By Barry Press, MD
High quality images from a digital camera are large files. They can be 6-10 MB or more. These will take a long time to upload and download, even with a high-speed Internet connection. Used in a PowerPoint presentation, they will take too long to load to be practical. For these applications, the images must be compressed.
Note: always save your original image for archiving and later use. Save compressed images with a different file name.
Computer monitors have a resolution of 72 pixels per inch (ppi). Theoretically, any resolution higher than this will not improve the image quality but will increase the file size. There are many ways of compressing images for viewing on monitors; this is a method that has worked well for me. I use Adobe Photoshop, but these manipulations can be made with any photo imaging software, including the one that comes bundled with your camera. Irfanview (http://www.irfanview.com) is a free Windows program which can handle these issues.
JPEG (Joint Photographic Experts Group) is a format that allows for compression with minimal loss in quality, and is a standard format for the web, email, and presentations. My method:
- Perform any sharpening, brightness, contrast or color adjustments necessary.
- Go to the menu item that sets Image Size: set the longer dimension of the image to 5 inches and the resolution to 72-150 ppi. Some images will look better at higher resolutions than 72 ppi; experiment until you are satisfied with the appearance. Small images to be used for presentations should be set at 125-150 ppi.
- For radiographs or scans, change the mode to Grayscale.
- For color images, consider changing the mode from RGB to 256 colors. This will decrease the file size without an appreciable loss in quality.
- Save the image as a JPEG with a quality setting of 5-8/12 (higher setting: better quality and larger file size.
- Give it a descriptive name different from the original image.