How to convert a color image to monochrome

Since the appearance of color film, photographers have started to obtain monochrome photos from color negatives or color transparency films. Although their first choice is always a monochrome negative film, it is indeed possible to obtain color without the use of monochrome film.

The process of making monochrome printed photos with color negatives is very easy. Images can be simply imaged on a special darkroom paper or darkroom multi-purpose universal paper. Contrast adjustment of multi-level universal paper produces a good dark tone range on monochrome photographs, sometimes no different from photographs obtained from black and white negatives. If it is a color transparency, this situation will be more complicated. A transparent film was re-photographed on a high-quality film with flash removed to obtain a monochrome intermediate film, in two steps: overexposure and under-washing were the key. Then print the film on the regular monochrome paper in the dark room. If these steps are done correctly, you will get very good results.

With the continuous development of digital imaging technology, there is now a better way to convert color images to monochrome. First of all, no matter how this image is obtained, ordinary film or digital cameras need to be input into a computer and saved as digital images in RGB format. Use Adobe Photoshop or other software with digital image processing capabilities, all with similar conversion tools for converting color images to grayscale images. As shown in FIG. 1, the color image is initially scanned into Photoshop using a film scanner and saved in RGB color mode. We can use the following series of methods to convert a color digital file into a grayscale image (as shown in Figure 2).

One or four different conversion methods

1. The quickest way to turn a color image into a monochrome image is to use the grayscale mode option, which is to select "image/mode/grayscale", but the biggest drawback of this operation is that it puts all color information in the conversion process. All have been removed, and the results of the conversion will vary from image to image, which means that there is no guarantee of equal conversion and conditional consistency in the color conversion process. Therefore, if you want a good color conversion, try not to adopt this method.

2. Saturation method, or rather dilute the color, ie select "Image, Adjustment, Hue/Saturation" as shown in Figure 2. Move the Saturation slider from 0 to -100. Then click "OK". The resulting image is very similar to an image obtained using a monochrome negative film without using a color filter. The image at this time tends to be even smoother, which is equivalent to obtaining a completely different original color image with the same grayscale dark light (as shown in FIG. 3). However, this method is not the best, so it is not recommended.

3. This method is considered to be the most perfect so far, that is, use Lab color to transform. First select "Image/Mode/Lab Color"; then select "Mode/Grayscale" so that the color image will become a monochrome image.

In the channel palette, the color information is contained in the a channel and the b channel. The brightness value is in the brightness channel (as shown in Figure 4). The highlights and images of the luminance channel appear in monochrome. This one step is the biggest improvement of the above two steps is that it uses the Lab color space, which has the largest color gamut, uses a large color space to match a small color gamut, and Lab color space is a device-independent color expression , so do not need to consider the changes brought about by the display conditions. However, although this method is popular with most photographers, but personally think this color conversion is not the most appropriate way, the following describes a new method to convert color images into monochrome images.

4. Mixed channel conversion method. Select “Mode/Adjustment/Mix Channel” as shown in Figure 5 and check Monochrome in the dialog box. The default value for this channel is +100% Red, 0% Green, and 0% Blue. Adjust 3 sliders to change the monochrome value of the image. In the mediation process, sometimes 100% Red is the best choice. However, as a common basic starting point setting, the channel is set to: 60% Red, 40% Green, 0% Blue. Adjust the 3 channels until the color of the image is satisfied with you, then click "OK". Try to balance all the balance to 100%, but this is not an absolute point. In my opinion, this method is the best way to convert color images to monochrome.

Moreover, the biggest advantage of this method is that you can select different parts of the image and use mixed channels selectively. Take the above image as an example, it can be divided into three parts to adjust: the first step is for the hut, the second step is the sky, and finally the beach. Separately use the mixing channel to convert each part to monochrome, and the adjustment values ​​of these three parts are as follows:
Boathouse - Red: +116; Green: +2; Blue: 0;
Sky - Red: 0; Green: +54; Blue: +50;
Beach - Red: +68; Green: +52; Blue: 0.

This method allows you to control the final transition, similar to using different color filters to select different parts of the image. You can even control the density and tone values ​​of the smallest element of an image, such as a pebble on the beach. Therefore, this corresponds to selective control, which was previously only available in dark rooms.

Second, the matters needing attention

When we perform pre-press image processing, we hope to get the best results, but if we want to get the final high-quality print, we must consider the constraints of other factors, and it is not enough only for image processing. For example, when printing monochrome documents, experience tells us that a particularly heavy matte paper will produce the best results; cancel any high-speed printing tool in the printer operating software, use only black ink, set the highest resolution, paper It is also allowed. The resolution usually used when using this kind of paper is 1440dpi. Although many ink-jet printing papers are still capable of displaying green color shift, it is not yet mature to use black ink to remove undesired color shift. This aspect needs further study. Therefore, to obtain a high-quality monochrome photograph that can be obtained under a real darkroom, it is very important to select the proper paper.

Source: Ke Yin Network