Digital Learning > Digital Image Basics
Understanding Digital Image Terminology and Technology
Digital Image Capture Basics (continued)
Cameras are rated and marketed based on the number of megapixels (million pixels) that the camera's sensor can capture at the highest setting. Compact digital cameras normally capture pixels in a height to width ratio of 1:33, while digital SLR cameras normally capture images in an aspect ratio of 1:5. Multiplying pixel height by width (columns by rows) provides a total pixel count, and industry practice is to round to the nearest 1 million pixels to indicate a megapixel rating. Camera cost normally increases with megapixels. However, some argue that there is a point of diminishing returns beyond 5 megapixels for the non-professional, since files become very large and most non-professional uses do not require digital files in excess of 5 megapixels.
When scanning images, we are asked to select a pixels per inch (PPI) resolution at which to capture the image of the object to be scanned. For example, we may be scanning an old 4" x 6" print. Based on our intended use to print a certain size enlargement, we may have decided that we should capture the image information at 600 PPI. The pixel count for the scanned image will be (600 PPI x 4") or 2,400 pixels of height x (600 PPI x 6") or 3,600 pixels of width = 8,640,000 pixels = 8.64 megapixels.
Tip / Hint: You will often read articles, equipment manuals, and software settings that use PPI (pixels per inch) and DPI (dots per inch) interchangeably. This is the source of considerable confusion. It can eliminate a lot of confusion if you always think of image capture and display resolution in terms of pixels and pixels per inch. Only think of dots and dots per inch as related to a printer's mechanical ability to place dots of ink on paper. Pixels are not dots of ink. Pixels are best thought of as squares of image information. Dots per inch are seldom useful in our thinking, as the capability of current "photo quality printers" is well beyond our ability to discern a dots per inch difference. On the other hand, the number of pixels we have to work with in displaying an image on a monitor or in a paper print can make a big difference in our perception of the resolution quality of the image.
Deciding on settings for image capture pixel resolution will impact the size and manageability of your digital files, and will impact the ways in which you will be able to use and share the captured images.
Digital Image Files and Formats
Once image information is captured in pixels, the information must be formatted and saved as a digital file containing bits and bytes of data reflecting information about each image pixel.
Cameras, scanners, and image editing software often offer us a choice of digital image file formats to save and store our captured image information. In this section, it is recommended that Jpeg and Gif file formats be your typical choices for preservation and sharing needs. As described below, the choice will depend on the nature of the image. In this section, we briefly describe the alternative uses of these formats and the basic reasons for our recommendation. For a broader discussion of file formats, see our article titled Digital File Formats and Recommendations.
When to Use Jpeg - The Jpeg file format is recommended to record information about full color or subtle gray-scale realistic images, such as photographs. The Jpeg format can distinguish between blends of 256 shades of red, 256 shades of green, and 256 shades of blue that result in millions of color variations.
Jpeg has two major advantages. First, it is the most commonly used format for non-professionals. Therefore, it is the easiest format to use to share and transfer images. Secondly, the Jpeg format will compress the image file at a compression level largely determined by selectable settings on cameras, scanners, and available in image editing software.
Tip / Hint: Terminology used to set compression levels for Jpeg files varies. Simply keep in mind that a low compression level equates to keeping a saved image at a high quality. It is generally agreed that compression levels of up to 10:1 (compressed files 1/10th of uncompressed size) will retain a sufficient quality of detail for most non-professional needs. Jpeg compression algorithms compress information on each and every save. Each time a file is re-saved, even modest re-compression will result in some additional lost quality. Therefore, it is a good practice when working with Jpeg files to work with a copy of the original Jpeg file for each use, in order to avoid multiple re-saves of the original Jpeg image file.
When to Use Gif - Gif is the recommended format when working with mages of limited colors and/or including sharp lines. These images would include line drawings, images such as cartoons, text documents, heavily captioned images, some maps, etc.
Although, Gif is a lossless digital image file format (no pixel information is eliminated in compression process), it is effective in consolidating stored information about areas of the same color. Therefore, depending upon the nature and complexity of an image, Gif will often result in a much smaller file than Jpeg for the types of images recommended for Gif.
