While reading the Cnet review of the Canon EF-S 55-250mm f4.0-5.6 IS, they made a point regarding coupling the lens with an EF-S 18-55mm f3.5-5.6 IS:
When coupled with Canon's EF-S 18-55mm f3.5-5.6 IS lens, you have an effective range of 29mm to 400mm with image stabilization
As far as I can understand, this seems to be about using both lenses together. How would I do this?
I have a Canon 550d.
Can anybody explain how this sort of coupling works?
In this instance they are merely referring to taking both lenses with you and using them interchangeably to provide continuous coverage from 18 - 250mm. The article uses the word "effective" to indicate the full frame equivalent range of 29 - 400mm, which covers the vast majority of photographic subjects.
There are types of lenses that can be coupled, for example teleconverters can be coupled to regular lenses to increase the focal length. The coupling is accomplished by the teleconverter having a standard lens mount on the front - teleconverters cannot be used to take images by themselves as they are unable to focus an image onto the sensor.
Another type of coupling that occurs with regular lenses is reversal of a wide angle lens in order to produce a high magnification macro lens. A wide angle lens is usually mounted onto a normal lens by the use of a reversing ring, which screws into the filter threads on each lens.
All that they mean by this is that you swap the lenses when appropriate. The 29-400 numbers come from the 1.6x crop factor of the 550D's sensor (18 * 1.6 = 29, or close enough; 250 * 1.6 = 400).
