Using Solvepnp with rvec fixed and only optimizing tvec?

[Eduardo]

You cannot fix the rotation and only refine for the translation with SOLVEPNP_ITERATIVE.

But you can for sure code it yourself (or modify the OpenCV code):

• 16. Gauss–Newton method
• Pose Estimation for Augmented Reality: A Hands-On Survey
• since the Jacobian when optimizing for the full 3D pose is:
  
• you can also optimise for just the translation part