[The_bluester (Paul)]

I am sure there is a simple formula but i have not found it so far.

When looking at changng the spacing between a reducer/corrector and image sensor (Or Eyepiece I) experience suggest that the change in position of the focuser to bring the assembly to focus is greater than the change in spacing.

If I am not leading myself astray and seeing an affect that is not there, is it as simple as dividing the change of spacing by the reduction ratio? So for instance if you changed reducer to sensor spacing by 10mm using a 0.7 reducer the change in focuser position to bring it back to focus would be about 14.2mm?

I am fiddling around with my ED72 and trying to work out what bits and pieces I can put where in the image train, being of a technical bent I would prefer to be able to measure things first and have a good idea if they will work rather than just chuck it on and see.