Utility of low-index high-dispersion glasses for Cooke triplet design

Four Cooke triplet systems have been designed for an 85-mm focal length and f/2.8 relative aperture for use as a 35-mm slide projector objective. Two designs use the existing glasses. In the other two designs the EDF glass of the negative lens was replaced by hypothetical low-index high-dispersion glasses. The performance of the triplet systems is found to improve considerably by using low-index high-dispersion glasses for the middle negative lens.