chart results the narrower lowest density area on the test print, the lowest density area appearing on the reproduced test pattern consisting of the plurality of rows of lines with gradually increased line densities displays a substantially triangular shape. Therefore, it becomes possible to determine the deviation of the lowest density area from the center of the test pattern as a definite quantitative value. 
     According to another aspect of the present invention, a focusing method for a printing lens that is determined to print an image of an original on a photosensitive recording material placed in a predetermined position when the original is placed in a predetermined reference focal plane that is perpendicular to an optical axis of the lens, the method comprising the steps of: holding a focusing chart with a test pattern thereon in a position where the focusing chart is inclined to the reference focal plane with a center of the test pattern aligned with the reference focal plane; making at least a test print from the focusing chart through the printing lens; detecting a deviation value of an object focal plane of the printing lens from the reference focal plane in view of sharpness of the test pattern reproduced on the test print; and adjusting the object focal plane of the printing lens to the reference focal plane in accordance with the detected deviation value of the object focal plane. 
     It is preferable to make test prints from the focusing chart through the printing lens while setting the printing lens at different printing magnifications, detect a deviation value of an object focal plane of the printing lens from the reference focal plane at each of the different printing magnifications in view of sharpness of the test pattern reproduced on the test prints, and calculate correction amounts from the deviation values detected from the test prints, for focusing the printing lens in accordance with the correction amount. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when read in association with the accompanying drawings, which are given by way of illustration only and thus are not limiting the present invention. In the drawings, like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
     FIG. 1 shows a perspective view of a focus inspection device according to an embodiment of the present invention; 
     FIG. 2 shows a vertical section of the focus inspection device in connection to a lower mask of a film carrier of a printer; 
     FIG. 3 shows a top plan view of a focusing chart of the focus inspection device; 
     FIG. 4 schematically shows a printer-processor using the focus inspection device for adjusting the focus of a printing lens thereof; 
     FIG. 5 shows a block diagram illustrating a focusing device of the printing lens of the printer-processor; 
     FIG. 6 shows an explanatory diagram illustrating a test print obtained from the focusing chart; 
     FIG. 7 shows a graph illustrating changes in position of the focal plane along the optical axis of the printing lens at different magnifications, caused by correcting a second lens set value β; 
     FIG. 8 shows a graph illustrating changes in position of the focal plane along the optical axis of the printing lens at different magnifications, caused by correcting a first lens set value α; 
     FIG. 9 shows a vertical section of a focus inspection device according to another embodiment of the invention, in connection to the lower mask of the film carrier; 
     FIG. 10 shows a vertical section of a focus inspection device according to a further embodiment of the invention, in connection to the lower mask of the film carrier; and 
     FIG. 11 shows a perspective view of the focus inspection device of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A focus inspection device  10  shown in FIG. 1 is constituted of a focusing chart  11  and a holder  12  for holding the focusing chart  11 . As shown in FIG. 2, the focus inspection device  10  is placed in an opening  14   a  of a lower mask  14  of a film carrier  13 . 
     The focusing chart  11  is constituted of a glass plate  20  and a test pattern  21  consisting of parallel lines that extend in parallel to a widthwise direction of the focusing chart  11 , and are formed on an obverse surface of the glass plate  20  by aluminum deposition. As shown in detail in FIG. 3, the test pattern  21  is constituted of first to fourth rows  22 ,  23 ,  24  and  25  of parallel lines, wherein each row extends in a lengthwise direction of the focusing chart  11  and is aligned with other rows in the widthwise direction of the focusing chart  11 . The parallel lines of one row are arranged at a different interval from the parallel lines of another row. More specifically, each row consists of obscure or black parallel lines  22   a ,  23   a ,  24   a  or  25   a , and transparent parallel lines  22   b ,  23   b ,  24   b  or  25   b  which are arranged alternately with the black parallel lines, wherein the parallel black lines and the transparent parallel lines of the same row have an equal width. That is, the line width of each row is equivalent to the spacing between the lines of that row, and the line width of one row is different from the line width of another row. In the present embodiment, the first row  22  in FIG. 3 has the smallest line spacing or the highest line density, and the fourth row  25  has the largest line spacing or the lowest line density. 
