A composition that is photopolymerizable upon absorption of light and/or heat, the composition including a binder, a polymerizable compound, a sensitizer, and a photoinitiator, characterized in that the composition includes, with respect to its non-volatile compounds, at least about 0.01 wt. % of a polythiol compound, and wherein the composition when coated on a support has a sensitivity of about 150 .mu.J/cm.sup.2 or higher, in combination with a high hardness.