using teleconverters?

Hi

I'm thinking on getting a teleconverter, but Im not sure if this is what I need.
I have an 18-200mm sigma lens (with 72mm filter thread) so i dont have to carry around several big lenses with me just one mounten on the camera. The only thing is that it goes up to 200mm, and I like to take photos of wild animals and birds that always keep a distance to humans.

So, is a 2x teleconverter what I need?

Most teleconverters are with a 37mm thread. What is that mean? Most lenses have a 55mm-72mm filter thread.

What is a diopter? Is it the same as the teleconverter? like this one: http://cgi.ebay.co.uk/Brand-New-Macr...item20b7b9dcf9

A diopter is basically a filter that you screw onto the front of your lens that acts like a magnifying glass. These are generally used for macro photography for increasing the size of a subject when it is really close to your lens.

A teleconverter is a special type of "lens" that mounts directly to the camera, and then you attach your regular lens to the teleconverter. A 2x teleconverter will double the focal length of any lens that you attach to it. So your 18-200mm lens will act like a 36-400mm lens. This is what you would want in order to photograph wildlife from a distance, not a diopter.

I'm sure that sounds great, but there are a few things you need to understand about how teleconverters work. First of all, teleconverters cause you to loose light. With a 2x teleconverter, you are going to loose two stops of light. I'm guessing that the maximum aperture on your lens is about f/5.6 at 200mm. By adding a 2x TC your maximum aperture becomes f/11. This loss of light may cause your camera's AF to not work very good, or possibly disable it completely, depending on the lens/camera combo you are using. (Most cameras need an aperture of f/6.3 to auto focus.) The two stops of light loss is also going to cause your shutter speeds to drop quite a bit so you are going to have to increase your ISO or shoot when it's really bright outside to get shutter speeds fast enough to freeze motion.

Another issue with TC's is that they can cause a loss in image quality. Any problems with image quality your lens may have, will be magnified by using a TC. If your lens is soft without a TC, it's only going to get worse with one.

TC's are a great way to increase your focal length without spending a ton of money, but they do come at a price. This is why most people who use TC's use them with very high quality lenses with fast apertures. This is really what they are designed to be used with. TC's on consumer grade zoom lenses generally aren't the best choice.

thanks.
this was a very comprehensive explanation.
regards,
Istvan