The ZOOM parameter controls the camera in a way similar to a telephoto lens on a standard camera. A zoom value of 1.0 is standard, with a 90 degree field of view. By changing the zoom factor to 2.0, the relative size of any feature in the frame is twice as big, while the field of view is decreased slightly. The zoom effect is implemented as a scaling factor on the height and width of the image plane relative to the world.
The ASPECRATIO parameter controls the aspect ratio of the resulting image. By using the aspect ratio parameter, one can produce images which look correct on any screen. Aspect ratio alters the relative width of the image plane, while keeping the height of the image plane constant. In general, most workstation displays have an aspect ratio of 1.0. To see what aspect ratio your display has, you can render a simple sphere, at a resolution of 512x512 and measure the ratio of its width to its height.
The ANTIALIASING parameter controls the maximum level of supersampling used to obtain higher image quality. The parameter given sets the number of additional rays to trace per-pixel to attain higher image quality.
The RAYDEPTH parameter tells the raytracer what the maximum level of reflections, refractions, or in general the maximum recursion depth to trace rays to. A value between 4 and 12 is usually good. A value of 1 will disable rendering of reflective or transmissive objects (they'll be black).
The remaining three camera parameters are the most important, because they define the coordinate system of the camera, and its position in the scene. The CENTER parameter is an X, Y, Z coordinate defining the center of the camera (also known as the Center of Projection). Once you have determined where the camera will be placed in the scene, you need to tell the raytracer what the camera should be looking at. The VIEWDIR parameter is a vector indicating the direction the camera is facing. It may be useful for me to add a "Look At" type keyword in the future to make camera aiming easier. If people want or need the "Look At" style camera, let me know. The last parameter needed to completely define a camera is the "up" direction. The UPDIR parameter is a vector which points in the direction of the "sky". I wrote the camera so that VIEWDIR and UPDIR don't have to be perpendicular, and there shouldn't be a need for a "right" vector although some other raytracers require it. Here's a snippet of a camera definition:
CAMERA ZOOM 1.0 ASPECTRATIO 1.0 ANTIALIASING 0 RAYDEPTH 12 CENTER 0.0 0.0 2.0 VIEWDIR 0 0 -1 UPDIR 0 1 0 END_CAMERA