     For example, in the first row  22 , the black lines  22   a  are arranged at a frequency of 40 lines/mm, and have a width W 1  of 12.5 μm, so the transparent lines  22   b  have a width W 2  of 12.5 μm. In the second row  23 , the black lines  23   a  are arranged at a frequency of 25 lines/mm, and have a width W 3  of 20 μm, so the transparent lines  23   b  have a width W 4  of 20 μm. In the third row  24 , the black lines  24   a  are arranged at a frequency of 16 lines/mm, and have a width W 5  of 31.25 μm, so the transparent lines  24   b  have a width W 6  of 31.25 μm. In the fourth row  25 , the black lines  25   a  are arranged at a frequency of 12 lines/mm, and have a width W 7  of 41.67 μm, so the transparent lines  25   b  have a width W 8  of 41.67 μm. 
     Provided along a top margin of the test pattern  21  is a scale  26  that represents a degree of inclination of the focusing chart  11 . As shown in FIG. 1, the holder  12  is shaped into a rectangular frame having a rectangular center opening  30  for holding the focusing chart  11  therein. A pair of steps  31  and  32  are formed in the margin of the opening  30  along opposite short sides thereof, such that short sides  11   a  and  11   b  of the focusing chart  11  are held on the steps  31  and  32  when the focusing chart  11  is put in the opening  30 . The steps  31  and  32  have a difference in height from each other, as shown in FIG. 2, so the focusing chart  11  is inclined in its lengthwise direction, i.e. the direction along which the scale  26  is provided, as it is held in the holder  12 . Thereafter, the focusing chart  11  is affixed to the holder  12  by an adhesive agent or another fastening device. The inclination of the focusing chart  11  in the holder  12  is determined such that the difference in height from the lower end to the upper end of the test pattern  21  is 1.6 mm. That is, the scale  26  is determined to indicate the height difference of ±0.8 mm from the center of the test pattern  21  to the opposite ends. 
     The center of the test pattern  21  is indicated by a number “0” in the scale  26 , and the scale  26  is provided with numbers “1” to “8” at regular intervals along the upper half of the test pattern  21  to show a height difference of 0.1 mm each. On the other hand, the scale  26  is provided with alphabets “A” to “H” at regular intervals along the lower half of the test pattern  21  to show the height difference of 0.1 mm each. Between these numbers “1” to “8” and the alphabets “A” to “H”, the scale  26  is graduated in 25 μm divisions  26   a . It is alternatively possible to provide numbers “−1” to “−8” instead of the alphabet “A” to “H”. 
     The holder  12  also has stepped flanges  35  along its short sides. Horizontal portions  35   a  of the stepped flanges  35  are placed on a top side  14   b  of a lower mask  14  of the film carrier  13 , and vertical portions  35   b  of the stepped flanges  35  are fitted in the opening  14   a  of the lower mask  14  such that the vertical portions  35   b  are in contact with a vertical inner wall  14   c  around the opening  14   a . Thereby the position of the focus inspection device  10  is fixed in the film carrier  13  in the horizontal and vertical directions. Where the focus inspection device  10  is positioned in the film carrier  13 , the centers of the respective rows  22  to  25 , i.e. the center of the test pattern  21  as indicated by the number “0” in the scale  26 , coincide with a reference focal plane FSL in which a photo filmstrip is to be positioned flat in the film carrier  13 . It is to be noted that stepped flanges like the stepped flanges  35  may be provided along the long sides of the holder  12 , or around the four sides of the holder  12 . 
     FIG. 4 shows a printer-processor  40  that uses the focus inspection device  10  for adjusting the focus of a printing lens  44 . The printer-processor  40  consists of a printer section  41  and a processor section  42 . In the printer section  41 , a light source  43 , the film carrier  13 , the printing lens  44 , a black shutter  45  and a paper advancing device  46  are disposed in this order from a bottom side. The black shutter  45  is driven by a shutter driver  45   a.    
