Digital Image Literacy
Questions and Answers about Digital Imaging
Last updated May, 2008
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?
A flat-bed scanner is easy to use and can make very high-quality images of flat copy and graphic
artwork, but most are limited to covering a standard-size sheet in one scan. Scanning very
large artwork or solid objects is not practical. A
is more portable and can take pictures of large objects of any shape, but the results
depend on good photographic technique (adequate light and careful focusing and camera alignment).
A digital video camera (or an analog video camera or VCR connected to a computer with a video
digitizer) can capture still video frames, but these typically give an image size
no better than 320 X 240 pixels.
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.
Double-clicking on a document lets the computer decide which program to use to open the file. It can be a convenient short-cut for opening files
that were created with software than still resides on that same machine. But
image files are commonly created by separate hardware (e.g. on a digital camera) or by software that resides
on another computer (e.g. the computer with the scanner attached). In that case the computer
can't open the file with the original program that created it, and it will either choose another
program that can open files of that type, or it will give you an error message.
A more predictable way to open image files is to decide what software you want to use
to view the image, then launch that program and use the Open command
in the File menu. Alternatively, drag the icon of the image file and drop it on top of
the icon for the program that you want to open it.
Why are image files so darned big compared to text files?
Because images are much "denser" - after all, text is mostly white space (i.e. the
white paper or screen on which the text appears). Computer images are formed of thousands of
closely-spaced tiny dots called pixels. Each dot takes one, two, or three bytes of memory, depending on the "pixel depth"
(maximum number of colors) in the image:
How can the quality of digital images be maximized while
minimizing the file size and storage requirements?
|Number of colors ||pixel depth
|256 colors (8 bits) ||1 byte/pixel
|Thousands (16 bits) ||2 bytes/pixel
|Millions (24 bits) ||3 bytes/pixel
You can easily calculate the memory
requirements of any image by multiplying the height times the width (in pixels)
times the pixel depth. For example, a 480 X 640 image in "millions" of colors (24 bit) mode
would take 480 X 640 X 3 = 921600 bytes (nearly 1 megabyte!).
I'm confused by "resolution".
- 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.
- 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!
- 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.
- 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.
That's because the term is used in two different ways. In some cases it
means the total number of pixels in an image (really image size).
For example, digital cameras are rated this way. In
other cases it means the number of pixels per inch or dots per inch (dpi).
The resolution of scanners and display devices such as computer screens and
printers is rated in this way.
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!
The resolution of computer screens is only 70-90 dpi (dots per inch), so
if you scan a picture at more than that resolution, it will look big when all of
its pixels are displayed in another program. Solution: scan at 75 dpi. (You
can calculate the dpi resolution of your display by dividing the horizontal
screen width in pixels by the physical horizontal width of your computer screen measured
with a ruler: for example, 800 pixels/11.5 inches = 70 dpi). Most computer models
allow you to change the numbers of pixels on your screen: (Macintosh:
Control Panel > Monitors > Resolution. Windows: Control Panel > Display > Settings > Screen area).
It is sometimes useful to create an enlarged image of a small graphic (e.g.
a postage stamp) by scaning at high resolution (e.g. 300 dpi). When displayed
in other programs, the resulting image will be larger than life and will show all
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.
Modern (2 and 3 megapixel) digital cameras produce images that are 1600 - 2000 pixels wide.
but most peoples' computer screens are only 640 - 1000 pixels wide. Solution: reduce the image to 640
pixels width or less so it will neatly fit on their screen.
I scanned a photo, but when I zoom in on small details it looks all "blocky".
Those blocks are the individual "pixels" in the original image. You may need to re-scan the
photo with higher resolution (150 or 300 or even 600 dpi) in order to preserve the fine details
in the picture.
Why all those different file formats? Which is the "best" format?
Some formats are platform-specific (PICT on Macintosh and BMP on Windows)
and some are platform-neutral (TIFF, JPEG, and GIF). Some formats include
file compression (JPEG and GIF) and some save without compression. Some
programs accept only graphics in certain formats (e.g. The Writing Center
on Macintosh takes only PICT format). Web pages need graphics in either
GIF or JPEG format. GIF format is best for hard-edged computer drawings
(e.g. KidPix pcitures) and charts and graphs. JPEG format is best
for photographs. If you use the wrong format, your graphics
will look poorer and take longer to load (some examples).
You can use a shareware graphic utility program
(such as "Graphic Converter"
for the Mac or "LView" or
"Paint Shop Pro" for Windows) to convert
from one format to another by Opening in one format and Saving in another.
Why are the graphics on some Web pages so slow and how can I make mine faster?
The larger the image file size, the longer it takes to load. Also, the
larger the number of graphics on one page, the longer it takes the
whole page to load. Therefore it's a good idea to avoid putting too many
pictures on one page - break them up between several pages.
How can I capture pictures on the Web?
You should seek to reduce the file size of your Web graphics by
(1) making the picture smaller by cropping or shrinking, (2) using the
proper file format.
For digitized photographs, JPEG format
is best. JPEG images can be saved with different amounts of compression.
The greater the compression, the smaller the file but the poorer the image quality.
Here are some compression examples.
For computer-generated drawings, like KidPix drawings, GIF is the best. Here
some examples of JPEG vs GIF. In some cases
number of colors of simple images can reduce file size substantially.
