JPS6131866B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for obtaining a halftone positive or halftone dot converted into new halftone dots different from the original halftone positive or halftone dot.

Conventionally, if you want to publish different sizes using net positive or negative net, if you reduce the size as is, the tone will become hard on the positive, and the number of screen lines will become finer, which will make the tone even harder when printing. Had to have. Furthermore, if the image is simply enlarged, the number of screen lines will become coarser and the halftone dots will become more noticeable. Therefore, even if the screen was enlarged or reduced, it was required to have the same number of screen lines as the original screen.

Furthermore, there are cases in which it is necessary to make only the screen line number finer or coarser while the magnification is the same, or it is necessary to change the screen line number at a magnification different from the magnification. Furthermore, there have been cases where it has become necessary to convert the screen angle. In such cases, conventional methods have been to blur the image using a camera or scanner, or print it out using color film and reduce it to such an extent that the halftone dots are difficult to distinguish, and then print the color image as a manuscript using normal plate-making methods. Methods such as enlarging the image and reinstalling the screen have been used, but since both methods utilize blurring of the photo, it is difficult to quantify it, and there are many elements of trial and error. Not only does it require a lot of materials and labor, but its finished product also lacks sharpness, so it is currently not used except in special cases.

The present invention relates to a method for obtaining halftone positives or halftones that are easy to quantify and have excellent sharpness by using an ordinary plate-making camera and utilizing the resolving power of a lens.

In general, the resolving power of a lens is expressed by the number of black or white striations per mm that can be determined to be striations on the image plane, considering black and white striations of equal width, and is expressed by the number of black or white striations in 1 mm that can be determined as striations on the image plane. The resolving power R _L is given by R _L =1/1.22λF(1+m) (1). Here, λ is the wavelength of the photographing light, F is the F number of the lens, and m is the photographing magnification, and it can be seen that the larger the F number, the worse the resolving power R _L becomes. Moreover, the dimension l between the lines where the striations become indistinguishable can be expressed as l=1/2R _L (2). Using this formula, calculate the required dimension to make it indistinguishable from the number of screen lines of the original positive halftone or negative halftone screen that becomes the manuscript, find the resolving power R _L at that time, find the F value from equation (1), and When shooting using a fixed aperture, halftone dots will disappear, and conditions are sought that will cause the least reduction in resolution.

For example, if the original has halftone dots formed by 150/inch screen lines, when photographing at original size, approximately
If you do not resolve the 120μ line, the halftone dot will disappear, and in 2/3 reduction shooting, it will be about 80μ.
If you do not resolve the lines, the halftone dots will disappear. When calculating R _L from formula (2), it is approximately
4.2 lines/mm, approximately 6.3 lines/mm at 2/3. Substituting this value into equation (1) to find the F value gives 179 for the original size and 143 for 2/3, which means that you can shoot with the lens aperture set to this value.

However, the original net positive (or net negative) is closely inverted onto a squirrel-shaped film to create a net negative (or net positive), and the net negative (or net positive) obtained by inversion is used as the manuscript for the above-mentioned F. If you narrow down the aperture of the lens so that the value is the same, and take halftone shots through a contact screen to obtain a new halftone positive (or halftone negative) after screen conversion, the curve A in Figure 1 will be used. It turned out to be as shown.

In other words, what should ideally be a straight line as shown in B in Figure 1 ends up being a positive net with insufficient shadow and light areas, and if such a positive net is used, it is difficult to obtain good prints. It is not possible.

It is possible to perform auxiliary exposure such as shadow exposure or light exposure to compensate for the lack of halftone dots in the shadow and light areas, but determining the amount of exposure for auxiliary exposure is complicated and Even if the exposure amount deviates, the size of the halftone dots will deviate greatly, which tends to cause extremely large variations, making stable plate making impossible. Furthermore, by performing auxiliary exposure, not only the shadow and light areas but also the intermediate tone areas are affected, resulting in a reproduced halftone positive (or halftone negative) that is faithful to the original halftone positive (or halftone negative). ) cannot be obtained.

In order to solve this problem, the present inventors conducted various research and found that they first reversed the halftone dot original of lithographic film to a soft tone film for continuous tone such as gravure film, and determined the density range by adjusting the exposure amount and development time. Using the inverted halftone negative or halftone positive as a document, the aperture of the camera lens is narrowed down so that the halftone dots on the document disappear, a contact screen is applied, and a squirrel-type film is produced. The present invention was completed by discovering that it is possible to obtain halftone positive or halftone negative with completely new halftone dots by halftone photography.

