Teleconverters Versus Extension Tubes
Teleconverters and extension tubes can increase the reach of your lens, allowing you to get closer to your subject. But while the two are widely different in achieving those ends, their uses aren't quite so clear-cut.
A teleconverter functions like a magnifying glass, sitting between your camera and lens. It enlarges the central portion of what the lens projects to fill the frame, cropping out what lies outside that new frame. It takes a high-quality converter to do a good job, producing an image without softer areas around the edges or other optical defects. The best teleconverters are specifically paired with the lens in use, made to go with it by the lens manufacturer. I have yet to find a third-party teleconverter that doesn't come with compromises. But no matter what a Nikon teleconverter costs, it pales compared to what it would cost you to buy a lens equivalent in focal length any other way. And the combination of shorter telephoto plus teleconverter will weigh far less. You save in both ways.
But a teleconverter will cost you in other ways. As already mentioned, using a teleconverter will cost you at least some image quality. But since it uses the light from only the central portion of what passes through your lens, it will also cost you brightness. A 1.4x teleconverter will cost you one stop of light. You'll lose two stops with a 2x converter. With a small enough maximum aperture, you could lose autofocus. I generally have one in my bag, though, since they take up so little space. I can pull out my teleconverter if I need a little more reach. Wildlife shooters love teleconverters to get those full-frame images of their elusive and sometimes dangerous quarries.
Like a teleconverter, an extension tube sits between your camera and lens. But unlike a teleconverter, a tube contains no glass elements. "Truth in advertising" at its finest, an extension tube is just what it claims to be: a hollow tube. Since they lack any glass elements, an extension tube won't degrade your image the way a teleconverter can.
But if this all sounds too good to be true, it is. A lens focuses by moving a group of elements inside, forward and back. When pushed all the way forward, a lens will focus as closely as possible. Turn the focusing helicoid to the end of its range in the opposite direction, and you will be focused at infinity, as far away as possible. But since an extension tube moves the lens away from the body, it will cost you infinity focus. You won't be able to turn the ring far enough to pull the far horizon into sharp focus anymore.
Macro photographers love extension tubes since they let them focus closer. If I have even a passing notion of encountering macro subjects, I'll throw a set of tubes in my bag before I head out. When no other subject look appealing, I can always sit down and do some close-up photography. A macro lens is just a regular lens with enough extension built-in to get close. A conventional optic design isn't optimized for such work, so I'd still prefer a dedicated macro lens if I know I'll be doing close-up work. But a set of tubes sure weighs less. No matter what you decide to carry, there are always compromises. So, choose wisely.
So, a teleconverter magnifies things at a distance so you can see them more closely, while an extension tube allows you to move closer to achieve higher magnification. The obvious conclusion is that teleconverters and extension tubes are suited to different kinds of subjects. But if you think one is for wildlife photography and the other for shooting wildflowers and tiny insects, you'd only be half right. The truth is, either one can have a role in the domain of scale typically viewed as the domain of the other.
I like shooting wildflower images, but it's challenging to get the flower sharply in focus against a blurred background. Most mountain flowers grow in groupings, or even whole fields or mountainsides. Even with their infamous shallow depth of field, macro lenses often leave the background too jumbled when focusing on a single blossom. But I've found an unconventional alternative that can work wonders. Fasten a short extension tube behind a long telephoto lens, and it becomes a wildflower monster. The minimum working distance for that big glass drops considerably, letting you shoot subjects. You won't be able to shoot wildflowers across the valley anymore, but you will be able to focus on flowers just feet or yards away. And you'll have a beautifully blurred background, thanks to the shallow depth of field. Wildlife photographers sometimes use a short extension tube to get closer to their prey, too. I wouldn't recommend this technique for every subject, of course. You'd be well advised to keep your distance from grizzly bears, charging bison, and the like. Don't say I didn't warn you.
And you can also use a teleconverter in pursuit of macro images. Not every combination will work well, but there's no reason you can't use a macro lens with a teleconverter to zoom in even closer. Few macro lenses can achieve greater than 1:1 on their own. If there's enough light to see what you're doing, a teleconverter adds its magnification to what your lens can do. Remember to use manual focus. Coupling the loss of light from the teleconverter with the shallow depth of field inherent in high magnification yields conditions that confuse even the best autofocus systems. And as magnification increases, so does the need for a good tripod. Even the slightest camera shake can ruin your chances for good results without one.
So who wins in this match-up between teleconverters and extension tubes? Potentially, you do, if you know to use them. Both are helpful tools that take up little space in your bag or add much weight. And their uses go beyond what most people consider. If you have some spare time one day, try a few combinations with your gear and see what you can work out.