The Poor Man’s Super-Telephoto

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Canon Extender EF 2x II

The lens of choice among the serious pro wildlife photographers I know seems to be the 600mm ƒ/4 super-telephoto. It’s great for subjects that won’t let you get close, is incredibly sharp, and autofocuses quickly and accurately. However, it costs over $7,000. That being just a bit beyond my budget, when I really need “reach,” I turn my $1,200 300mm ƒ/4 lens into a 600mm ƒ/8 by attaching a $300 2x teleconverter between the lens and camera body.

Also known as tele-extenders, teleconverters are available from the major lens manufacturers for their long lenses, and offer three major benefits.

First, as just cited, they’re an economical way to get superlong focal lengths. And they’re not just for the budget-challenged. Pros use them, too—a 1.4x converter turns that monster 600mm into an 840mm; a 2x converter, into a 1200mm.

The second benefit of the teleconverter is that it doesn’t change the lens’ minimum focusing distance. Add a 2x converter to a 300mm lens that focuses down to five feet, and you have a 600mm lens that focuses down to five feet. (For comparison, my camera manufacturer’s 600mm super-telephoto won’t focus closer than 18 feet unless you attach it to an extension tube; but then it won’t focus out to infinity.)

Sigma APO Teleconverter 1.4x EX

The third teleconverter benefit is lack of bulk. A 300mm lens with a 2x teleconverter is much more compact than a 600mm ƒ/4 super-telephoto lens. (A 600mm ƒ/8 prime lens also would be smaller than the 600mm ƒ/4, but currently no one makes a 600mm ƒ/8.)

Of course, teleconverters do have some drawbacks, the biggest being that they reduce the amount of light transmitted to the film or image sensor—by 1 stop for a 1.4x converter, 1.5 stops for a 1.7x converter, and 2 stops for a 2x. Add a 1.4x converter to a 300mm ƒ/4 lens, and it becomes a 420mm ƒ/5.6. Add a 2x converter to the 300mm ƒ/4, and it becomes a 600mm ƒ/8. TTL metering automatically compensates for the light loss, but the necessarily slower shutter speed reduces action-stopping power.

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Why the light loss? Because the focal length increases while the maximum aperture diameter doesn’t. For a 300mm ƒ/4 lens, the “wide-open” aperture diameter is 75mm (300mm focal length divided by ƒ/4). Add a 2x teleconverter, and the focal length becomes 600mm, but the maximum aperture diameter is still 75mm (600 divided by 75 equals ƒ/8).

This loss of light not only requires longer shutter speeds, but also slows or cancels autofocusing, depending on the lens/converter combo and the camera body. For example, using the 2x converter with my 300mm ƒ/4 lens on my Canon EOS 40D switches off autofocusing, while the same combo on the pro Canon EOS-1D Mark III will autofocus, but noticeably more slowly than the 300mm lens alone (and much more slowly than the 600mm ƒ/4 prime lens).

Nikon AF-S Teleconverter TC-14E II 1.4x

Pro-Optic Teleconverter 2x C/AF

(Teleconverter manual-focusing tip: If your D-SLR has a Live View monitor, that image is much easier to focus than the dim SLR viewfinder image.)

The last teleconverter disadvantage is that it does reduce image quality—by a very small amount when you’re using a quality converter with lenses it was designed for; by a lot if you use a poor-quality converter or a good one with a noncompatible lens. For best results, use a converter made by the manufacturer of your lens, and check that the manufacturer recommends it for that lens.

If your lens has a built-in stabilizer, it will function normally with the converter attached, and I’ve produced some sharp images handholding my stabilized 300mm/2x combo, but I’d recommend mounting such long focal lengths on a tripod for maximum sharpness.