Simon and others,
I don't have any answers for you but have struggeled with the same dilemna. It's been mostly trial and error, even after reading some articles on digital camera focal lengths and their relationship to a standard focal length.
My digital camera's EXIF data, indicates a 6mm focal length. I've read that to convert it to "standard" focal length numbers, you basically multiply the digital camera focal length by 6. So, my 6mm is converted into 36mm. I thought then, I could use that number and plug it in to SU, which should superimpose nicely on my photo, but it never works out. It seems more often than not, that I need to bump up my SU focal length to almost 60mm to get the same "feel" as my photo.
Anyone know any other tricks, if there is a better conversion or if I'm missing a step?
Ben