[APML]: Barlow/Teleconverter

[ems@lucent.com]

I learned (by reading) that the effective magnification of a Barlow or 
teleconverter depends on where it is placed relative to the primary's focus.
This makes sense to me. 

Now, I assume that these lenses are optimised for use at a particular position.
However, it occurred to me that the magnification could be reduced by placing 
extension tubes between converter and primary.

Is this already a well-known trick?
Has anyone tried it?
How much effect does it have on aberrations?

Regards

Edwin Spector
Bath, UK.