Just wanted to comment on Monsterzero's comment about camera positioning.
If you are trying to accurately represent a real world camera (as most ACT users are) you have to understand how the ACT represents the camera and how real world cameras work. The thing to make clear is that the ACT camera is positioned at the focal point and not the film plane of the camera. Unfortunately the focal point of a camera moves as the focal length changes. So as monsterzero points out, if you want to position a camera's film plane 5 meters away from a target and you are using an 85 mm lens then you would move the origin (5m - 85mm) = 4.915m away from the target. This difference in positioning can make a big difference with "long" lenses (i.e. large focal lengths) and when you are very close to your target.
For those out there curious about how ACT works in more detail read on. All of ACT was built using the existing ruby API with no modifications or additions. That is why it works like any other plugin without an update to SketchUp. You only need 3 numbers to specify a real world camera. The image width (this is the actual film or sensor width), the aspect ratio of the film or sensor (w/h), and the focal length of the lens (in mm). These can be set on a SketchUp camera through the ruby api. Thats it. When you set a non 0 aspect ratio to a camera in SketchUp it shows the masking bars on the screen to maintain the correct aspect ratio. Also when these are set the horizontal and vertical field of view stays constant unless you change the focal length.
One of the trickiest parts of writing the plugin was acquiring all of the camera data. Getting accurate data from the web can be difficult so most of the best data was requested directly from the camera manufacturers.
Brian "Not the thought-police" Brown