Bower MC4 2x AF teleconverter

• 2x autofocus teleconverter/extender for Canon EF lens system
• Magnification: 2x
• Optical Construction: 4 elements in 4 groups
• Fully Multi-coated, automatic diaphragm coupling
• Made in Japan, Lifetime Garantee

Performs as expected:
First off let me say that South Bay Camera handled my order very well. I really chose a teleconverter based on price. With that in mind I am very satisfied with the product. I confirm what other reviewers mentioned that the AF "hunts" for a focus at higher magnifications so I had to use manual focus. Other than that, I had no other problems with it. FYI, I used the converter on my Canon 75-300mm f4-5.6 III and my kit lens (18-55mm). It worked on both.

Autofocus problem is not a problem:
Some people have complained about autofocus problems. After reading a little about this, people must understand that for most autofocus systems to work the aperature must be 6 or greater (that means a number smaller than six). But, a 2x converter adds two f-stops to the aperature (decreases the aperature) so you must have a lens that goes no lower than f/4 for the aperature. Most zoom lenses go to somewhere are 5.3 to even higher numbers and that means that as you zoom in on a subject the autofocus will quit working. The answer is that with a teleconverter expect to use manual focus when using a zoom lens.

Some good with some bad:
I am please that the Bower 2X lens mounted to my EOS Rebel XT and performed well with my Canon EF 70-300mm zoom lens. But when my Canon EFS 18-55mm lens would not mount to the Bower 2X, I was very disappointed. The extra 1/8" of rubber-like material on the inner Canon EFS 18-55 lens will not allow it to fully seat in the Bower mount.

Mounting Problems for EFS lens:
The Bower tele-converters work only on Canon's EF lenses. It will not work on the Canon EFS lenses. There's a difference in the way the EF and EFS lenses are made. The Canon camera body will accept both EF and EFS lenses, but Bower tele-converters only accept the EF lenses. Also, I am trying to get the Bower converter to work with a Canon EF 50mm f/1.8 on my EOS Rebel XSi. But I'm having very limited success. The auto-focus will not work. The camera searches and searches, but to no avail. It must be manually focused. And that's a big pain because the ring you must hold in order to manually focus the lens is tiny and I end up touching the lens filter about half the time. If I could get my money back on the bower tele-converter, I would.

Auto-Focus is unpredictable with this:
I have had this teleconverter for several years. It does work, however as others have mentioned, autofocus is troublesome. Every auto-focus lens I have tried will endlessly hunt if your in medium to low light situation and you ultimately have to manually focus. Previous reviews have explained "why" it happens, however it also slows down an otherwise fast autofocus lens in bright daylight which limits its use. One thing I did notice is if you get everything just "right" the images still are not as sharp. I would recommend using a different name-brand teleconverter.