Volume rendering is now a common tool for multi-dimensional data exploration in biology, medicine, meteorology, geology, material science, and other fields. In order to perform volume rendering, users are often forced to preprocess and segment their data. This step of processing before visualization often inhibits the use of volume rendering gas it can be quite cumbersome and can also introduce undesirable artifacts. In order to enhance the use of direct volume visualization, powerful, yet easy-to-use methods need to be developed. In this paper, we present an approach that offers the user data-dependent control over the focal region of the visualization. This approach enables the user to easily visualize interior structures in the dataset by controlling physically defined parameters, without performing segmentation.