Peter Zack , Jun 19, 2007; 05:56 p.m.
I would recommend a 1.4x converter if you are getting one, Tamron makes a good version as does Sigma (APO EX). Both will cost more but produce better results. These will multiply a 300mm to roughly a 420mm which has an effective focal length of 630mm on a DSLR and you only loose 1 stop (at f4 the lens becomes f5.6 etc). You may loose the auto focus though, unless the lens is below f4. Most converters are designed for faster lenses to retain the auto focus function. And that is also assuming you have enough available light for auto focus to begin with.
These converters do reduce the lenses image quality as well. Any converter will show a loss in the final image.
You may just want to save the money and wait to buy a better long lens and then you keep the AF as well as the image quality.
That being said I have a Vivitar 2x macro converter that I never use to extend the telephoto length but as a macro lens. It's convenient but still not as good as a dedicated macro lens like the 50mm f2,8 or 100mm f2.8 etc. It will convert a 50mm lens to a 100mm 1:1 macro. It basically works like a set of extension tubes with variable control to frame the shot. They usually sell for around $60-80 and there is a version with all the contacts so that the lens keeps full auto control again if it is faster than f4.