Prepress Color Image Processing: Choosing the Right Mode
In the typesetting of newspapers or magazines, handling color images is often required. When opening a color image, it may be in RGB mode or CMYK mode. So, when using Photoshop, should one work in RGB mode or CMYK mode for color image processing? This article shares my thoughts on this issue.1. RGB and CMYK Modes Should Not Be Converted Frequently
When processing images in Photoshop, the first thing to keep in mind is that, regardless of whether an image is in CMYK or RGB mode, it should not be converted back and forth between the two. Each conversion between these color spaces results in the loss of some of the original image’s detail. If an image is switched repeatedly between RGB and CMYK, the loss of information will be substantial.It must be noted that, in the process of newspaper publishing, images used for plate-making must ultimately be in CMYK mode; otherwise, they cannot be printed. However, this does not mean that processing images in CMYK mode will necessarily yield the best printing results. In fact, working in RGB mode in Photoshop generally produces better results. As long as the image is processed properly in RGB mode, converting it into CMYK afterward will still allow for successful plate-making and printing.
2. To Ensure Print Quality, Images Should Be Processed in RGB Mode
If the source image is already in RGB mode, or if the scanner allows capturing images directly in RGB, this is ideal for color newspaper typesetting. When scanning with Photoshop, simply select the RGB option under Color Settings, and the scanned image will be in RGB mode. In short, whenever possible, images should be obtained and processed in RGB without unnecessary mode conversion. This is particularly true for images downloaded from the internet—processing them in RGB is essential to ensure good print results. The following points illustrate why:(1) RGB mode covers a wider color gamut than CMYK.
All optical devices, such as monitors, work in RGB. RGB can reproduce many colors, especially bright and vivid ones, that CMYK cannot. This is why images appear darker after conversion to CMYK. In Photoshop, one should edit images in RGB while using CMYK Preview (View > Proof Colors) to simulate how they will look in print. This method provides both editing flexibility and accurate color expectation. Working directly in CMYK is less efficient and can result in lower quality.(2) Some Photoshop filters do not support CMYK mode.
Image editing often involves detailed adjustments, compositing, and applying filters. Because RGB provides a much larger color space, editing in RGB allows for richer, more nuanced effects. Even though conversion to CMYK will inevitably cause some color loss, starting in RGB preserves as much detail as possible compared to working in CMYK from the outset.(3) Color separation parameters are crucial when converting to CMYK.
Photoshop allows control over separation settings during conversion, which can minimize color loss. Thus, it is best to process in RGB and fine-tune the conversion to CMYK later.(4) RGB is better for long-term image archiving.
Corrected and adjusted RGB images can serve as high-quality master files, reusable across different devices and workflows. Whether conversion is done by system-level color management or within Photoshop, RGB archives provide flexibility for future publishing needs.(5) Gray balance and device output consistency.
In printing, consistent reproduction depends on balancing cyan, magenta, and yellow to achieve neutral gray. If an image is stored only in CMYK, adjustments to black levels, tonal ranges, or saturation may be required for different presses or paper types, complicating the process. RGB, with its broader tonal range, ensures more consistent color reproduction across devices until the final separation into CMYK.From these points, it is clear that images should generally be processed in RGB in Photoshop. However, care must be taken to preview in CMYK and ensure accurate colors at the final output stage. Always keep a copy of the original RGB file before converting.
3. Photoshop’s Lab Color Mode Also Offers Advantages
In addition to RGB, Photoshop’s Lab mode has unique strengths. RGB is based on light, CMYK on pigment reflection, but Lab compensates for the weaknesses of both. For example, RGB has uneven transitions between certain colors, and CMYK loses even more detail during editing. Lab mode provides a more balanced structure, consisting of three channels:- L channel for lightness (controls brightness and contrast),
- a channel from green to magenta,
- b channel from blue to yellow.
Lab mode is device-independent, allowing editing of any image type quickly—faster than CMYK—and ensuring that colors within the CMYK range remain intact during conversion. A common workflow is to convert RGB to Lab before converting to CMYK, which helps retain more color fidelity.
In non-color publishing, Lab mode is often used to convert images to grayscale. For example, an RGB image downloaded from the internet should first be converted to Lab, then its L channel extracted for use as a grayscale image. Direct conversion to grayscale may cause problems in some publishing software.
Conclusion
In image editing, it is best to work in RGB or Lab mode, and only convert to CMYK when necessary. Once an image is converted to CMYK, avoid converting it back. If further editing is required, consider converting to Lab instead. For scanned color images intended for grayscale layouts, always scan in RGB, process in RGB, then convert via Lab to grayscale.Many operators take shortcuts by scanning directly into the needed mode, making minor adjustments, and sending images straight to print. While this may “work,” the results are often less than optimal. For truly high-quality printing, it pays to follow proper workflows and not shy away from the extra steps.
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