Canon 2XEF II 2x Extender

Let's get straight to the point: teleconverters and extenders are a cheap way to get a longer lens while sacrificing quality. Here's a typical buying decision:

Canon 100-400mm f4L = around 4x the cost of the converter alone

or

Canon 70-200mm F2.8L + the extender = 140-400mm f5.6 for 1/4 the cost (assuming you already have the 70-200)

Which should you do? That really depends on how often you will use that longer focal length - use it a lot and the extra investment in the lens will pay off; use it occasionally and that's quite a bit of money saved. Personally I would use a 100-400mm rarely so can't justify the expense nor the weight of carrying it around. The extender gives me an extra lens while taking minimal space.

You get what you pay for. You will lose two stops in speed, you will lose some image quality and, depending on the lens you connect it to, you may lose some functionality. Make sure you check on the Canon website as to the lens compatibility. Finally, how much image quality is lost will depend on the camera body, more specifically the sensor size. A full frame sensor such as in the 5D Mk II will result in less deterioration than using it on an APS-C sensor like the 50D or 7D. This is because you are effectively using only a central portion of the lens surface.

Brilliant on the 70 - 200 L series, not so good on the 100 - 400, but for the cost is very good

I use it with my Canon IS f2.8 70-200mm lens and I love it. I photograph mostly wildlife and now I can get much much closer withour disturbint the subject. "F" changes to 5.6 but everyone know that now :)

Autofocus takes longer to lock but still very good.

Pictures slightly softer than normal

I do recommend.