I've seen filters that just looked like a double-convex lens. I guess it allows you to focus closer to the lens than without it. So I'm not quite sure if this is exactly a telephoto effect or just a focus effect.

Also, you can easily find 2x teleconverters for 35mm cameras.

But I have some fixed-lens medium format cameras. Is there any way to add a lens to make a wide-angle attachment? A "wideconverter"?