Axial and lateral optical-trapping forces on polystyrene and glass microbeads are measured as a function of sphere size and axial trapping position inside a specimen chamber containing water. A strong decrease of the light forces with increasing distance of the trapping position from the coverslip of the chamber is found. It is shown that beyond a certain maximal distance the trapping efficiency decreases substantially but trapping becomes possible in different, axial positions. We consider these effects to be accounted for by spherical aberration of the focused laser beam.