DOF - Dedicated macro vs tubes etc.

Normally you produce the focus stack by moving the whole camera/lens assembly on a focusing rail. Trying to do it by changing the lens focus will change the magnification between frames, as well as the plane of focus.

If you're serious about focus stacking then there are automated rails such as the Cognisys Stackshot : http://www.cognisys-inc.com/stackshot/stackshot.php

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Check out my galleries (lots of macros) at :
http://www.pbase.com/cjed