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|Right: A teleconverter fits between your lens and camera body, and unlike extension tubes, it has optical elements that create the magnification. Because of the optics involved, matching appropriate lenses to converters is a good idea. Left: Examples of teleconverters from Sigma, Canon, Tamron, Pro-Optic, Nikon, Kenko and Olympus.|
A really long lens will fill the frame with the subject, but really long lenses are really expensive and bulky. Many of us just get as close as we can, use the longest lens we have, and then crop the image in our image-editing program. But that throws away pixels, reducing image quality and the size at which we can print the image. There's another way, which provides you with supertelephoto focal lengths, but without the same cost or bulk: the teleconverter (or tele-extender).
A teleconverter is a small tube containing glass elements that fits between your camera body and tele lens, and it increases the focal length. There are 1.4x, 1.7x, 2x and even 3x converters, each increasing the focal length of the lens to which it's attached by the stated amount. The big advantage is that a teleconverter costs a small fraction of what a lens 1.4, 1.7, 2 or 3 times the focal length of your longest lens costs. Attach a $150 to $500 2x converter to a 300mm lens, and you have a 600mm lens.
|Teleconverter Vs. Extension Tube
The primary difference between a teleconverter and an extension tube is that the converter contains glass elements and the extension tube is just a spacer that moves the lens away from the image plane. The elements in the converter actually increase the focal length of the lens to which it's attached. The extension tube doesn't optically increase the focal length; by moving the lens farther from the image plane, it allows you to focus closer than would be possible with the lens alone, and that increases the obtainable magnification. Like converters, extension tubes reduce the amount of light transmitted to the film/image sensor; the longer the extension tube, the greater the magnification and greater the light loss. As with teleconverters, built-in TTL metering automatically handles the light loss, but you have to take it into account if using an external meter.
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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.)
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.
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.
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Finally, teleconverters also reduce AF performance. Except in some high-end pro cameras, DSLR AF systems were designed to function at apertures of ƒ/5.6 and larger. When the aperture is smaller than that, the beam of light transmitted to the AF module isn't wide enough to assure the required focusing accuracy, so most cameras deactivate the AF system when a lens or lens/converter combination slower than ƒ/5.6 is used. This means you'll lose AF when you use a 1.4x converter on a lens slower than ƒ/4 or a 2x converter on a lens slower than ƒ/2.8. Additionally, all AF systems slow down as the light gets dimmer, so using a teleconverter will slow AF performance noticeably from that of the lens alone. That said, AF performance with fast pro supertelephotos and teleconverters is still good enough to do birds in flight.
|Effect Of Teleconverters
In these illustrations, you can see how the subject-to-lens distance stays the same with and without a teleconverter. This is one of the main reasons why macro and supermacro photographers use them. You can also see the magnification at the image sensor. Notice that in Figure 2 the subject-to-lens distance is the same as in Figure 1, but the sensor, and, therefore, the photographer, is slightly farther away.
As with everything in photography, there are plusses and minuses associated with teleconverters. Their small size, versatility and relatively low cost make them tools every nature photographer should have in his or her bag.
Teleconverter + eBay = Inexpensive BIG Telephoto
If you're willing to focus manually, you can save some money on your telephoto setup and still get excellent optical quality. A new 300mm ƒ/2.8 supertelephoto lens can cost upward of $6,000. A new 300mm ƒ/4 is around $1,400. But an old, non-AF Nikon 300mm ƒ/4.5 sells for around $200 on eBay. In some cases, you can find an old telephoto lens that mounts directly on your camera (Nikon, for example); in other cases, you may need an adapter.
Lens adapters let you mount lenses for other cameras on yours, such as old Canon FD lenses on EOS bodies, Nikon lenses on Canon bodies and just about any lens for which you can find an adapter onto mirrorless bodies. The big concerns regarding lens adapters are availability for the lens and camera body in question, and flange-back distance (the distance between the lens mount and the image plane). If the camera body plus the adapter's flange-back distance is too great, the lens won't focus all the way to infinity, but you can use it for close-up work. This isn't a problem with mirrorless cameras and their short flange-back distances. If the flange-back distance for the body and adapter is too short, the lens will focus past infinity—not a big a deal, but be aware of it. Many adapters are just connectors with no linkage; they allow you to attach a given lens to a given camera, but focus and exposure are manual (or aperture-priority AE). Some adapters (generally, those from the camera manufacturer) provide metering and even AF capability.
Depending on the lens-teleconverter-adapter combination, you may or may not be able to use all of your auto-exposure functions. There are so many possible combinations available that we can't cover all of the nuances of each here. This can be an excellent solution, but you have to do some homework. Check your camera's manual to determine compatibility or try contacting the manufacturer directly.
All-In-One Superzoom + Teleconverter = Compact Do-It-All Package?
The all-in-one, wide-ratio superzooms—typically, 18-200mm, 18-250mm, 18-270mm, 18-300mm, 28-300mm and the like—very conveniently package focal lengths from wide-angle to telephoto in a single surprisingly compact unit. That's a wonderful thing for anyone who wants to travel very light while retaining focal-length flexibility in the field. Can you add a teleconverter for even more versatility? Yes, but it's probably not a great idea. First, with some lens/converter combinations, damage can occur from rear lens elements contacting the converter front element should you zoom to a wide setting. Second, and more importantly, designing and manufacturing a lens with the huge range of an all-in-one superzoom requires a number of compromises. Optical performance at the longest focal length isn't as good as with a prime lens of that focal length or with a shorter-range zoom (say, a 70-300mm) at that focal length. Add a converter, and image quality is compromised further. A third consideration is that superzooms have maximum apertures of ƒ/5.6 to ƒ/6.3 at their tele end, which means your DSLR's AF system likely won't function with a converter added. For these reasons, we don't recommend using a teleconverter with a superzoom. (Note that Sigma's 1.4x and 2x converters work well with the company's 50-500mm superzoom, with manual focusing only, but that's a normal-to-supertele lens, not a wide-to-tele lens.)