@John and John, Thanks for the detailed explanation and formula's. Sorry I didn't get back to you earlier but this weekend has been a little crazy. It's only been 30 years since I took Algebra 101, but I think I can fight my way through this [:^)]. I do understand that focus distance is measured from the CCD and not the front element on the lens----Canon was even kind enough to put a little mark on my camera so I knew exactly where the ccd is.


I'll wrestle with this tomorrow and let you know what I discover, i.e. Calculation vs reality.


Thanks again,


Bob