EF Extension/teleconverter stacker distance tube

Canon Extenders or Teleconverters can usually not be stacked without a spacer between them. You can solve this problem by using any extension tube. By doing so you change the backfocus between the teleconverters and therefore loose the ability to focus to infinity. The longer the distance between the teleconverters the shorter the furthest focus distance gets. Therefore you want the shortest tube possible. I found 18mm to be the minimal distance between them without touching.

Most lenses can focus a short distance further than infinity so depending on your gear you might be able to focus to infinity as well. On my lens (Sigma 120-400mm F4.5-5.6 APO HSM II) it is still possible to focus to infinity, but that might be different for yours.

By stacking teleconverters you not only multiply the focal length but you also multiply the F-stop therefore reducing the light you can capture therefore you need much light to beginn with.

Disclaimer: The connection is only based on friction, be sure that they are properly conected and be aware that the connection might loosen at any time especially with heavy lenses/cameras. Always hold/secure both lens and camera at the same time. Usage is at your own risk and I won't take any responsibility for damaged equipment.