The original mental ray camera is a pinhole camera. Therefore, some of the terminology differs with a normal camera. I will write them down here.
aperture: This is the film width size. This has no relationship with lens aperture/opening. mental ray's camera was a pinhole camera, there is no lens aperture.
focal: This is the distance from lens to the film (sensor). Pinhole camera always focused, so there is no focal length meaning. But using parameters aperture and focal length, we can compute the horizontal field of view. I think these two parameters exist for this purpose.
aspect ratio: This is film(sensor)'s aspect ratio. There is no relationship with pixel resolution and camera geometry. aspect ratio = film width/film height. Note: If we use aperture, focal, and aspect ratio, we can compute the vertical field of view.
|mental ray/iray camera model (pinhole camera) without lens shader|
Current mental ray/iray can have depth of field effect, etc., this terminology is confusing. For the depth of field effect, some lens shaders have parameters like lens_radius and focus_plane_distance.
I recommend to read Andy Kopra's Writing mental ray Shaders, chapter 24 to know about the camera model.
By the way, mental images has completely integrated with NVidia. (http://www.mentalimages.com) The name, mental images, has been changed to NVidia ARC (NVidia Advanced Rendering Center).
At this year's SIGGRAPH, NVidia announced they will support the mental images products (http://www.nvidia.com/object/siggraph-2011.html) and enhance the products with more GPU/cloud support. But, it is a bit sad for me that there is no company called mental images anymore.