像差（英语：Optical aberration）是光学中，实际像与根据单透镜理论确定的理想像的偏离。这些偏离是折射作用造成的。像差是由透镜对色光的不同弯曲能力所致，并造成带有色晕的像。单色像差与是与色无关的像差，包括使畸变、像场弯曲等变形像差和面像、形像、散光等使像模糊的像差。像差在照相机、望远镜和其他光学仪器中可以通过透镜的组合减小到最低限度。面镜也有与透镜一样的单色像差，没有像差。

初阶像差分为五种：球面像差、 彗形像差、 散光、 场曲、 畸变。

An optical aberration is a departure of the performance of an optical system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.