     A photographic filmstrip  47  is carried through the film carrier  13 , such that picture frames to print are seriatim placed in a printing station that is defined by the opening  14   a  of the lower mask  14 . The picture frame placed in the printing station is illuminated by the light source  43 , and light components traveling through the picture frame are projected as an optical image through the printing lens  44  onto a color photographic paper  48 , recording a latent image on the photographic paper  48 . Prior to exposing the photographic paper  48  to the optical image by opening the black shutter  45 , a scanner  49  measures three-color separated photometric values of the picture frame in the printing station, and an exposure calculator  50  determines a proper exposure value on the basis of the three-color separated photometric values. Then, a controller  51  controls color balance and light intensity of the illumination light from the light source  43  by adjusting degree of insertion of three color filters  52   a ,  52   b  and  52   c  of a light quality control device  52  into a light path from the light source  43  to the printing station. 
     In the paper advancing section  46 , the color photographic paper  48  is pulled out from a roll and is placed in an exposure position  54  through feed rollers  53 . After being exposed, the color photographic paper  48  is fed to the processor section  42 , to be subjected to ordinary photofinishing processes to develop the latent image into a visible positive image. Thereafter, the color photographic paper  48  is cut into an individual photo-print piece. A cutter  56  and a paper reservoir  57  are disposed between the printer section  41  and the processor section  42 . The cutter  56  cuts the color photographic paper  48  when the printing process is interrupted or when a test printing process is executed. The paper reservoir  57  absorbs difference in advancing or processing speed of the color photographic paper  48  between the printer section  41  and the processor section  42 . 
     For inspecting and adjusting the focus of the printing lens  44 , the focusing chart  11  is placed in the printing position instead of the photographic filmstrip  47 . As shown in FIG. 5, a focusing device  60  is connected to the printing lens  44 , and is controlled by the controller  51 . The printing lens  44  is composed of a first lens group  62  fixedly held in a first lens barrel  61 , and a second lens group  64  held in a second lens barrel  63  that is mounted in the first lens barrel  61  and movable relative to the first lens barrel  61 . The first lens barrel  61  is moved along an optical axis L of the printing lens  44  by means of a first lens shift mechanism  65 , and the second lens barrel  64  is also moved along the optical axis L by means of a second lens shift mechanism  66 . The first and second lens shift mechanisms  65  and  66  are driven by respective pulse motors  67  and  68  which are controlled by the controller  51  through respective drivers  67   a  and  68   a.    
     In the present embodiment, the focal plane of the printing lens  44  is displaced about 20 μm in the direction of the optical axis L per one drive pulse applied to the pulse motor  67  of the first lens shift mechanism  65  at any magnifications. On the other hand, by rotating the pulse motor  68  for the second lens shift mechanism  66 , the second lens barrel  63  is moved relative to the first lens barrel  61 , so the position of the second lens group  64  relative to the first lens group  62  changes. With a change in the position of the second lens group  64 , the focal plane of the printing lens  44  may also be displaced in the direction of the optical axis L. Therefore, the focus of the printing lens  44  is adjusted by controlling the number of drive pulses applied to the pulse motor  67  and the number of drive pulses applied to the second pulse motor  68  as well. 
     Now the method of inspecting and adjusting the focus of the printing lens  44  by means of the focus inspection device  10  will be described. 
     First the focus inspection device  10  is placed in the opening  14   a  of the lower mask  14  of the film carrier  13 , so that the center of the test pattern  21  as indicated by “0” of the scale  26  is aligned with the reference focal plane FSL that is equivalent to a photosensitive surface of the photographic filmstrip  47  in the printing position, and the lengthwise direction of the focusing chart  11  is adjusted to the lengthwise direction of each picture frame in the printing position. 
     Then, a first lens set value “α” and a second lens set value “β” are entered in the controller  51  through a keyboard  70 , as reference values for controlling the first and second lens shift mechanisms  65  and  66  respectively. The entered lens set values are displayed on a screen of a display device  71 . The lens set values α and β are predetermined for each printing lens  44 , and are recorded on the printing lens  44 . 
