Hand The Hand Tool scrolls the image within the window. Thus, it only works when the window is smaller than the image. It is bound to the H key, but you can temporary activate the hand by holding down the space bar. Unlike most tools, double clicking the hand on the palette does not open the option palette, rather it zooms the image to the screen.
Command temporarily activates zoom-in.
Option temporarily activates zoom-out. When working in full screen mode, the hand is the only way to scroll an image. When I'm retouching a photograph, the hand tool is perfect for navigation because the two modifiers also allow me to zoom in and out.