Digital Image Literacy

Questions and Answers about Digital Imaging
Last updated May, 2008

Tom O'Haver
The University of Maryland at College Park

What's the best way to capture images: a flat bed scanner, a digital camera, or a video camera?

Why is it that sometimes I can't open my images files - I just get a "The application couldn't be found" error message when I double-click on my file.

Why are image files so darned big compared to text files?

How can the quality of digital images be maximized while minimizing the file size and storage requirements?
  1. Make the picture smaller by cropping or shrinking. Reducing a image by half reduces the memory requirement by a factor of four. For example, a 240 X 320 image in "millions" of colors (24 bit) takes only 240 X 320 X 3 = 230400 bytes (230 K). Even a modest 30% reduction in image size reduces the memory requirements by half.

  2. Save in a compressed file format, such as JPEG format for photographic images or GIF format for line drawings, graphs, and cartoons. For example, a 480 X 640 color photographic image might take up nearly one megabyte if saved without compression, but if saved in JPEG format might take only 40 or 50 Kbytes of disk space (depending on the complexity of the image). A 475 X 323 pixel KidPix drawing would take 151 Kbytes if saved without compression but only 6.5 Kbyes as a GIF file. Those are big savings!

  3. Adjust the compression settings of JPEG photographic images. When you save a JPG image, you will usually have some way of adjusting the amount of image compression. For example, in Paint Shop Pro, click on the Options button in the Save dialog box and use the slider to select a compression number between 1 (highest quality image and largest file size) and 99 (lowest quality image and smallest file size). Try different settings yourself and choose the highest compression number that gives an acceptable image quality.

  4. Reduce the number of colors: Thousands of colors (16 bits) is enough for most photographic images. 256 colors (8 bits) is enough for KidPix drawings. 16 colors may be enough for computer-generated charts and graphs. Fewer colors often reduces the image file size and memory requirements. All image processing programs have a way to reduce the number of colors in an image.

I'm confused by "resolution". I scanned a photograph in my flat-bed scanner at a resolution of 300 dpi (dots per inch) but when I view it in another program the picture is HUGE! What happened?

I took a photo with my new digital camera and emailed it to a friend, but my friend said the picture was way too big.

I scanned a photo, but when I zoom in on small details it looks all "blocky".

Why all those different file formats? Which is the "best" format? Why are the graphics on some Web pages so slow and how can I make mine faster? How can I capture pictures on the Web? How can I save graphics from programs that don't have a Save command? My image was too small so I enlarged it using the "resize" function of my graphics program. Now it looks all blocky Why do some of my images look too blurry/dark/light/ washed-out/grainly/blotchy/blocky and how can I fix them? Do I need Adobe Photoshop to edit digital images? How can I move an image between programs? How can I get better color print-outs?