Adobe Photoshop CS5 for Photographers - Martin Evening [306]
It is true that if a TIFF file is saved using JPEG file compression there are some instances where this can cause problems when sending the file to some older PostScript devices. Otherwise, the image degradation is barely noticeable at the higher quality compression settings, even when the image is viewed on the screen in close-up at a 2:1 view.
Figure 14.4.
Here are two JPEG images: both have the same pixel resolution and both have been saved using the same JPEG quality setting. Yet the Sahara desert image compresses to just 21 kilobytes, while the gas works picture is over three times bigger at 74 kilobytes. This is because it contains a lot of extra detail. The more contrasting sharp lines there are in an image, the larger the file size will be after compression.
Keeping files small
Only one thing matters when you publish images on the Web and that is to keep the total file size of your pages as small as possible. The JPEG format is the most effective way to achieve file compression for continuous tone photographic images, but graphics that contain fewer, distinct blocks of color should be saved using the GIF format. Some web servers are case sensitive and won't recognize capitalized file names. If this is likely to be the case, you can go to the Photoshop Preferences File Handling section and make sure the ‘Use Lower Case Extensions box’ is checked.
Lossless and lossy compression
The LZW and ZIP methods of compression are lossless. They can reduce the file size, but without degrading the image. The JPEG compression is lossy and you can only use it when saving TIFF images that have been converted to 8-bits per channel (although you can preserve the layers when saving a TIFF image using the JPEG compression method).
Choosing the right compression type
JPEG compression offers the most effective way to reduce file size, but this is achieved at the expense of throwing away some of the image data (as was demonstrated in Figure 14.2). JPEG is therefore known as a lossy format, so you need to be careful not to apply any more compression than is necessary. If you refer to Figure 14.5 below you can compare the different file sizes that were obtained when saving a 500 × 600 pixel image using different JPEG settings. As an uncompressed, 8-bit RGB TIFF, this file was 1.8 MB in size. When saved using the highest JPEG quality setting there was barely any degradation to the image, yet the JPEG file size was just 232 kilobytes, or 12% of its original file size, which is quite a saving. When a medium (8) JPEG quality setting was used the file size was reduced further to just 72 kilobytes. This is probably about the right amount of compression to use when preparing photographs to go on a website where you wish to strike the right balance between maintaining decent image quality, yet still keep the files compact in size. The lowest compression setting squeezed the image down to just 34 kilobytes, but at this level photographic images are likely to appear extremely ‘mushy’ and the lower quality settings are therefore best avoided.
Figure 14.5 Here we have one image saved six different ways, where each method produces a different file size. The opened image measured 500 × 600 pixels and the uncompressed TIFF file size was 1.8 MB. Below that are the JPEG versions which were saved using different quality settings. Lastly, a GIF version was saved, which as you can see does not offer the most efficient compression method and is unsuitable anyway for saving most kinds of photographic images.
TIFF compression for FTP transfer
I mention the TIFF format again here because it is the standard file format used for transferring files used for pre-press work. If you save a layered