     Next, printing conditions, including the print magnification, are set up. Initially, the print magnification is set at a minimum value, e.g. 3.96× in this instance, and an image of the focusing chart  11  is printed on the color photographic paper  48 . Thereafter, an image of the focusing chart  11  is printed at a maximum magnification, e.g. 13.75×. By processing the color photographic paper  48  in the processor section  42 , a couple of test prints are obtained. FIG. 6 shows a test print  80  as an example. In view of these test prints, a deviation of an object focal plane of the printing lens  44  from the reference focal plane FSL is detected in a manner as set forth below. 
     Since the focusing chart  11  of the focus inspection device  10  is inclined relative to the reference focal plane FSL that is perpendicular to the optical axis L of the printing lens  44 , only those parallel lines of the test pattern  21  are reproduced sharply on the test print  80  which are placed proximate to the object focal plane of the printing  44  on the focusing chart  11 . Hereinafter, the position of the test pattern  21  that is placed in the object focal plane of the printing lens  44  on the focusing chart  11  is called a just focus position JPP, and the position where the lines are most sharply reproduced may be determined as the just focus position JPP on the test print  80 . 
     Concretely, on the test print  80 , a just focus area  81  is produced around the just focus position JPP, where black and transparent parallel lines  85  and  86  are printed sharply, and blurred areas  82  containing blurred parallel lines are produced on opposite sides of the just focus area  81 . Because the magnitude of blurs of the printed parallel lines increases with the distance from the just focus area  81 , the density of the test print  80  increases with the distance from the just focus area  81 . Thus, the test print  80  has high density areas  87  outside the blurred areas  82 , wherein the black lines cannot be distinguished from each other. 
     In the example shown in FIG. 6, the just focus area  81  centers around an intermediate position between the grades “1” and “2” of the scale  26 , and the blurred areas  82  extend substantially symmetrically about this intermediate position, so this position may be judged as the just focus position JPP that corresponds to the object focal plane of the printing lens  44 . As described above, the parallel lines of the test pattern  21  are arranged at different intervals from one row to another such that the line spacing decreases from the fourth row  25  to the first row  22 . Because the smaller line spacing means the higher line density, high density areas  87  get wider from the fourth row  25  to the first row  22  in the test print  80 . That is, the just focus area  81  as a whole has a substantially triangular shape tapering toward the just focus position JPP. Since the scale  26  is provided along the top margin of the test pattern  21 , it is easy to determine the just focus position JPP in the scale  26  on the test print  80  with naked eyes. In addition, because the blurred areas  82   b  and  82   c  also are converged toward the just focus position JPP on the opposite sides of the just focus area  81 , the inspector can make use of the blurred areas  82   b  and  82   c , to determine the just focus position JPP with accuracy. 
     Since the focusing chart  11  is inclined from the perpendicular plane to the optical axis L, the lines  85  and  86  in the left-hand blurred area  82   b , which correspond to the parallel lines of the lower half of the test pattern  21 , have a yellow-greenish tinge, whereas the lines  85  and  86  in the right-hand blurred area  82   c , which correspond to the parallel lines of the upper half of the test pattern  21 , have a tinge of blue. The lines  85  and  86  are seen to be black-white stripes only in the just focus area  81 . Accordingly, the inspector can distinguish the just focus area  81  from the blurred areas  82   a  and  82   b  in view of the tinges, in addition to the sharpness of the lines  85  and  86 . Which makes it easier for the inspector to determine the just focus position JPP. 
     If the focal plane of the printing lens  44  coincides with the reference focal plane FSL, the just focus position JPP would be located at the center of the test pattern  21  as indicated by “0” in the scale  26 . If the focal plane of the printing lens  44  is higher than the reference focal plane FSL, the just focus position JPP deviates from the center of the test pattern  21  toward the upper end as indicated by “8” in the scale  26 . If the focal plane of the printing lens  44  is lower than the reference focal plane FSL, the just focus position JPP deviates from the center of the test pattern  21  toward the lower end as indicated by “H” in the scale  26 . The inspector reads the deviation of the just focus position JPP from the center position “0” on the scale  26 , and inputs a value of the deviation in the controller  51  through the keyboard  70 . Then, the controller  51  calculates a correction amount on the basis of the deviation value, for correcting the focal plane of the printing lens  44  to be coincident with the reference focal plane FSL. Formulas for the calculation of the correction amount are previously obtained on the basis of design values for the printing lens  44 . 
