Note that the returned value describes the multiplication factor applied to the inputs by the forward transformation of the ZoomMap assuming the ZoomMap has not been inverted. If the ZoomMap has been inverted (e.g. by using AST_INVERT), then the reciprocal of the returned factor will, in effect, be used by the forward transformation of the ZoomMap.
In general, Mapping attributes cannot be changed after the Mapping has been created (the exception to this is the Invert attribute, which can be changed at any time). However, several of the oldest Mapping classes - including the ZoomMap class - were introduced into the AST library before this restriction was enforced. To reduce the chances of breaking existing software, the attributes of such Mappings may still be changed, but only for Mapping instances that have exactly one active reference. In other words, an error will be reported if an attempt is made to set or clear an attribute of a Mapping (other than the Invert attribute) if that Mapping has been cloned. Mappings are cloned when they are incorporated into another object such as a CmpMap or FrameSet, or when the AST_CLONE function is used.
The Zoom attribute may not be set to zero.