Originally posted by Thomas The title asks the question. The 645 600mm A* has 12 elements in 11 groups . Compare that to some other Pentax lenses: 35mm 600mm A*, 8/6; DA 560mm, 6/5; 67 M* 800mm, 9/8.
Any thoughts?
Each lens design is different. The simple design of the 560mm means that the lens has to physically be longer, and there are fewer ED elements, which will result in more fringing. However, it's also sharper because it has fewer elements.
Essentially there is always going to be a trade-off, but modern lenses typically have more elements than older lenses, as it allows for better correction of aberrations and distortion, and generally more compact designs.