For Christmas, I purchased a set of Kenko Extension Tubes in these sizes: 12mm, 20mm, and 36mm. These are straight tubes, with no glass or optics inside. They have the connections to allow for your AF functions to still work. I conducted an experiment to see what the effects and benefits were to their use. I used these sizes individually; they may be stacked for additional gain. I wanted to see how I might be able to use this lens for macro work.
Conditions: Canon Rebel Xsi body with a Canon 55 - 250mm IS EF-S lens on a tripod. I set the focal length to 100mm. Aperture was fixed at f/6.3. ISO = 800. Using ambient lighting from a nearby lamp. Used a 2 second delay to take the picture. Note, the closest distance to an object in this scenario is 1.1m or 3.6 feet. I used a Christmas ornament as my object.
Initial findings: as you increase the length of the tube and move closer to the object, you get an increased object size in exchange for requiring a longer shutter speed (less light).
This first shot is taken with no extension tube. Aperture is f/6.3 and shutter is 1/40 sec, and the distance from the hot shoe to the front of the object is 43.5".
This second shot is with the 12mm extension tube. Aperture is f/6.3, and the shutter is at 1/25 sec. The distance from hot shoe to object is now 39". Can't really see a benefit or improvement in object size.
This shot is taken with the 20mm extension tube. The aperture is f/6.3, and the shutter is now 1/20 sec. The distance from hot shoe to front of object is now 27". Note the signficant improvement in object size.
This shot is taken with the 36mm extension tube. Aperture is f/6.3, and the shutter speed is 1/8 sec. Note the greatly magnified object size. Distance from hot shoe to front of object is now just 18", compared to the earlier 43.5" with no tube.
Conclusion: Assuming that I have good lighting and possibly a tripod, I can use this technique to magnify object sizes. Note, that with extension tubes, they will fit my other telephoto lenses--unlike magnifiers placed on the front of the lens, which must match the size of the lens filter.
Next experiment will be with my 30mm lens, with and without a 1.4 teleconverter to see the benefits and restrictions of this combination.