The scale_factor CF property.
This property is only used when writing to a file on disk.
Examples
>>> f.scale_factor = 10.0
>>> f.scale_factor
10.0
>>> del f.scale_factor
>>> f.setprop('scale_factor', 10.0)
>>> f.getprop('scale_factor')
10.0
>>> f.delprop('scale_factor')