The Computer is not a Microscope
There is a common misconception that the computer screen can be used as a microscope, and that images can be viewed at a range of magnifications. There is a seductive illusion of doing this with images of large objects without distinct boundaries, such as photographs of people or of landscapes, where a very large image allows "zooming out" to view the whole image and "zooming in" to view fine details.
The fallacy of this is quickly revealed in images of fine line drawings, small text, or in histological detail: where "zooming out" renders lines jagged and interrupted, type becomes illegible, and histology distorted. For good on-screen viewing and for prints, separate images should be captured for low-magnification and high-magnification views.
To look good on-screen, an image should have a resolution of 72 pixels per inch (ppi) and a size of 640 X 420 pixels. Such optimally sized images are about 0.8 Mb in size, and are readily managed by generally available computers. Good prints, however, require higher resolution, larger images.
Our best printing results have been obtained with images of 200 ppi over 8 x 10 inches (1600 x 2000 pixels), with a file size of 8 Mb. To compromise between these alternatives of optimal image size, our tests have shown a size of 2.1 Mb on screen, with a resolution of 150 ppi over 5 x 7 inches (1050 x 750 pixels) yields good on-screen images as well as good prints (after expanding the file by interpolation to 8 Mb using Photoshop).