Grateful for pointers to what I may have missed. The set-up: A single Yongnuo YN-560, set to Manual. Yongnuo RF-603 transceivers Canon EOS 5D ("Vintage.") (Not the MK2.) All batteries are freshly charged. The result: sync @ 1/160, 1st curtain. The product specs say (at least) 1/200. Again, grateful for ideas, pointers to threads I missed, and links.