Have you ever had the problem that yours is not long enough? Fear not, no pills involved and they actually work with only minor side effects! In August 2010 Canon announced the 3rd installment of their ever so popular EF lens extenders in the usual flavours of 1.4x and 2.0x enhancement. As we are speaking they are finally hitting the shelves in stores near you, effectively replacing the old v2 models from 2001.
What is the difference you ask? First and foremost the price. The new versions will leave you with the decision of spending 500€/500$ versus approx. 320€/300$ for the old ones. So where is the extra bang for the buck? Better weather resistance and a new fluorine coating on the glass to resist liquids and dirt more effectively. More glass elements for improved optics and a integrated processor for better data communication between your camera and lens for faster and more accurate focus and metering.
The downsides are common to all extenders, but you should be aware of them nonetheless: the 1.4x takes 1 stop of aperture away, the 2.0x whooping 2 stops. That is even more important when keeping in mind that all but the top of the line cameras only support auto focus up to f5.6. If you are slamming a 1.4x extender on a f4.0-5.6 lens, it will become a f5.6-8.0 for example, loosing the ability to auto focus. Adding more glass to your existing glass will also cause a small to moderate decrease in picture quality, since you are enhancing the flaws of your lens, the 2.0x being prone to this more than the 1.4x obviously.
Why go for an extender in the first place? It is a relatively cheap, flexible and compact way to gain the extra reach you were missing for that wildlife and to some extend sports shot; due to the higher f-stop = slower shutter speed: normally no problem during the day, but in the evening/indoors it will be. Also keep in mind that if you migrate from an APS-C sensor DSLR to a full frame one you will loose reach due to the crop factor being reduced from 1.6x to 1.0x leaving your 70-200mm for what it is, not a 320mm.
They will work on most EF primes from 135mm and zooms from 70mm on, make sure to check this list.
I will most definitely get one of the new 1.4x mark 3s for that added value on my 7D and when I add a 5D to my gear. The 2.0x does not appeal to me personally, since I am all f2.8 and even remotely thinking about a fixed f5.6 terrifies me :).
If you are not convinced yet or want to learn more about the new extenders have a look at the brilliant reviews below from Bryan at The Digital Picture: