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Tuesday, April 21, 2009

The Poor Man’s Super-Telephoto

Using a tele-extender can give your long lenses even more punch for wildlife and landscape photos

Labels: LensesGear

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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.


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