I am using S3D regularly, and developed a USB user interface that allows me to adjust S3D parameters such as focal plane using a contour express dial. I have stopped using scale factor by disabling the keyboard scroll, and switching to z translation. I am finding that I need to adjust the focal plane a lot.? Usually it is between the clipping planes, but when I do z translations it normally gets between the camera and the near clipping plane, or out past the far clipping plane. For S3D animations I can work around it. For interactive use, people are having major eye strain. I would like to modify the chimera code to automatically adjust the focal plane as the clipping planes change. Can you point me to the area of code where I could make the change? Matthew Dougherty National Center for Macromolecular Imaging Baylor College of Medicine ===========================================================================