of the lens glass material in other words condition 3 serves to 34 uncorrect 34 or compensate for the excessive color aberration correction provided by the front lens group and establishes a positive lens to correct other aberrations when f 1 2 3 4 is shorter than the lower limit f 0 8 of condition 3 in conjunction with condition 2 the sum of petzval 39 s lenses is reduced the back focus is decreased and various aberrations increase which leads to diminished color aberration correction the aberration can be lowered but only at the expense of more costly lens glass material when f 1 2 3 4 is longer than f 0 5 the color aberration correction is too excessive to maintain a balanced color aberration correction throughout the entire lens system which in conjunction with condition 4 necessitates a change in the power of the fifth lens condition 3 is also useful for preventing irregularities in the image formed by light beams having a low incident height condition 4 determines the power of the fifth lens together with the above relationships and corrects color aberration spherical aberration and coma aberration when f 1 2 3 4 5 is longer than f 0 65 the power of the fifth lens is weak and it is difficult to correct the color aberration whereby it is desirable to use high dispersion lens glass material such glass.