LightValue is a Composite tag, not EXIF. There is a DNG Lens tag which could be written to the EXIF, but since you're not talking about DNG files, I think you're probably talking about another Lens tag here. As for the IPTC, it should be copied.

I can figure out what is going on if you send me the image and tell me the command line you are using. (my email is phil at owl.phy.queensu.ca)

- Phil