     The method of correcting the focus of the printing lens  44  on the basis of the deviation value of the just focus position JPP will be described in more detail below. 
     In the present embodiment, the first lens set value α is represented by the number of drive pulses applied to the pulse motor  67 , and the second lens set value β is represented by the number of drive pulses applied to the pulse motor  68 , each from a predetermined reference position of the printing lens  44 . For example, the first lens set value α is “119”, and the second lens set value β is “105”. On the assumption that a correcting direction to elongate the distance between the conjugate points of the printing lens  44  is regarded as a positive direction, the focal plane of the printing lens  44  is designed to be displaced +20 μm with a correction of +1 pulse to the first lens set value α. On the other hand, with a correction of +1 pulse to the second lens set value β, the focal plane is designed to be displaced −86 μm at the magnification of 3.96×, unchanged at a magnification of 9.08×, and displaced +12 μm at the magnification of 13.75×. 
     FIG. 7 shows how the focal plane of the printing lens  44  is displaced with the correction of the second lens set value β at the respective magnifications “3.96×”, “9.08×” and “13.75×”. FIG. 8 shows how the focal plane of the printing lens  44  is displaced with the correction of the first lens set value α at the respective magnifications “3.96×”, “9.08×” and “13.75×”. 
     In FIG. 7, a curve S 1  shown by a solid line represents initial positions of the focal plane of the printing lens  44  along the optical axis L at the respective magnifications, that is, where the first and second lens groups  62  and  64  are placed in those positions determined by the first and second lens set values α and β. On the other hand, a curve S 2  shown by dashed line represents axial positions of the focal plane obtained by subtracting  1  from the second lens set value β, a curve S 3  shown by dashed line represents axial positions of the focal plane obtained by subtracting  3  from the second lens set value β, and a curve S 4  shown by dashed line represents axial positions of the focal plane obtained by subtracting  7  from the second lens set value β, respectively at the above three magnifications. As seen from the curve S 4 , the positions of the focal plane at the different magnifications approach to each other when the second lens set value β is corrected by −7 pulses, in the example shown in FIG.  7 . 
     In FIG. 8, a curve P 1  shown by a solid line represents axial positions of the focal plane at the three different magnifications after the correction of −7 pulses to the second lens set value β, before correcting the first lens set value α, and a curve P 2  shown by dashed lines represents positions of the focal plane at the respective magnifications after correcting the first lens set value α with a correction value of +5 pulses. That is, the curve P 1  is equivalent to the curve S 4 . It is to be noted that the grade “0” in the vertical axis of the graphs of FIGS. 7 and 8 means that the focal plane coincides with the reference focal plane FSL. The first lens set value α should be corrected such that the focal plane approaches to the reference focal plane FSL at any magnifications. In this example, the focal plane approximates to the reference focal plane at any magnifications by correcting the first lens set value α with a correction value of +9 pulses, as shown by a curve P 3  in FIG.  8 . 
     From the characteristic curves shown in FIGS. 7 and 8, it is proved that a correction value Δα for the first lens set value α and a correction value Δβ for the second lens set value β may be calculated on the basis of a deviation value X of the just focus position JPP from the center of the scale  26  at a minimum magnification, and a deviation value Y of the just focus position JPP from the center of the scale  26  at a maximum magnification, wherein these deviation values X and Y may be determined by the visual inspection on the test prints obtained at the minimum and maximum magnifications respectively. 
     Concretely, the correction value Δβ for the second lens set value β is calculated by use of the following equation: 
     
       
         Δβ=( X−Y )/(β2−β1) 
       
     
     wherein β1 represents a change in the axial position of the focal plane per one pulse added to the second lens set value β at the minimum magnification, and β2 represents a change in the axial position of the focal plane per one pulse added to the second lens set value β at the maximum magnification. 
     The correction value Δα is calculated by use of the following equation: 
     
       
         Δα=−( Y+ 12·Δβ)/α1 
       
     
     wherein α1 represents a change in the axial position of the focal plane per one pulse of increment in the first lens set value α. 
