Panning within a camera view

I am creating CG objects to composite in to portraits I have taken (steampunk) and I do it by creating a camera to the same angle as my real camera was when I took the object (along with the same mm measurement from the lens). Often I need to zoom in to work in more detail but I need to remain within that main camera’s view in order for the perspective to match up while I’m positioning things.

My question is:

I can zoom in, but what about when I need to view something outside of the center? How can I move along the X or Y axis of the viewport while staying within the camera’s view?

Shift + Middle Mouse Button (or Alt Shift LMB if using two button mouse with Emulate 3 Button Mouse preference on).