|Some File Formats:
.BMP - A format native and almost exclusive to PC/Windows
.PICT - A format native and almost exclusive to Mac
Neither of the above are recommended for any serious image manipulation or file sharing.
PSD - The native file used in Photoshop which allows retention of all Photoshop manipulations
.TIF - A complete (raster) image file with all information retained (also called a "lossless" file, even when compressed with LZW. This is the "ideal" file format for retaining image quality. However, it is typically very large file size for a given image.
With the introduction of Photoshop 7, TIFFs can now be saved with layers (and can be compressed). These options has become the scourge of most every print house and image user as most programs can NOT recognize or use these types of files. Unless specifically asked, do NOT save TIFFs with layers or any compression (TIFFs with layers can also become enormous file sizes.)
.EPS - A vector based file using mathematical relationship to describe the image. Vector images can be varied in size, from a postage stamp to a billboard, without any loss of quality (i.e. jaggies). Most often used to design logos, illustrations and other graphics (with programs like Illustrator or Freehand). Some printing houses also prefer receiving final Photoshop files as eps, because once the raster image is converted to vector, it is much more difficult to induce changes and (unwanted) alterations.
.GIF - A format limited to between 2 and 256 colors and used primarily for logos and minimum color variant work for the web. Advantages are that it allows transparent backgrounds, animation and often smaller file size (though not always) than a comparable JPG. Generally not recommended for web photos (and definitely not for printed images).
.PDF - An Adobe Acrobat file. These files are primarily intended as a "universal" document format which can allow almost any type of computer system (with the associated software "reader") to view or read the document (whether it be text or images) and regardless of original application source. These files are not intended to allow manipulation by the recipient. As with .eps, some printing houses prefer receiving final Photoshop files as .pdf.
.JPG - A raster image which essentially represents a TIFF, with various amounts of information deleted from the image in order to reduce the file size. This is called a "lossy" format. A nominal compression or size reduction retains the highest image quality. The higher the compression (more file size reduction), the lower the image quality. Deleted information, resulting from a JPG format can NEVER be recovered! Lost information can be synthesized, but never reconstituted.
In an attempt to offer a very simplistic explanation: If you had a beautiful TIFF image with 10,000 variants of color, out of millions of possible colors, a TIFF save would not only save the 10,000 existing colors, it would also save the potential space for the rest of the colors as well. If you save that TIFF image as a maximum quality JPEG image, it would save the 10,000 existing colors but not the reserve space for the remaining potential values. Hence the file size would be reduced or "compressed".
On the extreme, if you saved the same image at the highest compression (lowest quality), the compression scheme would actually average the colors, and other image information down to the bare minimum representation. For example a hundred shades or variants of green might be averaged to just one green, - the same with all the other information, so the the final compressed image may only have a a few hundred colors, of what used to be thousands. (You can actually see this by enlarging a highly compressed JPEG and comparing it to the original). Again remember, deleted information can NEVER be recovered!
When you open a JPEG, you actually are expanding it back to its original TIFF file size. The extent of the JPEG compression will determine how much original information will still be retained in the display (and how much quality you will recover after opening).
Do NOT work in a JPEG format! Every time the same image is saved as a JPEG, the image quality progressively gets worse. If you have a JPEG, save it as a TIFF or PSD before you do any work or manipulation with it. Only save it again as a JPEG when all the work is completed and NO more manipulation will be done on the image - by anyone!
JPEGs are best for minimizing the storage and transfer file size (i.e. emails, web pages, etc.)
In association with a local printing house, we have done various test printing on a high end color press, using both TIFFs and high quality JPEGS. As long as no (or minimal) manipulation was done to the JPEGs, there was NO discernible difference between a press run four color 150 lpi TIFF and a JPEG, even under a loupe. However, the big difference was having a 2 MB JPEG file versus a 14 MB TIFF file. The former could be emailed while the later had to be shipped on a CD or hand delivered.
(There is now a JPEG 2000 being released. Best bet is to let it mature a bit.) (Back to Questions)