     In the present embodiment, as shown in FIGS. 7 and 8, β1 is −86 μm, β2 is 12 μm, X is −800 μm, and Y is −100 μm, wherein the minimum magnification is 3.96×, and the maximum magnification is 13.75×. Thus the correction value Δβ is: 
     
       
         Δβ={(−800)−(−100)}/{12−(−86)}=−700/98=−7.14 
       
     
     Since α1 is 20 μm in this embodiment, 
     
       
         Δβ=−{(−100)+12·(−7.14)}/20=9.28 
       
     
     According to these correction values Δα=9.28 and Δβ=−7.14, the first lens group  62  is displaced from the initial position by 9 drive pulses in the positive direction, i.e. the direction to elongate the conjugate distance, and the second lens group  64  is displaced from the initial position by 7 drive pulses in the negative direction, wherein the initial positions of the first and second lens groups  62  and  64  are determined by the first lens set value α and the second lens set value β respectively. 
     After correcting the focus of the printing lens  44  with the calculated correction values Δα and Δβ, test prints are made from the focusing chart  11  not only at the minimum and maximum magnifications of 3.96× and 13.75×, but also at an intermediate magnification, i.e. 9.08×. Then the just focus position is determined with respect to each of the three test prints in the same way as set forth above. 
     If the deviation of the just focus position from the center position of the scale  26  is not within a tolerance at the minimum magnification or at the maximum magnification, the correction values Δα and Δβ are recalculated on the basis of the newly determined deviation values X and Y on the test prints obtained at the minimum and maximum magnifications. After correcting the focus with the newly calculated correction values Δα and Δβ, test prints are made from the focusing chart  11  at the minimum, intermediate and maximum magnifications, and the just focus positions on the test prints are inspected. In this way, the focus of the printing lens  44  is adjusted to the optimum condition. 
     In the present embodiment, tolerances at the minimum, intermediate and maximum magnifications are predetermined as set forth below: 
     ±50 μm at 3.96× 
     ±25 μm at 9.08× 
     ±25 μm at 13.75× 
     In addition to these tolerance ranges, a tolerance of 25 μm is required on the difference in height between the focal plane or the just focus position at the magnification of 9.08× and that at the magnification of 13.75×. However, the tolerances may be set at other approximate values. 
     Although the lens set values are automatically corrected with correction values which are determined on the basis of the deviation values detected from the test prints and entered in the controller in the present embodiment, it is possible to manually calculate correction values from the deviation values, revise the lens set values with the correction values and input revised lens set values through the keyboard. 
     In the above embodiment, the lengthwise direction of the focusing chart  11  is aligned with the lengthwise direction of the picture frame in the printing station, it is possible to inspect the focus of the printing lens in another condition where the focusing chart  11  is turned by 90 degrees from this position. 
     Although the focusing chart  11  is held inclined in the opening  30  of the holder  12  in the focus inspection device  10  of the above embodiment, it is alternatively possible to hold the focusing chart  11  horizontal in an opening of a holder with respect to a flat top surface of the holder, and provide the inclination of the focusing chart  11  by other means. According to an embodiment of a focus inspection device  105  shown in FIG. 9, the focusing chart  11  is held horizontal in an opening  101  of a holder  100  with respect to a flat top surface of the holder  100 , and bottom stepped portions  102  and  103  of the holder  100  have different heights from each other relative to a top flat surface of the holder  100 . Thus, the focusing chart  11  is inclined to the perpendicular plane to the optical axis L of the printing lens  44  when the focus inspection device  105  is fitted in the opening  14   a  of the lower mask  14  of the film carrier  13  with the bottom stepped portions  102  and  103  in contact with the top surface of the lower mask  14 . 
     Instead of providing a difference in height between the stepped portions  102  and  103 , it is possible to provide a focus inspection device with an inclination adjusting device. According to an embodiment shown in FIGS. 10 and 11, the inclination adjusting device is constituted of three screws  111 ,  112  and  113  put through opposite ends of a holder  110  of a focus inspection device  114  wherein the focusing chart  11  is held horizontal to a top flat surface of the holder  110 . By turning one or more of the screws  111  to  113 , the height of the top surface of the holder and thus the height of the focusing chart  11  are varied relative to the lower mask  14 , so is adjusted the inclination of the focusing chart  11 . It is preferable to use the screw  111  for adjusting the inclination, and the screws  112  and  113  on the opposite end for adjusting the center of the focusing chart  11  to the reference focal plane FSL (see FIG.  2 ). 
