JPH043102A - Automatic focus control method for plate making camera - Google Patents

Abstract

PURPOSE:To improve the quality, size accuracy of images by calculating respective target positions based on respective focal lengths according to the respective magnification range segments of the plate making camera. CONSTITUTION:While the aberrations of a plate making lens 5 increase with a deviation of a photographing magnification from a magnification side to a reduction side, the practical focal length at which the finest image quality and image size are obtainable is previously calculated within the respective magnification range segments of the plate making camera divided to a plurality. Namely, the 1st practical focal length Fa and the 2nd practical focal length Fb are separately regulated for each of the respective magnification range segments. The target positions A, B of the respective moving frames, and as lens frame 4 and original frame 7, are calculated in accordance with the respective practical focal lengths according to the respective magnification range segments. The finest quality, size accuracy of the images are maintained over the entire magnification range of the plate making camera in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an automatic focus control method applied when focusing a plate-making camera using an automatic focus device.

(Background Art) A plate-making camera is required to have higher image quality and stricter image dimensional accuracy than a general copying machine or the like. Therefore, unlike general lenses, plate-making lenses are strictly manufactured to meet such requirements, and are usually set so that various aberrations are minimized at the original size (imaging magnification m=1).

On the other hand, the focal length of a lens is generally defined by the position at which light parallel to the tip axis of the lens enters the lens and the emitted light intersects the optical axis, and plate-making lenses are no exception.

However, even if the focal length thus defined (hereinafter referred to as the nominal focal length) is applied as is to the imaging formula, the high image quality and high image dimensional accuracy required of a prepress camera may not be obtained.

In other words, general prepress cameras are configured to be able to enlarge and reduce the magnification using the same lens, using the original size as a reference magnification.
As the image is enlarged or reduced from the original size, various aberrations of the plate-making lens also increase. For this reason, even if you shoot at the focus position of the desired magnification based on the nominal focal length, due to the effects of distortion, etc.
The required image dimensional accuracy cannot be obtained.

Furthermore, although the nominal focal length defined above can be strictly applied at the imaging magnification m = Ilo:l, some kind of correction may be necessary in the normal magnification range of a prepress camera. known from experience.

In other words, in order to obtain the best image quality and image dimensional accuracy with a prepress camera, it is necessary to determine in advance a practical focal length (hereinafter referred to as practical focal length) within the relevant magnification range (conventional technology). In consideration of the above points, the autofocus device of the plate making camera has conventionally been configured as follows, for example.

As shown in FIG. 6, these are lens rack positioning means 120 for positioning the lens rack 4, document rack positioning means 130 for positioning the document rack 7, and the positioning means 12.
Drive control means 140 for controlling the drive of 0 and 130, respectively.
and a drive circuit 145 that drives each positioning means 120 and 130 using an output signal from the drive control means 140.

The drive control means 140 includes each positioning means 120 and 1.
Based on the position pulses Pb-Pa from the pulse generators 124 and 134 of 30, the lens rack 4 and document rack 7
A pulse counter 141 reads the current position B3, A of
, the arbitrary magnification m when focusing in the manual operation mode for basic data setting, and the lens rack position B8, B and document rack position A1, A at that magnification. Origin position B. and the origin position A of the manuscript shelf 7. and the practical focal length F of the plate-making lens 5, and the basic data A thereof. , B
a storage unit 143 that stores o and F; the desired imaging magnification m set by the input operation unit 111; and the basic data A. ,Bo,
The drive amount calculation unit 144 calculates the lens rack target position B and the document rack target position A in the automatic focus mode based on It is configured to perform positioning control to target positions B and A.

Then, the driving amount calculation unit 144 calculates the document side distance a between the document surface 7a in the document shelf 7 and the plate-making lens 5, and
The image side distance between the image forming surface 2a and the plate-making lens 5 is defined as b, and based on the image forming formula % formula (1), the following formula A=a+b-Ao, B=b-B.

The document rack target position A and the lens rack target position B are calculated by the following.

In other words, the practical focal length etc. near the actual size are determined in advance in manual operation mode, and the results are used for automatic focusing.

Note that reference numerals 123 and 133 in FIG. 6 indicate lens rack positioning means 120 and document rack positioning means 130, respectively.
The drive motors 122 and 132 constituting the lens frame are respectively located at the origin position B of the lens frame. and original position A of the original shelf. This is the position sensor that defines the

(Problems to be Solved by the Invention) According to the above-mentioned conventional technology, the image quality and image size near the original size are completely comparable, but when the imaging magnification range is relatively wide, Since adjustments must be made with emphasis on image quality (resolution),
The required image dimensional accuracy cannot be obtained.

