The width is treated as constant and the height set to match your aspect ratio. You can use Martin Breidt's Overscan script to adjust your field of view. You might need to use a camera, though.
Overscan should adjust the Fov according to the chosen resolution change, editing the canvas is a good comparison. If I remember correctly, it might get confused if you change resolution outside the opened script or use it repeatedly without closing.
Turns out that Overscan doesn't work with VRay-Cameras, so sorry for that. http://www.scriptspot.com/3ds-max/scripts/adjustrenderwidth-for-vray does, though. (I assume you meant this with "adjustwidthrender".) You need to turn off vertical and horizontal shift first, though, and adjust your camera to match the view without…