Simple model explaining the angular resolution effects on the derived masses and sizes of the sources located on top of filaments or other background fluctuations. The model image, consisting of an unresolved source on top of a filament, was convolved to different resolutions using Gaussian beams (FWHM), indicated in the panels. Shown are the radial profiles of the model across the filament (left), the getsf -interpolated background of the source (middle), and the background-subtracted source (right). The true radial profile of the model source and filament is shown by the thick dotted line. The background was interpolated within a radius of 1.25 times the resolution, which fully encircles the true source convolved to the resolution. Derived mass of the background-subtracted source increases by the factors 1.08, 1.55, 2.45, 3.48, and 4.12, when degrading the resolution by a factor of two. At each resolution, the measured half-maximum size of the source is larger than the size of the true convolved source by the factors 1.005, 1.077, 1.13, 1.31, 1.45, and 1.50, correspondingly.