Understanding and increasing bit depth in photoshop to help you have a hight quality picture. You may have heard the terms 8-bit and 16-bit tossed around in graphics circles (and neither has anything to do with Photoshop being a 64-bit program). When people refer to bits, they’re talking about how many colors an image contains. Photoshop’s color modes determine whether a document is an 8- or a 16-bit image (other, less common options are 1-bit and 32-bit). Since you’ll run into these labels fairly often, it helps to understand more about what these numbers mean. A bit is the smallest unit of measurement that computers use to store information: either a 1 or a 0 (on or off, respectively). Each pixel in an image has a bit depth, which controls how much color information that pixel can hold. So an image’s bit depth determines how much color info the image contains. The higher the bit depth, the more colors the image can display. And the more colors in your image, the more info (details) you’ve got to play with in Photoshop. To understanding bit depth, you need to know a little about channels, where Photoshop stores your image’s color info on separate layers. For example, in an RGB image you have three channels: one each for red, green, and blue. If you combine the info contained in each channel, you can figure how many colors are in your image. With all that in mind, here’s a quick tour of your various bit choices in Photoshop:
- In Bitmap color mode, your pixels can be only black or white. Images in this mode are called 1-bit images because each pixel can be only one color—black or white (they’re also known simply as bitmap images).
- An 8-bit image can hold two values in each bit, which equals 256 possible color values. Why 256? Since each of the eight bits can hold two possible values, you get 256 combinations. (For math fans, it’s two to the eighth power, which equals 256). Images in Grayscale mode contain one channel, so that’s 8 bits per channel, equaling 256 colors. Since images in RGB mode contain three channels (one each for red, blue, and green), folks refer to them as 24- bit images (8 bits per channel x 3 = 24), but they’re still really just 8-bit images. With 256 combinations for each channel (that’s 28 x 28 x 28), you can have over 16 million colors in an RGB image. Since CMYK images have four channels, folks refer to them as 32-bit images (8 bits per channel x 4 = 32), but again, these are still 8-bit images. Over 200 combinations per channel and four channels add up to a massive number of possible color values, but since you’re dealing with printed ink, your color range in CMYK is dictated by what can actually be reproduced on paper, which reduces it to about 55,000 colors.
- 16-bit images contain 65,536 colors in a single channel and are produced by some high-end digital cameras (digital single-lens reflex, or DSLR, cameras) shooting in Raw format and by really good scanners. These files don’t look any different from other images on your screen, but they take up twice as much hard drive space. Photographers really like 16-bit images because the extra colors give them more flexibility when they’re making Curves and Levels adjustments (Chapter 9), even though the larger file sizes can really slow Photoshop down. Also, not all of Photoshop’s tools and filters work with 16-bit images.
- 32-bit images, referred to as high dynamic range (HDR), contain more colors than you can shake a stick at. For the most part, you’ll deal with 8-bit images, but if you’ve got a camera that shoots at higher bit depths, by all means, take a weekend and experiment to see if the difference in quality is worth the sacrifice of hard drive space (and editing speed). And if you’re restoring a really old photo, it may be helpful to scan it at a high bit depth so you have a wider range of colors to work with.