That's (almost*) exactly what it does.
Originally Posted by 2F/2F
(* it magnifies the entire image projected by the prime lens, but since the film size remains the same, and there is a lot of vignetting ... )
It has been mentioned that teleconverters do not affect working distance, and that the physical length of such a thing does not matter.
But they do and it does: the front lens is moved closer to your subject by the length of the converter.
In 'normal' photography not a big deal. But it can be in close-up photography.
The extra loss of light David mentiones should only occur in uncoated converters. Put a decent coating on their elements, and that extra light loss is negligible.
Diopters work by reducing the focal length of the combined optical system. The lens' physical extension obviously is not affected.
And same amount of extension + shorter focal length = closer focussing distance.
The good thing is that they indeed do not lead to a loos of light. The bad thing that the thus changed lens loses its correction.
Diopters are quite usuable when not trying to get too close. But the closer you get, the worse these thingies are.