The most noteworthy improvement to these two lenses are significant weight reductions compared to their predecessors—no small benefit when talking about primes of this size and weight. The EF 400mm f/2.8L IS III USM weighs in at 6.26 pounds, making it 2.2 pounds lighter than the previous generation. The EF 600mm f/4L IS III USM is 1.9 pounds lighter than its predecessor, at 6.73 pounds.
The minimum focusing distance has also been improved in both lenses, from 8.8 feet to 8.2 feet for the EF 400mm f/2.8L IS III USM, and 14.8 feet to 13.8 feet for the EF 600mm f/4L IS III USM. Both lenses also incorporate improved image stabilization, providing up to 5 stops of correction.
The lenses will be available starting in December. List prices: $11,999 (EF 400mm f/2.8L IS III USM); $12,999 (EF 600mm f/4L IS III USM).