A combined scanning tunneling and force microscope (STM/FM) was built which allows for simultaneous measurement of forces and force gradients along STM constant current contours. Measuring Au(111) surfaces with this combined STM/FM, very low tip-sample interaction forces and variations of the local stiffness (force gradient) in the vicinity of single adsorbates and domain boundaries of the Au(111) (square-root 3X23) surface reconstruction were found. On Au(111) substrates covered by a compact thin organic film the interaction force is higher, indicating repulsive contact between tip and sample. An elastic theory for the deconvolution of real topography from local electronic and sample deformation effects on STM constant current contours of soft samples was developed.