[wim]

;Any lens which does not change focal length through the focus range does show focus breathing, Wim. They narrow the FOV.

There are two "types" of focal breathing: The lens can narrow the FOV and the lens can winden the FOV. In my above image example, you see two IF lenses. One widens, one narrows.

 

To calculate FL correctly, one needs to know the nodal points. How did you figure those out?

 

You don't necessarily need to know the nodal points, as long as you know focusing distance, size of object, size of object in the image, maximum image size and therefore magnification anf FoV. I guess you could calculate the nodal points based on those, but I was just trying to figure out the effective FL

 

Kind regards, Wim