Cross post from Autodesk Revit ng:-
Leonid Raiz posted :-
Digital camera focal lengths are different to traditional 35 mm cameras.The button In/Out of the Dynamic View dialog changes the focal length of the camera. However Revit does not provide focal length numeric value because of possible confusions. Focal length in photography usually relates to pictures taken with cameras that have 35 mm wide film. Internally cameras in Revit effectively shoot pictures on film that has width equal to height of crop region.