The EF12 II extension ring comes between the camera mount and any lens with an EF or EF-S mount.
It allows to reduce the lens minimum object distance to focus on object at very close range.

It doesn’t contain any optical element that could altered the image, the lens maintain its total optical quality.

The overall lens control and information it delivers to the camera are preserved by maintaining all the electrical contacts on the mount. Extending the flange back by 12mm, it won’t allow to focus at long distance. It also induced a one stop loss.

If the minimum object distance is still not enough, it’s possible to use the EF25 extension ring.