Canon EF APO 2x (c543781i) Lens Converter

Pros: Big jump in reach for many telephoto lenses
Cons: Makes your lens twice as slow, an f/2.8 becomes an f/5.6

The Canon 2x teleconverter is great for getting closer to subject without spending more on longer lenses. Telephoto lenses over 400mm are priced significantly higher than their shorter brethren. The Canon 300mm f/2.8 lens is $4000 which sounds like a lot until you look at the 600mm lens at $7200. By using the 2x teleconverter the 300mm f/2.8 lens becomes a 600mm f/5.6 lens which will allow most camera bodies to retain auto-focus.

The 2 stops you loose by using the 2x teleconverter can be problematic though. Many cameras lose auto-focus capabilities at speeds of f/8 or slower, so if your lens is f/5.6 or slower you will have to focus manually which may or may not be a problem for you.

You do lose some sharpness also, but the Canon 2x converter is regarded by many as the highest quality converter available and I agree with that. Its use is limited to certain lenses though, so make sure your lens is on the list at Canon’s web site before buying. Other manufactures make converters that will work with any lens, but the quality of the final images may not be as high.