It is not a good idea to use a Web page editor or HTML editor to re-size
graphics. It is better to re-size the graphics beforehand using a graphics
editor. Displaying an image smaller than its real size does nothing to speed up image loading
because the file size in not reduced, only the displayed image size.
Finally, avoid putting large numbers of graphics in a single table, because the entire table has
to load before it is displayed on the Web page. This creates a very
annoying blank-screen delay for the user. If you must use tables, break up
large multiple-row tables into several separate tables of one row each. That
way the first rows will be displayed while the later rows are loading. Here is an
Position the mouse pointer on the picture, hold down the (right) mouse button,
and select "Save image as..." from the pop-up menu. Graphics will be saved in
their original format (JPEG or GIF) but can be changed into other formats
by using a shareware graphics utility program.
How can I save graphics from programs that don't have a Save command?
Try using a "screen capture" utility such as HySnap for Windows
(which is shareware) or the Capture function that is built into Paint Shop Pro
version 6 or higher. This allows you to capture the image of any program's
windows, menus, dialog boxes, or even the entire screen.
My image was too small so I enlarged it using the "resize" function of my graphics program.
Now it looks all blocky
On the Mac, there is a built-in screen capture: press Command-Shift-3.
This will save an image of the entire screen on the hard disk as "Picture 1".
"Picture 2", etc., in PICT format. You can crop it to the portion that you want using
Enlarging a image in software just makes the pixels larger; it does
not create more pixels or improve the resolution. Take (or scan) the
picture again, but this time use a higher scanning resolution or zoom
in on the part that you are interested in.
Why do some of my images look too blurry/dark/light/
washed-out/grainly/blotchy/blocky and how can I fix them?
The first thing to do is to check you monitor color setting. (Macintosh:
Control Panel > Monitors > Color depth. Windows: Control Panel > Display > Settings > Color
Palette). Set to the highest color setting you can for the best color display.
If the display is set to too-few colors, all photographic images will look grainy and
splotchy, no matter how good the picture actually is.
(Unfortunately, some programs - like the older versions of KidPix -
force you to use 256 color mode.)
Do I need Adobe Photoshop to edit digital images?
Don't forget that your monitor has contrast and brighness controls. You
may need to adjust those controls if everything looks wrong.
If you created the picture on a Mac and it looks too dark when
viewed on a PC, or if you created the picture on a PC and it looks too
light and washed-out when
viewed on a Mac, then you probably need to adjust the
of the image. You can do that in Graphic Converter,
Paint Shop Pro.
All of the shareware graphic utility programs will let you adjust the
and color balance and saturation, which can sometimes
improve an image. You can even make images look slightly
sharper by using the sharpen function.
However, it may be that your image was flawed to begin with, perhaps
scanned with too low a resolution or with two few colors. In that
case your only choice is to scan it again with higher resolution and
No. Photoshop is a heavy-duty professional program that is expensive
and requires considerable computer hardware. You can do most of
the routine image editing tasks using a shareware graphics utility program
(such as "Graphic Converter"
for the Mac or "LView" or
"Paint Shop Pro" for Windows).
Another good low-cost program is Adobe PhotoDeluxe,
which is a simplified and less expensive ($49)
version of Photoshop for home use.
How can I move an image between programs?
Save the image in a format that the receiving program accepts. You may find it
useful to use a graphic utility program that can Open and Save in many different
formats (such as "Graphic Converter" for the Mac or
"LView" or "Paint Shop Pro" for Windows).
How can I get better color print-outs?
Try Copy and Paste. Sometimes you can just select a graphic in one
program, Copy it to the Clipboard, go into another program, and Paste
the graphic in.
Buy a newer printer and use better paper.
Ink jet printers have improved
greatly in recent years; they are now better than color laser printers for
photographic printing. For the best results with photographic images,
look for a model with "photo-enhanced" printing -
variable dot-size and/or six-color printing. Printers often have two or more
settings for print quality - a faster, lower quality setting and a slower,
higher-quality setting. Print quality is selected in the print dialog box.
For example, on a Windows machines you click on the Properties button.
With Hewlett-Packard Deskjet printers, the quality selections are Best,
Normal, and Econofast. Use Best or Normal for printing photographs; Econofast
for printing Web pages and text documents.
Paper selection is also very
important for ink-jet printing - much more so than for laser printing. Use
coated ink-jet paper rather than plain copier paper, and for the best
results with photographic images, use a paper with a glossy
or semi-glossy finish, such as Kodak Glossy Inkjet Paper (#17416);
this particular paper also has the advantage that it is coated on both sides, so
you so you can print on either side or both sides. (Most other coated paper is
coated only on one side, and you must print only on the coated side, meaning
that you have to load the paper properly into you printer so it prints
on the correct side).
For best results,
it's important that you
select the type of paper you are using in the print dialog box. The
print dialog box for your printer will have a pop-up menu of paper types (plain
paper, inkjet paper, glossy photo paper, transparencies, etc). This selection
influences how much ink is deposited on the paper. Make sure this selection matches the actual paper
type you are using. Either follow the recommendations of the paper manufacturer
or make you own series of test prints with various papers and paper settings
and choose the combination that you like best. Don't neglect this; it can have
a major effect on the quality of your photographic prints. Even low-cost
ink jet printers are capable of good photographic image quality with the right
paper and printer/paper settings.
- Sources of Images and Image Processing Software
Maryland Collaborative for Teacher Preparation
The University of Maryland at College Park