When Photoshop writes an image to disk, it doesn't save the picture itself; rather, it just saves a bunch of zeros and ones. But the zeros and ones that one program writes to disk may not be readable by another program. The same data can be written to disk in a variety of ways, called file formats. Different file formats may be as different as two languages (like Spanish versus Chinese), or as similar as two dialects of the same language (like American versus British English).
The world would be a simpler place if everyone (and all software) spoke the same language, but that's not going to happen. Fortunately, programs such as Photoshop, QuarkXPress, and InDesign can read and sometimes even write in multiple file formats. The important thing, then, is not for us to understand exactly what makes one different from the others, but rather what each file format's strengths and weaknesses are, so that we can use them intelligently.
In the first part of this chapter, we're taking an in-depth look at each of the many file formats that Photoshop understands. (Note that we won't cover Camera Raw or DNG here, because Photoshop can't write these filesit can only read them; they're discussed in Chapter 11, Building a Digital Workflow.)
Tip: Hide Formats You Don't Use
Photoshop itself actually only knows how to read and write about half of the file formats we're discussing in this chapter. It can read and write the other types because of plug-ins that came with the program. For instance, when the CompuServe GIF and FilmStrip plug-ins are in Photoshop's Plug-ins folder, Photoshop can read and write in these "languages." But if you don't use these formats, you don't have to leave them cluttering up your Save As popup menu. Instead, move any formats that you don't want out of the File Formats folder (inside the Plug-ins folder in the Photoshop folder) into some other folder outside the Plug-ins folder. Don't just hide them inside another nested folder; Photoshop can still find them there.
However, before we get to our discussion of file formats and compression, we need to cover three sets of options that appear in the Save As dialog box: Save, Color, and Image Previews.