Thanks very much for your replies, gents, very much appreciated! The metadata also said something about the camera, some Canon, I'll have to look it up. Very understandable, the article about cropfactor.
And thanks for the offer for more help on this, but I just got a mail from the client that they (suddenly) have budget problems, and would I mind letting it rest for a while
Well, it's an interesting subject, and may come up again anyway (or later), so efforts weren't wasted at all.
I'll check out LW 15 too, Klaus, thanks!