On the other hand, if the apparatus is adjusted with emphasis on image dimensional accuracy over the entire magnification range, the desired image quality cannot be obtained.

The present invention has been made in consideration of these circumstances, and its technical object is to further improve image quality and image dimensional accuracy over the entire imaging magnification range of a plate-making camera.

(Means for Solving the Problems) The present invention is configured as follows to solve the above problems.

That is, the document side distance between the document surface and the plate-making lens is defined as 3, the image-side distance between the image forming surface and the plate-making lens is defined as b, the practical focal length of the plate-making lens is defined as F, and the imaging magnification is defined as m. , image formation formula % formula %) Next, in the automatic focus control method for a prepress camera in which each target position ASB of the first moving beam and the second moving beam of the prepress camera is calculated, the practical focal length F of the prepress lens is calculated. , a first practical focal length F6 for calculating the target position A of the first moving beam, and a second practical focal length Fb for calculating the target position B of the second moving beam.
At the same time, the imaging magnification range of the prepress camera is divided into a plurality of magnification range divisions, and the first practical focal length F6 and the second practical focal length Fb are separately defined for each magnification range division. , This method is characterized in that the target positions A and B are calculated based on each of the above focal lengths according to each magnification range division of the prepress camera.

(Function) As the imaging magnification shifts from the original size to the enlargement side or the reduction side, the aberration of the plate-making lens increases, but in the present invention, the best image quality and image size are obtained within each magnification range division of the plate-making camera divided into a plurality of parts. Calculate in advance the practical focal length that will give you .

That is, a first practical focal length F and a second practical focal length F5 are separately defined for each magnification range division.

Then, according to each magnification range classification, the target position A of each movable beam such as the lens rack and document rack is determined based on the above-mentioned practical focal lengths.
, B are calculated. Therefore, the best image quality and image dimensional accuracy will be maintained over the entire magnification range of the prepress camera.

(Example) FIG. 1 is a schematic diagram of a plate-making camera and its autofocus device to which the present invention is applied.

This plate making camera has a photographing rack 2 at one end of a base frame 1.
is fixed, and the manuscript shelf (first movable beam) 7 is mounted on the base frame 1.
and a lens rack (second moving beam) 4 are provided so as to be movable for variable magnification, the photographing rack 2 and the lens rack 4 are connected via a bellows 3, and focusing can be performed by an automatic focusing device 10. There is.

The automatic focus device 10 includes an input operation section 11, a lens rack positioning means 20 for positioning the lens rack 4, a document rack positioning means 30 for positioning the document rack 7, and a drive for controlling the positioning means 20 and 30, respectively. Control means 4
0, and a drive circuit 45 that drives the drive motors 23 and 33 of each positioning means 20 and 30 in response to an output signal from the drive control means 40.

The lens rack positioning means 20 includes a drive screw 21 that moves the mount 6 of the lens rack 4 forward and backward, and an origin position B of the lens rack 4. It consists of an origin position sensor 22 that detects the rotational position of the drive screw 21, a drive motor 23 that rotates the drive screw 2I, and a pulse generator 24 that outputs a rotational position pulse P5 of the drive screw 21.

Similarly, the document rack positioning means 30 includes a drive screw 31, an origin position sensor 32, a drive motor 33, and a pulse generator 3.
It consists of 4.

The drive control means 40 includes a pulse counter 41, a basic data calculation section 42, a basic data storage section 43, and a drive amount calculation section 44.

The pulse counter 41 is connected to the origin position sensor 22.
32 detects the origin position, and counts the position pulses Pl, .P from the pulse generators 24 and 34, and reads the current positions B and A of the lens rack 4 and document rack 7, respectively. It is.

The basic data calculation unit 42 is used to calculate necessary basic data in advance using the basic data setting operation mode, and divides the imaging magnification range of the prepress camera into a plurality of magnification range divisions, and The following basic data is calculated to obtain the best image quality and image dimensional accuracy.

That is, the necessary basic data is, for example, assuming that the photographing magnification range of the plate-making camera is M=0.6 to 2.0, and that this is M,=o,6.
Divided into magnification range classifications of ~1.0 and M2-1.0 ~ 2.0, basic data A03, BO corresponding to the first division M.
, , Fo, F bl and the basic data A corresponding to the second classification M. 2.13ot, FatSF le2.

Then, within the first magnification range division M, the target positions A-B of the document shelf 7 and the lens shelf 4 are calculated by the following equations.