To explain one embodiment of the present invention in more detail with reference to the drawings, first, as shown in FIG. Emits light from the direction. By developing this, an inverted mesh negative 2' as shown in FIG. 3 is obtained. Next, as shown in FIG. 4, this closely inverted mesh negative 2' is used as a manuscript,
A contact screen 4 is superimposed on a squirrel type film 5 and halftone photography is performed. In this case, the screen angle, number of lines, dot shape, etc. of the contact screen 4 can be appropriately selected and used as desired.

Furthermore, in this halftone shooting process, after keeping the lens 3 of the camera in focus, the aperture (not shown) is narrowed down to the point where the halftone dots on the document disappear, and exposure for halftone shooting is performed. . That is, by using equations (1) and (2) above, halftone photography is performed with an aperture of F number at which halftone dots disappear.

By developing this, a new halftone positive 5' is obtained as shown in FIG.

The new net positive obtained in this way is the 6th net positive.
As shown in the figure, the reproducibility is as shown in curve C, which is extremely similar to ideal reproducibility curve B.

In the present invention, when the original halftone film is closely inverted, a continuous tone film is used instead of a lithographic film. This is due to the following reasons.

First, the characteristic curve of a contact screen used for halftone photography generally has a curve as shown in FIG. Although each contact screen has different characteristic curves, they are not extremely different. In order to perform halftone photography using a contact screen having a characteristic curve as shown in FIG. 7, it is sufficient that the inverted halftone film corresponds to the ideal curve determined by back calculation from FIG. The ideal curve when halftone negative is used as the reversal halftone film is curve P in FIG. On the other hand, if you create a reversal net negative using a squirrel type film, the 80% halftone dot becomes 20% in the reversal net negative, so if you convert it into a density, it will be 0.10.
becomes. The curve obtained in this way becomes the curve Q shown in FIG. 8, which is far from the ideal curve P.

On the other hand, in the case of the inverted net negative obtained by closely inverting a soft tone continuous tone film as in the present invention, the curve R shown in FIG. 7 is obtained, which is extremely close to the ideal curve P. In this case, since the curve R changes depending on the exposure conditions and development conditions, it is preferable to set various conditions in advance so that it approaches the ideal curve P.

If the halftone dots are erased using the inverted negative having a gradation like the curve R obtained in this way and halftone shots are taken through a contact screen, a tone very similar to the original halftone positive can be obtained as described above. A new net positive is obtained.

A more detailed explanation will be given based on examples.

Example 1 A 150/inch offset net positive was placed in close contact with Sakura SG-100E film as a manuscript, exposed for 8 seconds at a voltage of 35 V with a printer, and developed at 24 inches per minute, resulting in a net negative. Using a normal contact screen of /inch at an aperture of 152, I took halftone shots at a magnification of 2/3, and the results were as follows.
At a screen rate of 150/inch, a positive screen with 2/3 sharpness and intact tone was obtained.

Example 2 A 175/inch offset net positive was placed in close contact with Sakura GUS-175 film as a manuscript, exposed at 45V for 12 seconds using a printer, developed at 30 inches per minute, and the obtained net negative was used. ,
A normal contact screen of 150/inch was applied with an aperture of 128, and the screen was reduced to 2/3 for halftone photography. As a result, the same results as in Example 1 were obtained.

Example 3 Using the net negative obtained in Example 2, aperture
As a result of applying a normal contact screen of 150/inch on 181 and enlarging it to 1.4 times, I took a halftone shot.
Results similar to those in Example 1 were obtained.

As can be seen from the above examples, this invention
This is a positive halftone screen that uses an ordinary plate-making camera without using any special new equipment or expensive equipment such as a scanner, and allows you to freely change the number of screen lines without sacrificing sharpness or tone using just the main exposure. is obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the net positive and the original net positive when the screen is converted after inversion to a squirrel type film, and Figures 2~
FIG. 5 is an explanatory diagram showing the flow of the method of the present invention, and FIG.
The figure is a graph showing the relationship between the positive screen and the original positive screen when the screen is converted according to the present invention, FIG. 7 is a graph showing an example of the characteristic curve of a contact screen, and FIG. It is a graph showing the relationship with positive. 1... Original halftone positive, 2... Continuous tone film, 2'... Inverted halftone negative, 3
...Lens, 4...Contact screen, 5...
...Squirrel type film, 5'...New net positive.

Claims (1)

[Scope of Claims] 1. A halftone negative or halftone positive is created by closely inverting a halftone positive or halftone negative on a soft tone continuous tone film, and the halftone negative or halftone positive obtained by the close inversion is used as a document to print the desired image. A contact screen having the angle, number of lines, and dot shape is applied, and the halftone dots of the document are erased and halftone photographed on a lithium-type film at a desired magnification to create a new halftone positive or halftone negative. A method for creating a net positive or negative net. 2. The positive halftone dot according to claim 1, wherein the halftone dots on the document are erased by focusing a camera lens and narrowing down the aperture of the lens until the halftone dots disappear. How to create a web negative.