[lxdude]

However these first applications were rather calculations than designing. As the early computers were more related to what we now call calculators than those selfsustained computers aiding in lens design today.

A very important distinction. The early computers replaced mathematicians who did calculations manually; they did them much faster and less expensively. This led to more complex designs being attempted, because the calculating power was there to facilitate them; before then they were impractical or unprofitable.

Later computing power improved to the point that they could support the programming necessary to become design tools.
Large cost reduction has also come from a reduced need for prototypes.