Kenko Teleplus PRO300 DG 2X Teleconverter:
Using the teleconverter with the Canon L series lenses, the Exif information included in the picture contains the correct focal length, with other lenses it contains the focal length of the lens only.
A two power teleconverter doubles the focal length of a lens while retaining the minimum focusing distance so it improves the macro/close-up performance of a lens.
 Canon EF 28-135 mm f/3.5-5.6 IS USM, 1/200, f/7.1, ISO: 1600, length 135 mm without teleconverter, hand-held using available light |  Canon EF 28-135 mm f/3.5-5.6 IS USM, 1/60, f/5.6, ISO: 1600, length 270 mm with teleconverter, hand-held using available light |
 Canon EF 70-200mm f/4L USM, 1/400, f/8, ISO: 1600, length 172mm without teleconverter, corner of a box, hand-held using available light in a store. |  Canon EF 70-200mm f/4L USM, 1/200, f/8, ISO: 1600, length 236mm incl. teleconverter, corner of a box, hand-held using available light in a store. |
 Canon EF 100-400mm f/4.5-5.6L IS USM, 1/2000, f/16, ISO: 1600, length 400mm without teleconverter, hand-held outdoors at noon. |  Canon EF 100-400mm f/4.5-5.6L IS USM, 1/1000, f/20, ISO: 1600, length 800mm incl. teleconverter, hand-held outdoors at noon. The spots are dust on the sensor. |
I briefly had an appartment on the seventeenth floor. From the balcony there was a clear view of a MacDonald's restaurant a full city block to the west.
 This is the overall view from the camera's location to aid orientation |  This is a 100% crop from the 400 mm + teleconverter image below, it is over a city block away! |
 Sigma 150 mm Macro lens without a teleconverter |  Sigma 150 mm Macro lens with teleconverter In this picture someone is using the phone. |
 Canon EF 100-400mm f/4.5-5.6L IS USM, without teleconverter |  Canon EF 100-400mm f/4.5-5.6L IS USM, with teleconverter, expand this to see the phone |
This is a fairly extreme example. The Nikon CoolPIX 5700 is a 5 Megapixel camera, the Nikon CoolPIX 8800 is 8 Megapixels as is the Canon 30D. The Canon with zoom lens and teleconverter is much larger, heavier and more expensive. The Canon has a much larger sensor with larger photo diodes, too. In the thumbnail images, there does not seem to be much difference between the images. Expand the images to their full size and look at the antennas on top of the BMO building which are about 16.5 Km or 10.4 miles from the camera. The cables supporting the antenna are pretty thick, probably similar in diameter to the ones used to support utility poles on the street. The CN Tower is about a kilometer further away.
Nikon COOLPIX 5700, 1/75, f/4.2, ISO: 100, 284.8mm optical, 4.0 digital zoom (4 * 71.2 = 284.8mm)
Nikon COOLPIX 8800, 1/28, f/5.2, ISO: Auto, 356mm optical, 4.0 digital zoom (89 * 4 = 356mm)
Canon OES 30D, Canon EF 100-400mm f/4.5-5.6L IS USM, Kenko Teleplus PRO300 DG 2X, 1/30, f/11, ISO: 1000, 800mm optical, 2X teleconverter (400 * 2 = 800mm)