As SilverFast works "frame related" (data distinguished by scan marquee)the function behind the Zoom button is that SilverFast will Zoom the data inside the active scan marquee. That data is requested anew from the scanner.
SilverFast can NOT zoom this data any further without requesting that data again (from the scanner).
However, as the zoomed prescan of the active frame is stored by SilverFast another click on the Zoom button will only call this saved data into the preview window.
In order to further zoom a portion of a given image it is necessary to resize (downsize) the scan marquee in the SilverFast preview. This way SilverFast detects that for this (changed) scan marquee no zoom data is available and will therefor request this data anew from the scanner.