What are zoom-through lens adaptors?

Is it possible I have a non-zoom through lenses. If so how do you tell the difference?

They come in two flavours. If it is not a zoom-through-lens adaptor, you will know as soon as you made your first zoom in – the image would beginning to look out of focus as you zoom in. Make certain that the adaptor is one that is designed to hold focus through the entire zoom range.