Lossy Versus Lossless Compression
Lossy JPEG compression removes picture information permanently from a picture file. At the highest quality and lowest compression, few people could tell the difference between an uncompressed file and a compressed one. At the lowest quality and highest compression, there is a noticeable difference. Yet JPEG is adjustable so that the level of compression can be varied according to the needs of the photographer.
Experiment with JPEG compression levels on your camera and in your image-editing software to see what best meets your individual needs. You may find that when you apply more compression, the quality remains quite high while saving your memory card space and time on downloads. Save two or three photos at different compression levels and compare.
Compression can be quite confusing to the beginner. Part of the confusion is caused by software programs that use inconsistent language for compression settings and that also hide the compression option deep in the program.
To set the JPEG compression you may have to save a test file. When you choose Save As, you may see an Options button. Click this button to go to a JPEG dialog box that will allow you to change the JPEG compression level.
This is the dialog box for JPEG compression in the freeware IrfanView program. If you do not make the proper settings, your photos will be saved with the default settings.
Keep the following points in mind when you change the compression setting:
Low compression means higher quality.
High compression means lower quality.
Small file size means lower quality.
Large file size means higher quality.
Better quality means a larger file size.
Lower quality means a smaller file size.
Graphics programs usually come with the JPEG compression set to a default amount. If you want to control the JPEG quality, you will need to change the default setting yourself. With some programs, just changing this number causes it to become the permanent setting. However, with other programs you must also select your new setting as the default.
Keep in mind that for most everyday applications, a high quality save, such as a number 90 compression quality known as the compression factor, can reduce the size of an image by 50 to 70 percent with virtually no loss in visible quality. While the JPEG compression will alter the image permanently, most people will not be able to tell the difference when looking at a photograph.
Sometimes lower compression is useful and desirable:
When you are e-mailing pictures, send images that are more compressed. They will take less time for you to send and less time for your recipient to download.
When you post original images on the Internet that you don't want others to steal, use a lower quality. If you have a great high-quality image, compress it significantly before posting it on the Internet. A high compression image will look okay on the Web but will not be good for printing or other uses. Therefore it will discourage people from taking your image and claiming that they shot it. Also, since you have the original best-quality photo, you should have no trouble asserting your ownership if necessary.
Whenever you post photos on the Web, you should always compress your images to a number 80 compression factor or less so that they upload and download much more quickly.
If you are going to work on a picture for a number of saves, convert it to a TIFF file, as repeated savings in the JPEG format results in degradation of that image. This happens because the JPEG algorithm strips data from the photo with every save.
To compress a TIFF file, chose the LZW option when saving the file. Black and white files will compress quite well, though the compression with color files will be less effective.
