I ordered a collar a bit too large for my Vivitar 500 mm lens; however, this is so well made I could adjust without any problem with magnetic strips I surrounded the inner core with to make the correct size. I am impressed as it made this very sturdy ring adaptable. The tripod mount part also fits well and is secure. The quality indicates it will last a long time.
This is well made and fits properly, but the ABS plastic construction is just not rigid enough to use with the EF 400mm F5.6 lens, it flexes quite noticeably and can't hold the view still enough for sharp photographs.