Focal Length – The obvious benefit of using a teleconverter on your camera is that it extends the effective focal length of whatever lens you use it with. A 1.4x converter will give you an extra 40% (extending my 200mm maximum to 280mm) and a 2x converter will give you a 100% boost (effectively give you a 140-400mm zoom). The benefits of this extra reach are obvious – it could turn the framing of a tennis player shot from court side from a full body shot to a tightly framed upper body shot which reveals rippling muscles, dripping sweat and the grimace of their face as they strike the ball.
Cost – In comparison to the cost of buying a 400mm lens a teleconverter is a much more economical way to go.
Weight – I don’t know if you’ve seen some of the longer lenses going around but they can be quite huge. Add a 2x extender to a smaller lens instead of getting one of the larger ones and you’ll save your back some hardship.
Minimum Focus Distance – using an extender means you can keep the minimum focus length of your lens. This can be handy when you don’t have a macro lens handy and want to get close in on a subject that isn’t far away. It won’t really compare to a dedicated macro lens – but can be handy.
The Cons of Tele-converters/Extenders
Lens Speed – The first thing to consider when using a teleconverter is the impact it has upon how much light gets into your camera. Using teleconverters means less light gets in which means your maximum aperture will be decreased. When using a 1.4x converter this means you’ll lose one stop and when using a 2x converter you’ll lose two stops.
Camera Shake - As you extend the focal length of a lens – any movement of your camera will become more noticeable. Using a teleconverter magnifies both your subject and any movement in your camera so you’ll want to think carefully about how to reduce it, either by increasing your shutter speed and/or using a tripod/monopod or some other technique to secure your camera.
Focusing Speed - Another consideration with tele-converters is that they slow down the speed at which your camera will focus. This will vary from lens to lens but is particularly an issue in lower light. Some lower end DSLRs will not be able to use Auto focusing at all with some teleconverters at certain aperture settings (or at all) – so do check your camera’s compatibility before buying. To get around slow focusing switch to manual focus mode and learn how to use it – you’ll be surprised how quickly you get the hang of doing it yourself – it’s a useful skill to have.
Image Degradation – Extenders multiply not only the focal length but also any aberrations of the lens you pair it with. As a result you’ll notice on many lenses that image quality suffers – I’m told this is particularly the case with longer extenders (x2) where sharpness and contrast suffer – particularly when shooting into light (where flare and ghosting can be a problem). Using the best quality lens possible will help keep such degradation to a minimum.
(source:http://digital-photography-school.com)
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