A ” F a + (2+m ” 1 / ”)
A o + %B=Fb+ (1+m) BOI
-■ The same applies to the second magnification range division M.

A=Fat (2+m+ 1./m)-Aat, B=F
bt (1+m) Bat - ■In other words, the required image quality and image dimensional accuracy cannot be obtained with just a single practical focal length F, so as a supplementary measure, the practical focal length MF of the plate-making lens is changed to the manuscript rack ( A first practical focal length F8 for positioning the lens rack (second moving rack) and a second practical focal length F8 for positioning the lens rack (second moving rack) are separately defined, and these practical focal lengths are set for each magnification. Range classification M
3, M, every F lll5 F at and Fゎ1, Fb
t, and on the other hand, the origin positions A0 and Bo of the document rack 7 and the lens rack 4 are also defined as the respective magnification range divisions M3 and M.
, every A. 3, Aot, and B ol and B at are defined.

These basic data are calculated, for example, according to the procedure shown in FIG.

In FIG. 2, in step S, the lens rack 4 and document rack 7 are manually moved forward and backward in the vicinity of one end magnification m = 0.6 of the magnification range division M to obtain the best image quality (resolution). Position it so that it can be obtained.

In step S, the test chart original is photographed on a photosensitive material to confirm the focus state and accurate magnification.

In this state, the following equation holds true from equation 0.

Az=FaI (2+m, + 1/mr) −Ao+ −
■Bz=Fb+ (] +m,) 130+
+・■Here, At5 and Bit are respectively manuscript rack 7
and the current position of the lens rack 4, m, is the exact magnification at that position.

In step S3, the current positions ff1ljA, .

In step S4, the other end magnification m of the magnification range classification M1 is
The above steps 81 to S are executed in the vicinity of t=1.0.

In this state, the following equation also holds true.

A+y=F-+ (2+rn,+I/my) x
0+...■B+y”Fb+ (1+mt) [
301.-■ Here, At2 and Bit are the current positions of the document rack 7 and lens rack 4, respectively, and m is the accurate magnification at that position.

In step S, the above equations are made to be simultaneous, and F,1,
Ao is calculated, and F1.13o+ is calculated by simultaneously using the 00 formula.

In step S6, the second magnification range classification M.

The above step S in the vicinity of one end magnification m, = 1.0
, ~S. Note that it is preferable to use the result of step S4 as is.

As a result of step S6, the following equation holds true.

At5-Fat (2+ms+1/ms) 77°! ,
,,■Bzs=Fbt (1+ms) Boy
−■ However, A vs= A +tr B t3
= B, z.

In step S7, the second magnification range division M, the other end magnification m
The steps SI to S3 are executed for the vicinity of 4.2.0.

In this state, the following equation also holds true.

A*t=Fat(2+m4+1/ms) Aot"'
■Bt*=Fbt (I0m4)-Bat-
1ti) Here, A , , B , are the current positions of the document rack 7 and the lens rack 4, respectively, and m4 is the exact magnification at that position.

In step S, the above ■ and 0 equations are combined to yield F at,
Calculate Aot, and calculate F,,2,Bot by combining the [stock] formula.

With the above, calculation of basic data is completed, and these basic data are calculated for each magnification range classification M, in step S8.

M, is stored in the basic data storage section 43.

FIG. 3 is a flowchart showing the operating procedure when the autofocus device 10 is operated in autofocus mode.

In FIG. 3, in step S I+, input operation section 1]
Set and input the photographing magnification m.

In step S It, it is determined to which magnification range division the photographing magnification m belongs and whether or not m<l.
], the process proceeds to step SI3 as corresponding to the first magnification range classification M, and if m≧1, the process proceeds to step S14 as corresponding to the second magnification range classification M.

In step S13, the target positions A and B of the document rack 7 and the lens rack 4 are respectively calculated using the above equation 0, and in step S+4, the target positions A and B are similarly calculated using the above equation 0.

In step S4, the lens rack positioning means 20,
and each drive motor 23, 33 of the document rack positioning means 30.
starts, and in step S+@, the lens rack 4 and document rack 7 stop at target positions A and B, respectively. Autofocus is now complete.

In the above embodiment, the magnification range of the prepress camera is divided into two divisions M8 and M, but the magnification range may be divided into three as appropriate depending on the required magnification range, image quality, and image size accuracy.
It goes without saying that it is also possible to classify more than just division. Further, regarding the classification of the magnification range, it is also possible not to provide a boundary near the original size.

FIG. 4 is a schematic diagram of a slit scanning exposure type plate making camera to which the present invention can be applied.

