Digital Learning > Digital Image Basics
Understanding Digital Image Terminology and
Technology
Digital Image Sharing and Display
With pixel information for photos and documents safely stored in Jpeg or Gif format, the possibilities for sharing, displaying, and printing the images has expanded dramatically.
Many of these possibilities are described in "How To" Guides and project descriptions. In this section, we will provide some basic concepts related to common uses of images for sharing and display.
An important concept to understand is how the number of image pixels we have captured and saved relates to our intended image uses. Two of the most common uses are to display an image on a monitor (as an email attachment or as a website image), and to print the image in one of several available sizes. We will focus on printing concepts as high quality printing objectives will often dictate the minimum number of pixels we need to capture.
Tip / Hint:
When possible, visualize and capture the maximum number of pixels you
need to meet your defined project objective. Capturing more
pixels than you need will result in excessively large files, and
capturing fewer than needed pixels will limit how you can use the files.
Having defined your pixel resolution requirements, you can confidently
set your camera or scanner for appropriate results. If future
uses are unclear, it is better to error on the side of more pixels.
You can
quite easily and effectively downsize / resample a file's pixel
dimensions to meet reduced pixel needs for certain types of projects. However, success is not nearly as predictable when
attempting to increase an image's pixel dimensions with
interpolative software.
Printing Considerations
To understand how digital image pixels relate to printing quality, it is important to differentiate clearly between dots per inch (DPI) and pixels per inch (PPI), since these terms are often confused and used as interchangeable in articles on the subject.
As described on page 2 of this article, dots per inch (DPI) only relates to a given printer's ability to place a certain number of "dots" of ink on an inch of paper. With the DPI capability of current printers, DPI is normally not a consideration, since the non-professional is hard pressed to discern printer differences over about 600 DPI for text and 1200 DPI for photo realistic images, and current printers exceed this level.
On the other hand, a digital image contains a finite number of pixels in columns and rows, and the size of the print will dictate how densely the printer can represent these pixels of image information on an inch of paper. This density of pixels is expressed as pixels per inch or PPI.
How many pixels we have in our digital image file and how far we spread them will mathematically result in how many pixels per inch (PPI) will be reflected in the print. For example, if we scanned a 4" x 6" photo at a scanner capture resolution setting of 600 PPI, our file will reflect pixel dimensions of 2,400 pixels by 3,600 pixels. If we spread this pixel information over an 8" x 12" print size, we could have a professional photo quality print pixel resolution of 300 PPI (2,400 pixels / 8 = 300 and 3,600 pixels / 12 = 300). If we chose to print a larger 16" x 24" print, we have enough pixels to have 150 pixels per inch of image resolution, which would provide a quality print, but not as high of a quality as at 300 PPI.
Tip / Hint: To help you understand how PPI resolution impacts
image print quality, it is generally agreed that:
300 PPI =
Professional Photo Quality
[generally agreed, the best the eye can do]
240+ PPI = Near Professional Photo Quality
[difficult for a non-professional to discern a difference from 300 PPI]
200+ PPI = High Quality
[very good images and high quality capture of subtleties}
150+ PPI = Quality
[good images in this range for routine non-professional uses]
100+ PPI = Acceptable Quality
[depending on nature of image and purpose]
100< PPI = Poor Quality
[may be acceptable for some larger prints at a distance]
