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Many objective lenses designed forvery high volume production now typically contain aspherical surfaces. One such example is an aspherized version ofthe traditional Cooke triplet. While these aspherical designs are better than their antecedents, the basic design limitations, astigmatism and manufacturing sensitivities, remain. An inverse triplet having a negative, positive, negative configuration and aspherical surfaces has no all-spherical counterpart, but instead depends on the aspheres to correct the primaiy aberrations. Ifa sufficient number of surfaces are aspherical, there are more degrees of freedom than required for aberration correction, so the lens can be optimized to reduce the manufacturing sensitivities by reducing the amount ofaberration correction contribution by an individual element. BACKGROUNDIt is well know that the classical triplet ( Figure 1) can be corrected for all ofthe primary aberrations, but because there is a residual secondary astigmatism that is not correctable, the performance is limited. ffthe specifications are "pushed" too far, the astigmatism is so severe that the depth offocus becomes very shallow (Figure 2), and the lens becomes very difficult to manufacture simply because any manufacturing variations will further reduce that already shallow depth. A further disadvantage ofthe triplet design is that the spherical aberration and astigmatism correction are both achieved because the negative element introducesjust the amount of aberration ofthe opposite sign to cancel the effects ofthe two positive elements. This means that strongly aberrated lenses must be located properly, without tilts or decentrations, or the aberrations will not completely cancel each other, demonstrated by the through-focus MTF for a decentered second element ( Figure 3). There are many four element lens types that are much better, and usually employed to achieve higher performance than the triplet, but with some increase in cost. Even employing aspherical surfaces does not yield much improvement for the three element form because the stop is located too close to the negative for the astigmatism to be corrected. 264