This slit scanning exposure type plate-making camera includes a document holder 52 that is provided so as to be horizontally movable along a travel path 51, a cylindrical drum 56 for holding a sensitive material that is provided to be rotatable in synchronization with the travel of the document holder 52, It comprises a variable magnification projection optical system 53 disposed in the outer peripheral space of the cylindrical drum 56, and photosensitive material conveying means 58 and 59 provided to be able to supply and remove a photosensitive material 57 to and from the cylindrical drum 56, By positioning the variable magnification projection optical system 53 via an automatic focusing device (not shown), a slit-shaped image is formed on the sensitive material held on the cylindrical drum 56.

The variable magnification projection optical system 53 includes a plurality of mirrors 54a to 54d for reversing the optical path at right angles, and a plate-making lens 55, and a set of mirrors 54b and 54 behind the lens depending on the imaging magnification.
By independently positioning C (first moving beam) and plate making lens 5 (second moving beam) using separate driving poles and screws 61 and 62, automatic focusing is possible. Note that the reference numerals 22 and 32 in the figure are origin position sensors.

In this embodiment, the distance C between the document surface and the image forming surface is determined by positioning the pair of reversing mirrors 54b and 54C, which are the first moving beams, at the target position A. The distance Ha between them is set by positioning the lens 55, which is the second movable beam, at the target position B, so as to obtain the required magnification m. In this embodiment, as in the case described above, two magnification range divisions M,=
It is divided into 0.6 to 1.0, M, = 1.0 to 2.0, and basic data is set in advance for each division. Therefore, for the first division, the following 0 formula is used. , and for the second section, the reversing mirror 5 is determined by equation 0.
Target position A of 4b and 54c and target position 11 of lens 55
B is calculated.

A=Fal (2+m+1./rr+) -A,,,B
=Fb+N+i/m) BOI-■A=F
, t(2+m+l/m) Act, B=pbt(1+
1/m)-Bat-=6 FIG. 5 is a schematic diagram of yet another plate-making camera to which the present invention can be applied. This prepress camera has a document rack 7 fixed to one end of the base frame 1,
Photographing rack 2 (first moving beam) and lens rack 4 (second moving beam)
It differs from the one in FIG. 1 in that it is configured to be movable, but in other respects it is constructed similarly to the one in FIG. 1.

Note that the format of the plate-making camera is not limited to the above-mentioned type, and the present invention can be similarly applied to a type of plate-making camera with a fixed lens mount.

Further, the arrangement locations of the origin position sensors 22 and 23 can be changed as appropriate.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the document side practical focal length F and the image side practical focal length F are separately defined for each magnification range division, and each magnification Depending on the range classification,
Target position A with respect to a movable beam such as a lens rack or document rack based on each of the above-mentioned practical focal lengths.

Since B is calculated, the best image quality and image dimensional accuracy can be maintained over the entire magnification range of the prepress camera.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a prepress camera and its autofocus device to which the present invention is applied, Fig. 2 is a flowchart showing the basic data calculation procedure, Fig. 3 is a flowchart showing the operation procedure in autofocus mode, and Fig. 4 5 and 5 are schematic diagrams of another type of prepress camera to which the present invention can be applied, and FIG. 6 is a diagram corresponding to FIG. 1 showing a conventional example. 2a... Image forming surface, 4... Lens rack (second moving beam)
, 5... plate-making lens, 7... original shelf (first moving beam), 7a... original surface, A... target position of second moving beam, B... target of second moving beam. Position, Fl・Fat・
・・First practical focal length, F,, 1・F□・・Second practical focal length, ・M! ...Magnification range classification.

Claims (1)

[Claims] 1. The document side distance between the document surface and the plate-making lens is a, the image-side distance between the imaging surface and the plate-making lens is b, the practical focal length of the plate-making lens is F, and the photographing magnification are respectively defined as m, and the target positions A and B of the first and second moving racks of the prepress camera are calculated based on the imaging formulas a=F(1+1/m), b=F(1+m). In the automatic focus control method for a prepress camera, the practical focal length F of the prepress lens is set to the first practical focal length F_ for calculating the target position A of the first moving rack.
a and a second practical focal length F_b for calculating the target position B of the second movable rack, and also divides the imaging magnification range of the prepress camera into a plurality of magnification range categories, and divides each magnification range. A first practical focal length F_a and a second practical focal length F_b are defined separately for each classification, and target positions A and B are calculated based on each of the above focal lengths according to each magnification range classification of the prepress camera. An automatic focus control method for a prepress camera characterized by