Use the Magnifying Glass Tool in Preview to Zoom Into Image Detail
Preview in OS X Lion has been updated to include a helpful magnification tool that follows the mouse cursor. To activate it, just hit the ~ (tilde) key with an image open in Preview, then by using the trackpad you can pinch or spread to increase or decrease the size of the magnifier.
This is pretty useful and offers a much more precise option than zooming in on a full sized image with the command + and – shortcuts and then navigating around.
Thanks for the tip Alejandro