Hi Nancy welcome to WPF.
To put a different spin on what Don has said.
The first rule of thumb with optics is you do pretty well get what you pay for.
In the real world there are probably only a dozen or so expensive Zoom lens that give very good quality with a TC. Secondly there are various grades of TC aswell. So as a general point zooms & TC are best avoided.
Budget Zoom lens do tend to suffer from CA in bright contrasty lighting conditions anyway. Expensive lens use different types of glass to correct for this. Budget Zooms also tend to give a softer focus at the extreme focal lenght. Their optical performance normal don't start to peak until stopped down to f8-11. (This lens peaking is often referred to as the lens sweet spot). The TC will just amplify these lens defects. Plus the lost of light introduced by the TC means you have to reduce the f stop and bring the lens out of it's sweet spot range.
So to get good results you need the bests lens optics and the best quaility TC. The TC is normally designed to work with longer telephoto lens (200 - 300mm and above).
To achive better results many people use a prime telephoto lens and not a zoom.
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