Lets start at the beginning. What is a teleconverter and what does it do?
A teleconverter is a type of lens that works in conjunction with another lens. Specifically with a telephoto lens. This special type of lens alters the image at the sensor in two ways. It modifies the effective focal length of the primary lens and it takes light away from the image.
Years ago, teleconverters came along as a cheap way to double or triple (yerk burble barf) the effective focal length of your lens. I remember working part time in a camera store and being told by a company rep that these new teleconverters were amazing. You could buy an 80-200 lens for $250 and for only an extra $60 make it a 240-600! To a young guy it sounded too good to be true.
Yup, Thats absolutely correct. Fortunately when the bottom of the crapola lens market dropped out so did the crapola teleconverters. Today we can find teleconverters from camera manufacturers and the better third party lens manufacturers. Yes, there is still junk out there but its less prevalent than it used to be. There exists the concept of pair matched teleconverters, highly specialized devices built to work with a specific primary lens and they do exist but are outside the scope of this article. Instead I want to look at five teleconverters, two from Canon and three from Nikon.
Nikon AF-S Teleconverter TC-14E II
This cool little unit takes the focal length of the lens you are using it with and increases it by 1.4x, so for example a 200mm lens captures like a 280mm when the teleconverter is mounted between the lens and the camera body. The light lost in the teleconverter is a single stop so a low price to pay for a decent pop in magnification. Consider you own a Nikkor 300/4, a nice and not stupidly expensive lens. Add the TC-14E II and you also have a 420/5.6 by only spending $429.95 (CDN MSRP).
Canon EF Extender 1.4X III
The Canon converter works the same way as the Nikon. Its slim and coloured like Canons pro telephotos in that off white finish that Canon uses. The converter provides a 1.4x factor and consumes a single stop of light. Its very sharp and has very minimal negative impact. I use mine regularly with the 70-200/2.8L II Canon and it performs extremely well. Like the Nikon, it is lightweight and easy to carry in a regular pocket. The unit sells for $579.99 (CDN MSRP)
Nikon AF-S Teleconverter TC-17E II
This midpoint teleconverter offers a 1.7x focal length increase at the cost of about 1.5 stops of light transmission. Thats not bad at all considering. Using our earlier example we would have a 510mm f/6.3 for an added cost of $429.95 (CDN MSRP). Thats a fair bit less than what you might have to pay for a Nikkor 500mm prime.
Canon EF Extender 2x III
Canons 2x converter is in its third iteration, like the Nikon 2x offering. I owned the Series II Canon converter before it was stolen and I have to credit Canon for the significant improvement between the Series II and the Series III. The current unit is excellent and while I have noted a reduction in sharpness at the edges, centre sharpness loss is minimal. It does consume 2 full stops of light so one thing to remember is that the primary lens should be optically fast. I have used this in conjunction with my Sigma 120-300/2.8 and liked the results very much. Theres no way I can afford a 600/5.6 prime and in the limited times I need this kind of reach, the pair works admirably. The unit sells for $579.99 (CDN MSRP)
Nikon AF-S Teleconverter TC-20E III
I really like this converter a lot. Couple it with 70-200/2.8 Nikkor and youve got a rocking kit with tons of versatility for the additional spend of $599.95 (CDN MSRP). The converter doubles the effective focal length and uses up two stops of light. So that 200mm f/2.8 becomes a 400mm f/5.6.
A teleconverter reduces the amount of light transmitted, so using one on a lens that is already optically slow will have an impact on your ability to make images because of too slow shutter speeds or may even prevent the AF in your camera from operating. I have a Canon 100-400/5.6L zoom lens that works just fine with the Canon 2x converter but because the effective maximum f stop to f/11 smaller than the f/8 that is the minimum maximum aperture that the autofocus system in my camera. Even then, its still the answer to getting an image or not getting it in some situations.
Teleconverters cannot be used with wide angle lenses or lenses where the rear elements move backward as they could impact the front elements on the teleconverter. Manufacturers publish compatibility lists. Best to check these before you buy a teleconverter, or a lens to use with an existing teleconverter.
Some reviewers rail against teleconverters complaining that they introduce softness and impact sharpness. They do as would any additional elements that are not permanently part of a lens, but the manufacturer converters are stunningly good and when you consider what you get in exchange for what is really a very low purchase price. If youre like me and cannot afford a 500mm or 600mm native prime but need that kind of reach for images you like, a quality teleconverter can really help you get there.