An achromatic lens system may consists of several cemented or separated groups of orown and flint glasses. If there is a value of chromatic aberration in each group, and a certain relationship between the positive or negative sign of chromatic aberration and the selection of glasses, the secondary spectrum of the whole lens system can be reduced to any small value. According to this principle, two complicated triplets with secondary spectrum corrected, are given as an example. Their focal lengths are 1.5 m, F numbers are 7, and field angles are 15° and 18° respectively. It is entirely common glasses used in one of the lens.