Photoshop 6.01 Tutorial
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Definition - Dots Per Inch (measured by pixels)

In graphics - such as in design, printing and photographic processes - resolution can be a deciding factor in the quality of the image. High-quality images can range from 300 to 600 dpi (dots per inch) or more for photographic quality images.

Dpi is a reference to the printing process - layers of colored dots are laid down on paper to make the image. The more dots correctly placed, the higher the resolution.

On a computer screen, these dots are squares of colored light, called pixels. The following graphic illustrated how an image appears when it is zoomed up. Note the little squares - the pixels.

However, on a computer screen, there are only ever 73 pixels per square inch. There is no point in saving high resolution for a file which will not be printed, but will only be mounted on the web. A graphic which has been saved at more pixels than 73 per inch will spread out over the screen. The difference in resolution is demonstrated below:

(higher resolution)

(lower resolution)

Print Images

150 dpi should be sufficient on the current Lehigh printers to make a decent print image. 200 dpi or higher should be saved for "heirloom" quality prints - photographic images meant to be preserved. If you decide to print at such higher resolution, the costs of printing will be more expensive. Remember that the higher the resolution, the larger the image file. For instance, a 6" by 8" file at 300dpi can easily be 5 M or higher.

Web Images

By reducing the pixels down to 73 and using the "save for web" function as a jpg file, this same 5 M file can be compressed down to less than 50 k. These adjustments can be made in the "Image Size" function under the Image heading on the toolbar. You can reduce the pixels in this window, while retaining the print size.

You can further compress the file by using the "save for the web" function, which can be found under the "file" heading on the toolbar. This function is available in Photoshop 5.5 and higher. It compresses the file, giving you a choice of alternatives.

Choose the "4 up" tab a better choice of different ways to save your file. Note that the type, size of the file, and time/speed of upload are noted on each version.

A good rule of thumb is that photographic images use the .jpg format, and graphic images use .gif.

Note that you can manipulate the quality and type of file by clicking on one of the versions and operating the "settings" functions in the upper right corner of the screen

Note that .bmp files will not work on the web. The photoshop files themselves -- .psd files - are not web ready, although you may want to save a version of the .psd file, with its layers intact, so that you can come back and modify the file later. Do this with discretion, as the .psd files are quite large.

.png files are for use in Macromedia programs like Flash and Fireworks.