Prime lens

The invention relates to a prime lens which comprises a first lens (L1) with negative focal power, a second lens (L2) with positive focal power, a third lens (L3) with positive focal power and a fourth lens (L4) with negative focal power which are sequentially arranged from an object side to an image side along an optical axis. The prime lens has the advantages of being low in cost, small in size, light in weight, free of virtual focus within the temperature range of -40 DEG C to 80 DEG C, capable of achieving infrared performance and capable of being used day and night. 本发明涉及一种定焦镜头，包括沿光轴从物侧至像侧依次排列的具有负光焦度的第一透镜(L1)、具有正光焦度的第二透镜(L2)、具有正光焦度的第三透镜(L3)和具有负光焦度的第四透镜(L4)。本发明的定焦镜头具备低成本、小型化、重量轻，在-40℃～80℃温度范围内不虚焦，并具备红外性能，且能够日夜两用的特点。