Understanding Photoshop is a biweekly column that takes an in-depth look at how digital photographs are built and manipulated. It is a college-level course in plain english for free at Photofocus. To learn more see this article.
Choosing an Image mode
Within Photoshop, you need to choose from one of eight image modes when working with a document. The mode you pick will depend on what you need to do with the image and how you intend to output it. For example the mode used for web graphics will differ from ones used for professional printing. The three most common modes used are RGB, Grayscale, and CMYK, but its worth taking a quick look at all eight.
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The most common mode for graphics in Photoshop is RGB mode. The RGB Color mode uses additive color theory to represent color (a 100% value of red, green, and blue light creates white light). Different intensity values of red (R), green (G), and blue (B) combine to form accurate colors. By mixing intensity values, virtually ever color can be accurately represented.
When working in Photoshop, most designers choose RGB Color mode for its wider range of available color (also known as gamut) and extensive support for filters and adjustments. Additionally, computer monitors use RGB mode to display color, and this is the native color space for onscreen display. Because you’ll most often be processing images on a computer, it is easiest to work in the same color space as your monitor.
Professional printing uses a four-color process to simulate color. The four inks are cyan (C), magenta (M), yellow (Y), and black (K, for key). The CMYK Color mode uses the subtractive color model to re-create color. Subtractive color explains the theory of how ink or dye absorbs specific wavelengths of light and reflects others. The objects color is based on which part of the light spectrum is not absorbed. Although print designers use CMYK Color mode for professional printing, they will work in RGB mode throughout the design stage. CMYK Color mode has a smaller color gamut, so CMYK conversion is saved until the last stage of image preparation.
A grayscale image uses different shades of gray to represent image details. For example, an 8-bit image is represented by 256 levels of gray (see Bit Depth later in this column).
Likewise, a 16-bit image would show 65,536 levels of gray (a substantial improvement, but it requires an output device that can utilize the data). Many printers are starting to support these higher bit depths and using them adds a significant improvement in quality.
When creating grayscale images, it is important to perform test prints with the output device and paper to see how contrast is maintained.
A duotone image can actually be monotone, duotone, tritone, or quadtone. Grayscale images that use a single-colored ink are called monotones. Duotones, tritones, or quadtones are grayscale images printed with two, three, or four inks, respectively. Using both black and gray ink to represent the tonal values, duotones create better-quality printed grayscales.
The most popular form of duotone is a sepia-tone image (often seen in historical prints). In modern times, a designer may use a duotone for style purposes or to save money by using fewer inks.
To create a Duotone in Photoshop:
- Open up a color or grayscale image.
- Choose Image > Mode > Grayscale to convert to a single tone image.
- Choose Image > Mode > Duotone.
- Choose a preset from the Preset Menu for a popular look or change the type. You can use Monotone, Duotone, Tritone, or Quadtone.
- Click on the swatch for each ink to set the color.
- If desired, click the Curves Icon for each ink to adjust the range for the Duotone Curve.
- Make sure the preview box is checked to judge your work.
- When satisfied, click OK to store your conversion.
A bitmap image uses only one of two color values — black or white (no gray) — to represent the pixel data. These 1-bit images have a very small file size. To create a bitmap, you first must convert the image to an 8-bit grayscale formula, and then convert to the Bitmap image mode. Do not confuse Bitmap mode with a bitmap image, which is another name for raster (or pixel-based) images.
Additionally, avoid confusion with the BMP file format, which is a standard Windows file format that dates back to the earliest version of Windows. An image in the Bitmap mode simply uses only black and white to represent image data.
Indexed Color mode severely limits the number of colors used to represent the image. In Indexed Color mode, up to 256 colors are available. To reduce file sizes (and download times), some Web designers use fewer colors in their graphics.
They will turn to specialized formats like GIF and PNG-8. Although this mode reduces file size, it also visibly lowers the quality of the image. Indexed Color mode works well for illustrations or logos but not so well for photos on the Internet.
Instead of converting your original image to Indexed Color mode via the Image menu, use the Export commands. You can either choose File > Export > Export As or File > Export > Save for Web (Legacy). This will convert the file to a GIF or PNG-8 (which use the Indexed Color mode), but leave the original image in its original mode.
L*a*b* Color is the most complete color mode used to describe the colors visible to the human eye. The three parameters of color are L for luminance of the color, a represents the colors position between red and green, and b represents its position between yellow and blue.
The Lab Color mode was created to serve as a device-independent, absolute model to be used for a reference. Lab attempts to simulate the full gamut of color; however, it is a three-dimensional model and can’t be represented properly within Photoshop. Therefore, the * after the L, a, and b is used to signify that it is a derivative model. Lab images can only be printed on PostScript Level 2 and Level 3 printers; for all other professional printers, Lab images must first be converted to CMYK mode. The Lab Color mode is generally only used by imaging professionals seeking the truest color fidelity because it supports all the colors in both the RGB and CMYK Color modes.
Multichannel mode is a highly specialized mode used for complex separations for professional printing. You may never need to use it. Photoshop automatically converts to Multichannel mode when you delete a channel from an RGB or CMYK image.
The color onscreen is no longer accurate because Photoshop cannot describe it. This is sometimes done for an effect or as part of the image repair process if one channel did not capture properly (such as from a malfunctioning digital camera). Most likely, you’ll never want to work in Multichannel mode.
Choosing a Bit Depth
Besides resolution (the number of pixels) and color mode (the way colors are processed) there is one other variable that affects image quality. Bit depth measures how much color is available for display or printing of each pixel. A greater bit depth means each pixel contains more information for describing the color.
A pixel with a bit depth of 1 can display the pixel as either black or white. The most common bit depth is 8-bit mode, which has a possible value of 256 intensity levels per color channel. However, depending on the version of Photoshop you are working with as well as the file type and image mode, you can access 8, 16, or 32 bits per channel. Its important to note that larger bit depth can limit image adjustment commands.
You won’t encounter 32 bits per channel images very often. They come into play when working with generated imagery (such as those from 3D modeling applications). They can also be created by merging multiple photos together into a high dynamic range (HDR) image. You’ll learn this process later in this course.
TIP: Shooting Raw
One of the major benefits of shooting images in a camera raw format is that you can often choose to work in 16 bits per channel in Adobe Photoshop. This offers superior options for manipulating color and exposure.
Theres a lot more ground to cover, but you’ll explore the topics discussed here and others in greater depth in future lessons. You’ll feel a bit more comfortable with the language used to describe images and color as you read on. With the knowledge you’ve gained so far, you can jump into using Photoshop and start to navigate its interface. We’ll cover how to navigate Photoshop in the coming weeks.