There are a few things you should keep in mind about teleconverters. They reduce image quality a bit, although pro wildlife shooters use them with pro lenses to good advantage. Converters provide best results when used with fixed-focal-length telephoto lenses and fast pro tele-zooms. Some converters were designed specifically to be used with a specific group of lenses or focal lengths and will deliver best results with these lenses/focal lengths. Used with cheaper zoom lenses, especially the wide-ratio superzooms, teleconverters produce lesser results. Teleconverters shouldn't be used with wide-angle lenses; depending on the design of the converter and lens, physical damage could result. (Check the instructions for your converter, lens and camera for compatibility issues and mounting instructions. With most systems, you must first attach the converter to the lens, then attach that combination to the camera.)
Adding a teleconverter to a lens like this Sigma APO 150-500mm F/5-6.3 DG HSM can give you extreme magnification, but you'll have to expect significant image degradation.
If you want to get extreme, you can get a supertelezoom for a reasonable price, and when you add a teleconverter, the magnification is spectacular. This isn't an everyday combination, and you need a steady hand and a sturdy tripod to use it. Also, image quality shouldn't be expected to be perfect. Think of this sort of combination as similar to using ISO 25,600. Sure, you can set ISO 25,600, but it's best left to situations when the only other option is no shot at all.
For example, the Sigma APO 150-500mm F/5-6.3 DG HSM carries a street price of $1,069, features a built-in optical image-stabilization system, and is available in mounts to fit Canon, Nikon, Pentax, Sigma and Sony DSLRs (full-frame and APS-C). The Tamron 200-500mm F/5-6.3 Di LD carries a street price of $949 and is available in mounts for Canon, Nikon and Sony/Konica Minolta DSLRs (full-frame and APS-C). The 500mm focal length takes you well into supertele territory, even on full-frame DSLRs and 35mm film cameras; on APS-C DSLRs, that 500mm setting frames like 750-800mm on a full-frame camera for even more "reach." Add a 1.4x teleconverter, and you're looking at a range of 700mm to over 1000mm, depending on your camera.
Teleconverters reduce the amount of light transmitted to the film or image sensor—by 1 stop for a 1.4x converter, by 1.5 stops for a 1.7x, by 2 stops for a 2x and by 3 stops for a 3x. This means a 300mm ƒ/4 lens attached to a 2x teleconverter becomes a 600mm ƒ/8 lens. And that means you'll have to use a tripod or a faster shutter speed (and thus a higher ISO) than without the converter. Actually, it's a good idea to use a tripod anytime you're shooting with a telephoto focal length for maximum image sharpness. Today's DSLRs and mirrorless interchangeable-lens cameras produce very good image quality at higher ISO settings, so this isn't as big a deal as it was with film or early DSLRs.
Using a teleconverter like this Tamron SP AF Pro model with a macro lens like this Canon EF 100mm ƒ/2.8 L IS USM Macro can bring you some wild supermacro images.
Although the term "macro" is thrown around loosely, true macro is generally accepted to be life-size, or 1:1 reproduction ratio, on the sensor. By attaching a teleconverter to a 1:1 macro lens, you can get into the realm of supermacro and shoot at higher magnifications. The result is very narrow depth of field and extreme close-up perspectives. The photographs can be spectacular. To shoot supermacro with a teleconverter, all of the same rules about keeping steady apply, only more so. Also, it can be quite challenging to find even a tiny subject looking through the lens, so a relaxed and patient demeanor is helpful. If you take your time, you can get some incredible photos.