JPEGs Explained

What is a jpeg?

Format        Description

.jpg            JPEG, (Joint Photographic Experts Group) is a compression technique for colour
                  images, making them more portable by saving on memory and loading time. 

Other formats for graphic files include:

.bmp           Bitmap - Microsoft Windows format. For example, monitor background images are
                  in .bmp format.

.gif              Graphics Iinterchange Format - The bit-mapped file format used by CompuServe
                  and a standard form for Web icons, buttons and other non photo quality images.
 
.pcx            A common graphics file format supported by many graphics programs. Most scanners and fax modems also use this format.

.tif              Tagged Image File Format - A standard file format for storing images as bit maps. It is used especially for scanned images
                  because it can support any size, resolution, and color depth.

Where do jpeg’s come from?

They are the most common format for digital cameras and photophones. It is can also be the standard setting on, entry level, scanners.
The majority of web based graphics are also saved in this format.

How much does a jpeg compress files?

The compression size of a jpeg is dependent on its quality but can be as little as 5% of the original size.

The lower the quality of the image the smaller the file size.

A high quality image has the largest file size.

How does a jpeg compress files?

When saving jpeg looks for areas of similar colour and saves them a single value. The lower the quality setting the broader the colour range that is saved as a single colour.

How does jpeg affect the image quality?

You may notice a lower quality in jpeged images.
Smooth colour changes, in skyscapes or between the highlight and shadows on fleshtones in portraiture, can be badly effected by low jpeg quality. You will notice that the colour steps through the graduation rather than changing smoothly form one colour to another.
Where there is a rapid change between light and dark areas of an image, round text or logos, you may see ghosting on the edge of the image.

Can image quality errors be undone?

Unfortunately when saving a jpeg you are making permanent changes to the file. This cannot be undone. If the image quality has not been too badly compromised the errors can be disguised by using noise, blur and artistic filters.

Should I use jpeg?

As long as the image size is set to its Maximum/Highest value a jpeg file should reproduce well. The more you compress the file to save disc space the more you compromise on image quality.

The Warning!

Any degradation in print quality due to jpeg formatted files is solely your responsibility.