We revisit the critical penetration of Pearl vortices in narrow superconducting flat rings cooled in magnetic fields. Scanning superconducting quantum interference device microscopy measurements showed how a magnetic field penetrates and vortices are trapped in flat rings made of amorphous MoGe thin films. Counting the number of trapped vortices for each image, we found that the vortices are completely excluded from the ring annulus when the applied field H is below a threshold field H p: Above this field, the vortices increase linearly with the field. The obtained values of H p depend on the annulus width w ring and follow the relation μ 0 H p = (1.9 ± 0.1) φ 0 / w ring 2 with the superconducting flux quantum φ 0. This relationship provides an insight into the effect of the net-current circulating in the annular region and also leads to a precise control to trap or eliminate vortices in flat rings.