For photographers who know nothing about extension tubes:
An intermediate ring increases the distance between lens and film (here now, of course, sensor) and ensures that the photographer comes closer to the object. Gengenüber a macro lens has the disadvantage that less light arrives at the sensor. However, the modern cameras are so light strongly that no longer falls into weight.
The depth of field can also, of course, to strong, but with the lens iris to correct yet. Since we have a vicious circle: From house stands with intermediate ring less light available and now is also the aperture will be closed further. This can be accomplished, which leads to less resolution only with setting a higher sensitivity.
Distortion: The extension tubes have no optics yes, so they can not help it. It depends purely on the quality of the lens used, whether Distortions occur. If so, you should make the lens manufacturers not too much blame, because he has certainly not planned intermediate rings in its calculation. With my Nikon with 18-55mm zoom I can not see any distortions. It's also so that the object is in the center of the image and the background eh seems completely out of focus due to the low Tiefenschäfe. Thus Distortions are well to recognize only test shots with graph paper.
I have used on my analogue SLR ever intermediate rings for macro shots and am happy with this one as well.
IF YOU BECAUSE BEWARE!