Using the Photoshop Zoom Tool
http://www.elated.com/tutorials/graphics/photoshop/zoom_tool/

Elated
2003

Abstract:

Photoshop's zoom facility is very versatile and allows you to zoom in and out of an image in many different ways. You can even have different views of the same document open at once, with different zoom levels! In this tutorial we will explore the zoom tool and its applications.