     It is also possible to provide a device for setting the focusing chart  11  inclined to the perpendicular plane to the optical axis in the film carrier, e.g. on the lower mask, instead of the holder  12 ,  100  or  110  of the focusing chart  11 . Such a device may be constituted of a pair of stepped portions having different heights, or positioning screws, or spacers disposed between the lower mask and the focus inspection device, or the like. 
     Furthermore, it is possible to provide the inclination adjusting device in the focus inspection device  10  where the focusing chart  11  is held inclined relative to the holder  12 , for the sake of fine-adjustment of the inclination of the focusing chart  11 . It is also possible to hold the focusing chart  11  to be adjustable in inclination relative to the holder by means of screws or the like. 
     Although the just focus position JPP is determined by the inspector inspecting the test prints with naked eyes, it is possible to measure a mean density in each of several divisions of each row  22 ,  23 ,  24  or  25  of the-test pattern  21  on the test print, to determine the just focus position JPP based on the measured densities. In that case, the divisions of one row should have the same length in the lengthwise direction of the pattern  21 , and the length of the division is preferably a multiple of the line spacing or line width of that row, so that each division consists of the same number of parallel lines of that row. Thus, if the density of one row were uniform on the test print, the densities of the divisions of that row would be equal to each other. Since the density of the test print becomes the lowest around the just focus position JPP, as set forth above with reference to FIG. 6, it is possible to regard the division with the lowest density as the just focus position. The mean density of each division may be determined by calculation, or by means of a line sensor having a low pixel density, or an area sensor having a low pixel density. 
     Since each division consists of the same number of parallel lines within a row and the parallel lines are arranged in the same frequency within a row, it is preferable to measure a mean density of each division after shifting the position of each division by a half interval, and detect a difference between the mean density of the division before the shift and that after the shift. Because the difference in density is larger in the just focus position than in the blurred area, it is possible to determine the just focus position more precisely. Integrating the density values at half cycles and comparing the integration results will lead to the more accurate determination of the just focus position. 
     Although the focusing chart  11  has the test pattern  21  consisting of the four rows having different line frequencies from each other in the above embodiment, the test pattern may have a single row of parallel lines, or two, three or more than four rows of lines. The line spacing is equal to the line width within each row in the above embodiment, it is possible to differentiate the line spacing from the line width. The opaque lines may have another color than black. 
     The materials for manufacturing the focusing chart  11  is not limited to the above embodiment. For example, a photographic film having the above test pattern recorded thereon may be used as a focusing chart. 
     Although the focusing chart used in the above embodiment is of a type that is illuminated from the opposite side to the printing lens, and has transparent lines between the opaque parallel lines, the present invention is applicable to a case where a reflective type focusing chart is used, whose test pattern consists of alternating black and white parallel lines. In that case, the reflective type focusing chart is placed on a printing station for a reflective original. 
     The test pattern is not limited to the above embodiment. The parallel lines may be slanted from the rectangular direction to the inclined direction along which the parallel lines are arranged side by side. It is possible to constitute the test pattern of other elements than the parallel lines. For example, it is possible to arrange dot patterns or a group of circles along an inclined direction of the focusing chart. In that case, the just focus position may be determined with ease just by inspecting the sharpness of the patterns on the test print in the same way as described above. 
     It is also possible to make a test print while placing the focusing chart in the reference focal plane without inclining it. Then, the focal condition may be inspected with respect to the entire area of the printed picture frame, to check if the reference focal plane deviates from its normal position, or inclines to the perpendicular plane to the optical axis of the printing lens. 
     Although the present invention has been described with respect to the focus inspection device for the printing lens, the present invention is applicable to any kinds of image forming lenses that require focusing, like an image forming lens of a scanner. 
     Thus, the present invention is not limited to the above embodiments but, on the contrary, various modifications will be possible to those skilled in the art without departing from the scope of claims appended hereto.