Modern Lens Design, 2nd Edition

Chapter 9Split Triplets

As discussed in Sec. 3.3, splitting an element into two elements allows the aberrations of the two to be substantially reduced from that of the original single element. If the balance of the system can be adjusted to take advantage of this reduction, a significant improvement in performance can be achieved.

In the Cooke triplet, the outer crowns are often split to reduce the zonal spherical aberration and thus allow the speed of the lens to be increased. An example of the split-rear crown type is shown in Fig. 9.1. Note that, as the speed is increased, the angular field coverage is reduced and the vertex length is increased. The split-rear crown form is often used in camera lenses for 8-mm,16-mm, small-format TV, and ...