Canon USA has published an interview with the developers of the brand new Canon EF 400mm f/2.8L IS III and Canon EF 600mm f/4L IS III super telephoto lenses, the lightest lenses of their kind in the world.
A lot more went on during the development than just weight reduction. New challenges lead to new materials, manufacturing processes and functionality.
Some of the design goals and challenges facing the developers:
Rather than simply making the lens lighter, we retooled the lens from the ground up for an overall balanced weight.
Improved mobility and portability, with reduced weight
A significant — not average — reduction in weight
We were convinced we could reach our goal thanks to the
proposal of a new optical system.
New challenges, one after the other, with tag-team efforts
between Optical Design, Mechanical Design...