The Joys and Frustrations of Teleconverters
A teleconverter is a handy little device that can sandwich between your lens and camera body to increase focal length. But as amazing as teleconverters can be, their magic has its limits.
Mount a lens on the front of your camera. Light entering the front exits from the rear and projects toward the camera sensor, forming an image. You can fine-tune the focus by repositioning select elements inside the lens body, a task aided by an electrical motor on modern lenses. In this regard, all lenses are the same. But there's more than just marketing hype behind why so many variations exist in the marketplace. One of the most notable differences is their focal length. If you want a lens that magnifies the image more, you need one with a longer focal length. But longer lenses are bigger, heavier, and generally more expensive than ones with shorter focal lengths. If you want to save money, carry more gear, or prevent backaches, that means you have a problem. There are good reasons most of us aren't out there packing a 500mm f/4 over our shoulder.
It is here that the teleconverter enters our story. Also known as an extender for those who speak Canon, a teleconverter is basically just a fancy magnifying glass. When mounted on the back end of your lens, the image now strikes the converter, not the camera sensor. The elements in the converter then magnify just the central portion of the projected image and send it off on the rest of its journey to your sensor, adjusting the focus to compensate for the added distance. The outer portion of the original image falls outside the view seen by the teleconverter, resulting in an image not otherwise possible without a much longer focal length. For those who have laid awake at night puzzled over why a 2x teleconverter makes a lens only slightly longer rather than double, there's your answer.
A 2x teleconverter doesn't actually double your focal length. It only seems like it does when you look through the viewfinder. Your camera sees a cropped result in the same way that a DX (APS) sensor body sees a cropped view when compared with a full-frame body. Your 200mm lens works as if 400mm when you add a 2x extender. Your mileage may vary, but the typical 2x extender will fit in your jeans pocket. Or at least I'm far more certain of that than to suggest you could squeeze in a 400mm lens. That would require some exceptionally baggy pants and no small discomfort.
So while few of us are lugging around "big glass" on a regular basis, it's not uncommon for photographers to throw in a teleconverter. You never know when you might need one. In some circles, you almost have to have one to be considered a serious shooter. Some would say that only a rank amateur would go out without a teleconverter. That seems a tad overkill, but the things can come in handy.
But they also represent a tradeoff.
First off, a teleconverter is unlikely to yield the same image quality you would get when using an actual long lens. When a converter magnifies the central portion of the image, it also magnifies any defects inherent in the mounted lens. If it has a scratch in the middle, that blemish will appear proportionally bigger. If a lens isn't tack-sharp, you'll be more likely to see any softness. Everything is bigger. And the glass in the teleconverter itself will add more softness or aberrations of their own. Some combinations of lens and converter work better than others, but some are downright awful. Nikon and other major manufacturers publish compatibility charts on their websites. But it's unusual to find a converter that works well with all your lenses. Your mileage will vary here, too.
Then there's the problem of light loss. A teleconverter will also alter the effective aperture of your lens. A wide aperture lets in a lot of light. Stopping it down makes the opening's surface area narrower, and less light will fit through while the shutter is open. When it magnifies and uses only the central portion of the image cone, it wastes the light that forms what gets cut off. A teleconverter that didn't suffer from light loss wouldn't magnify. There wouldn't be much point in carrying around one of those, even if they existed.
While any magnification would be conceivably possible, lens makers mostly stick to 1.4x and 2x teleconverters. Purely based on the equivalence of magnification and cropping, a 1.4x converter will cost you one stop of light, while a 2x will double that to two. Some manufacturers will split the difference by adding a 1.7x converter to their lineup, but the same concept remains. Your fast telephoto becomes an average ultra-telephoto when used with a teleconverter. Average telephoto lenses suffer even more. A telephoto with a maximum aperture of f/5.6 won't go beyond f/11 with a 2x extender. Don't even think of trying auto-focus at f/11. You may have a hard time seeing anything at all if you vote for manual focus.
And these two major tradeoffs aren't the end of the story, either.
Using a teleconverter means adding more glass elements, increasing the risk of lens flare. Diffraction and reflection are possible each time light transitions from air to glass or back again.
More glass surfaces and more lens changes increase the likelihood of fingerprints or other smudges. See a streak or spot in your viewfinder? Finding it can be challenging if there's a teleconverter in the mix.
I've also seen (and experienced firsthand) problems with teleconverters and lens caps. The front side of an extender has to mate with a lens bayonet in much the same way as a camera body. And by the same token, the back face has to present the same form factor as the back of a lens if it wants to mate with a camera body. And while it is possible to screw a body cap and rear lens cap onto your teleconverter, I wouldn't recommend it unless you are sure they are compatible. I've gotten a cap stuck more than once by using the wrong one. They fit when you put them on, but they may not come back off, at least not without great difficulty. Put a marking on your teleconverter caps to prevent complications.
All that being said, teleconverters sure are handy. And if you have a combination of lens and converter that work well together, go for it. But don't expect a cheap third-party converter to yield high-quality results without a good helping of luck. If you want the best results, save up and buy a premium teleconverter. They aren't as expensive as a premium ultra-telephoto, but they will cost you. Don't skimp.
In today's world, you may wonder about leaving the teleconverter at home and cropping digitally later in Lightroom. While this won't mean any light loss, it will mean losing a lot of precious megapixels. While everything comes down to a tradeoff, cropping after will surely cost you detail. You paid for all those megapixels. Why throw them away?
And yes, I do often carry a 1.4x extender. It takes up little space, and although I rarely pull it out, it has let me get shots I couldn't have otherwise. I leave it out when shooting on a sandy beach for obvious reasons, and when I know what I'll be shooting won't need it. I may not always guess right, but that's just part of the joys and frustrations of teleconverters.