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Decrypting Photoshop image modes – Photofocus


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 those used for professional printing. The three most common modes used are RGB, Grayscale and CMYK, but it’s worth taking a quick look at all eight.

RGB color

The most common mode for graphics in Photoshop is RGB. 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 every 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 on-screen 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.

CMYK color

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 object’s 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.

Grayscale

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 . 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).

When creating grayscale images, it is important to perform test prints with the output device and paper to see how contrast is maintained.

Duotone

A duotone image can actually be monotone, duotone, tritone, or quadtone. Grayscale images that use a single-colored ink are called monotones. Duotonestritones 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.

This mode should be used when you know the printer is set up specifically to handle the job. If you just want the look of a duotone, you can create that look by working in RGB mode and using the Black and White adjustment layer.

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.

Bitmap

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

Indexed Color mode severely limits the number of colors used to represent an 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 Save For Web command (File > Export > Save For Web). This will convert the file to a GIF or PNG-8 (both use the Indexed Color mode), but leave the original image in its original mode.

Lab Color

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 for the color’s position between red and green, and b for 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 La, and 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

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 on-screen 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.



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Rafael Jones

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