Working with Images

Understanding Image Resolution

Determining an image's resolution depends on the format of the image and its intention for use. For example, if the image is going to be used on screens, it is measured in pixels-per-inch (PPI). Conversely, if the image is going to be used for print, then it is measured in dots-per-inch (DPI).

An image's resolution is defined by the amount of pixel data it contains. The more pixel data there is, the more detail. Higher-resolution images have more pixels per inch (PPI), resulting in crisp, detailed images. Low resolution images are blurry or pixelated when enlarged. When selecting images for presentations or virtual exhibitions, be sure to choose an image that has a high resolution before making edits or resizing.

Quick Definition of Terms

• Image Resolution: the amount of detail an image has stored in its file (pixels or dots)
  • High resolution = more detail/clarity
• PPI: the number of pixels displayed per inch on screen
  • More pixels per inch (PPI) = more detail
• DPI: the number of dots displayed on print media per inch
  • More dots per inch (DPI) = more detail
• Image Dimensions: the size (width x height) of the image relative to its format

Examples

• PPI example:
  • 72 PPI (standard for most screens, but may be increased for special cases, such as when you are projecting on larger screens or if an image is on a retina screen (4K, etc.))
• DPI example:
  • 300 DPI (standard for most print applications, but can go up higher depending on printer/printing needs)
• Image dimensions: screen vs print
  • 1200 x 1800 pixels = 4 x 6 inches
  • 1500 x 2100 pixels = 5 x 7 inches
  • 2400 x 2400 pixels = 8 x 8 inches

Example Image

In the example image, which is open in the Preview app on macOS, you can see by clicking the information button that the image resolution is displayed, along with other data related to the file, such as the "Document Type" or file extension (.tif), File Size (21.1 MB), Image size (1500 x 2550 px), Image DPI (96 pixels-per-inch), and Color Mode (RGB).

This image is very large due to it being a .tif, which contains layer data, so you would typically want to resize and reformat the image for use in a presentation or embedded in a research paper. To do that, please reference the image resizing section of this guide.