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Why is the four-color black not the K100?
A: In Photoshop, the setting of four-color black is very special. As far as Photoshop 7.0 is concerned, the default black mark of the software is always: C75 M68 Y67 K90. This is a really unreadable figure-- Recently, some people have asked this question. Because of this, we welcome this discussion.
In fact, the default black in the software, Photoshop is the initial setting for the LAB mode, the value is L0 a0 b0, which is the calculated value in the software system, and the color of R0 G0 B0 of RGB color is exactly the same as L0 a0 b0. , so this color is also accurate "black" color.
However, interesting things happen in CMYK colors: in CMYK colors, K100, which is usually called single black, cannot accurately express this "system black", and has a larger RGB (Lab) color than CMYK color systems. In the department, the K100 is just incomplete dark gray, and this dark gray is reddish tone! - It's no surprise that this is true in the printing factory. The printing masters use black expensive ink, and the effect printed is far less than the black in the RGB color. Still reddish tone. With regard to reddishness, we can look at the "grey" parts of a large number of prints - careful, long observations will reveal that they are all impure gray red.
Now we can easily understand why there is this puzzling figure in Photoshop: C75 M68 Y67 K90. Now that the single black is not black enough, it will add more pigment. It turns out that when CMY and K share, it can be more Full black effect, but also to correct that unpleasant "reddish tone", CMYK color plus about 8 percentage points of the blue (C) ink balance, which is a simple and effective method.
If you are interested, we can also discuss in more depth the black (K) ink color cast:
When the black ink (with a white printing ink) is washed very lightly (note that it is not minus the net), the single black (K) ink is a bluish tone, but as the dilute is weakened, the color cast will slowly bias toward the blue-violet Blue-violet-purple-red.
Why can't we see blue in print? We can use a magnifying glass to look at the grayscale in prints. In fact, they are not produced with white printing inks. Instead, they are printed with 100% black ink through net reduction, so they all show the deepest The color cast: red tone.
The last two points to add are:
Because all single black gray colors are strictly reddish, so in Photoshop, you want to get the ideal gray color, it is best to add a certain proportion of blue (C) color;
For text, we all know that multi-channel black is easy to print and paste, so when designing (printed) text in Photoshop (especially the small print), we must use 100% single black.
If there is a gray block, it will be sent to the film company and there will be dots in the gray strip. The file is K20, and the dots on the display cannot be seen. The depth of the color is probably controlled by the density of the dots. The screen shows a color block, but the printing is based on dot. If you look at it with a magnifying glass, the color of each dot is actually a solid color, but their distribution is dense. How can we make ugly gray, it is recommended to use non-gradient spot colors.