Raster images refer to the way computerized photographic images are saved and displayed. On the web the most common are JPGs, GIFs and PNGs. In the print world raster data is generally saved as EPS or TIFF files.
Image Attributes
Aside from the color mode, there are three attributes that define a digital image. Print Size, Pixel Dimensions and Resolution. They are interdependent and with any two you can define the third.
Print Resolution
Resolution refers to the number of pixels in each inch when printed. The higher the resolution the sharper the image will resolve to be. Generally DPI for printing is 300 for photographic images and 600 for lineart.
Web Resolution
Monitors are a lot more forgiving than the printed page. A monitor only needs 72 dpi to show an image flawlessly. One thing to note is that if an image is over 72 dpi it will still display at 72 dpi, it will just be bigger on the screen. For example, an image that is 360 dpi will display five times larger than its Print Size on screen. It has to show each of those pixels, and since only 72 will fit in each inch, it adds more inches.
Print Size
This is how large the image would be if it were printed directly on paper. The Print Size is the Pixel Size divided by the DPI resolution with the result being in inches. On the first sample pic below the Pixel Size is 400 across. Divide that by 72 and you get 5.556 inches, the Print Size. It’s important to remember the Print Size of a document, because if the client needs something at 300 DPI, but the Print Size is half what it needs to be, the file is only 150 DPI when enlarged to the correct Print Size.
Pixel Dimensions
This refers to how many pixels across and down make up the image. For instance the sample pic below is 400 pixels wide by 300 pixels tall. On the web the Pixel Dimensions are of primary importance since the browser will show all the pixels.
The first of the images below is 400 pixels by 300 pixels at 72 dpi. The image below that is also 400 x 300 pixels, but only 40 dpi. The only real difference between the two files is the Print Size and Resolution. Since the Pixel Size is the same both display at the same size on a monitor.
