How do I figure out what focal length and f stop? Things I know : (or will know) - distance to object - depth of focus - angle of view ( is this diagonal? ) - CCD or CMOS sensor size - underwater How do I determine what lens I need? I think if I know the focal length and the depth of focus, then I figure the f stop. Then I can determine how much light I need. Can anyone point me in the right direction?