The Bower's 2x DGII Teleconverter (7 Element) for Canon will effectively double the focal length of your Canon lens, while maintaining the autofocus function - a 35 mm lens becomes a 70 mm lens. This 7-element teleconverter uses multi-coated optics to deliver improved image quality. With an 85 mm f/1.4 lens, for example, the focal length will become 170 mm, which effectively sets the maximum aperture at f/2.8 (cutting the amount of light captured by two f/stops). This teleconverter is a portable, high-quality and versatile accessory that's well worth having in your camera bag. It's only compatible with lenses with a maximum aperture greater than f/4.0 in order to fully utilize its autofocus capabilities.
Reviewed by 2 customers
For some reason this product drained the battery in my camera (6D) even while the camera was powered off. I confirmed this through a simple test of observing the battery level over time with different things attached to it. When I wrote in to Bower they never replied about a possible cause or solution. I returned the Item. I did try it, its hard to use because you lose 4 stops of light also the images it produced were not sharp.
I used this teleconverter for shooting the moon and worked great! Took some time using manual focus which takes a few shots to get it right.