I don't use them now, though I used to use a 1.4 teleconverter when I had a 500mm lens. 1.4 increases the focal length by .4x and only one stop of light is lost and the quality is higher than a 2x teleconverter which looses two stops of light.

If it's the only way to capture an image, but don't forget being able to enlarge and crop in Photoshop might be better than loss of image quality due to using a teleconverter. And the best quality is to use the proper lens for the job.

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Does the teleconverter double everything? Most importantly, does it double the minimum focusing distance?
Are these useful tools, or should an individual just fess up the extra cash and get another lense that would handle the job?
Do any of you guys use them?

Scott,

I've never used one, but everything I've read about them says that they degrade image quality somewhat. I believe a 2X teleconverter effectively doubles the focal length of the lens, so a 200mm lens essentially becomes a 400mm lens. I don't know how that affects minimum focusing distance, f-stop values, etc.

I guess is you really, really need a longer lens and can't afford one, and good teleconverter might be an option, as long as you're aware of the tradeoff.......

Minimum focus distance doesn't change with a teleconverter. But the object gets bigger at that distance by a factor of the teleconverter. For example, a 2x teleconverter would give you an image twice the size at the minimum focusing distance. But so would enlarging and cropping in your image editing program. Only a side by side comparison would tell which is better. As I posted before, I wouldn't use a 2x teleconverter. I'd stay with the 1.4 versions and try to purchase one from the same manufacturer whose camera and lenses I was using because they are usually a better match and would give you a higher quality image.

Another thing to consider is that using a 2x teleconverter on a variable aperture zoom lens will slow down the autofocus to the point where it may not work because of the drop in light and contrast.