Adobe Photoshop CS5 for Photographers - Martin Evening [305]
Photoshop compresses images on a scale of 0–12, where a setting of 12 applies the least amount of compression and yields the highest image quality, while a setting of 0 applies the greatest amount of compression and is therefore the most lossy. When you choose to save as a JPEG and have the ‘Preview’ option checked in the JPEG Options dialog (Figure 14.3), you are able to preview the effects of the JPEG compression in the image document window as you adjust the Quality in the Image Options section. This shows how the image will look when it is reopened as a JPEG. The JPEG Options dialog box also indicates the compressed file size in kilobytes, although this is just an approximation.
Figure 14.3 This shows the JPEG Options dialog box. The ‘Baseline (‘Standard’)’ option is the most universally understood JPEG format option and one that most web browsers will be able to recognize. The ‘Baseline Optimized’ option will often yield a slightly more compressed-sized file than the standard JPEG format and most (but not all) web browsers are able to read this correctly. The ‘Progressive’ option creates a JPEG file that will download in an interlaced fashion (the same way you can encode a GIF file).
If you save a master file as a JPEG and then decide the file needs further compression, you can safely overwrite the last saved JPEG using a lower JPEG setting. For as long as the image is open in Photoshop, all data is held in Photoshop's memory and only the version saved to the disk is degraded. It is therefore possible to repeat saving an image using the JPEG format. However, once an image has been compressed using the JPEG format and is reopened, it is not a good idea to repeatedly resave it as a JPEG again, since this will only compound the compression that's already been applied to the structure of the image. Having said that, unlike other programs, the JPEG compressor used in Photoshop converges, so that after repeated opening and saving using the same JPEG settings (and without modifying the pixels), the data loss diminishes with every save, to the point where there is little or no further loss.
Orphan works
With any photos that you upload to the Web, it is important to take precautions against future government legislation that may permit others to use photographs that are deeemed to be ‘orphan works’, because the orginal copyright owner cannot be found. It is possible to upload photos to sites such as Flickr and retain all the copyright metadata, while other upload processes may delete this data. One way to be absolutely sure is to incorporate a visible watermark in everything you publish to the Web.
Saving 16-bit files as JPEGs
For those who prefer to edit their images in 16-bit, it was always frustrating when you would go to save an image as a JPEG copy, only to find that the JPEG option wasn't available in the Save dialog File Format menu. The reason for this was because 16-bit isn't supported by the JPEG format. With Photoshop CS5, when you choose ‘Save As…’ for a 16-bit image, the JPEG file format is now available as a save option, where the file is converted on-the-fly to an 8-bit JPEG, without further intervention. This allows you to quickly create JPEG copies without having to temporarily convert the image to 8-bit mode. Note that only the JPEG file format is supported in this way.
The JPEG format can primarily be used to send smaller-sized email attachments and ensure that visitors to your website don't have to hang around while the images download. You can also use the JPEG format to archive images for faster electronic distribution or when you are forced to save a large file to a restricted amount of disk space. For example, a 10″ × 8″ RGB file at 300