Article by Jean-François Landry
They look very similar, at least physically, but what they are used for qualify them as two separate members of the photographic community. We could almost call them fraternal twins.
An extension tube is nothing more than an empty tube, void of optical elements, which is inserted between the camera body and the lens. The goal is to distance the lens from the image sensor in order to extend the minimum focusing distance of an optical group. The longer the extension tube, the closer you can get to your subject; your enlargement will be better or, if you prefer, your subject will fill the frame better. The tube itself closely resembles a simplified version of the professional bellows that wealthy macro photographers delight in using.
You can find tubes ranging from 7 mm to 50 mm in thickness. You have to understand that without a lens inside the extension tube, the quality is not really important, so as long as the contacts (mechanic and/or electronic) enable the autofocus and aperture diaphragm to work, the brand doesn’t add much.
Of course, nothing in this world is perfect. When using an extension tube, you’ll notice a loss in luminosity: the longer the tube, the lower the light, but, after all, it’s only half bad.
To take full advantage of the increase in enlargements, it’s good to use a short focal-length lens: 35 mm and 50 mm. We know from experience that the longer the focal length, the longer the focusing distance becomes. The addition of an extension tube to a 70-200 mm lens would make less of an impact. Wide-angle lenses are not good candidates either: to achieve the best enlargement, the subject would need to be sitting on the front glass of the lens!
Teleconverters (or Doublers)
But nothing is free in this world, and the teleconverter comes with three drawbacks: it degrades the quality of the optics (to a greater or lesser degree, depending on the brand) and it causes a major loss of light. You lose a full stop on the diaphragm when you use a 1.4x teleconverter and two stops (or four times the ambient light!) when you add a 2x teleconverter. This loss of light is significant, and some cameras will refuse to focus once the converter is in place.
There you have it. On one hand, you have the benefits, on the other hand, the drawbacks. It is due to these drawbacks that a 50-mm lens equipped with an extension tube can never replace a 100-mm f/2.8 Macro, and that a 70-200-mm f/2.8 with a 2x focal teleconverter can never beat a 100-400-mm f/5.6. So, what do you gain? The gain is almost all monetary, but that’s often more than good enough.
This article was originally published in Photo Life December/January 2010