Tuesday, August 21, 2012
Using A Teleconverter
Get extra reach with an affordable alternative to extreme tele lenses
|Teleconverters are great for macros from a distance, allowing you to get a close-up perspective on your subject without disturbing it. While there are trade-offs in performance, you do get the advantage of greatly extending your focal range without the extra weight, size and expense of a supertele lens.|
Supertelephoto lenses are really expensive, but worth it for those who can afford the cost, due to their superb optics and AF performance, and fast maximum apertures. But there's an accessory that can give those on a budget access to supertele "reach" at a much lower cost: the teleconverter.
PROS AND CONS
A teleconverter (or tele-extender) is a short tube that fits between the lens and camera body, and increases the focal length of the lens. Converters come in 1.4x, 1.7x, 2x and even 3x strengths. Attach a 2x converter to a 300mm lens, and you have a 600mm lens. But where the pro 600mm lenses cost upward of $10,000, good 2x converters can be had for under $500.
As an added bonus, adding a teleconverter doesn't change the lens' minimum focusing distance. If you add a 2x converter to a 300mm lens that focuses down to five feet, you get a 600mm lens that focuses down to five feet—close enough to produce a half-life-size magnification at the image plane and good for popular macro subjects, like butterflies and flowers. This is especially nice when you consider that those incredibly expensive 600mm pro lenses won't focus closer than about 15 feet.
|Pro-Optic 2x Converter|
Another drawback of converters is that AF performance slows—and with most cameras will disappear altogether with lens/converter combinations that are slower than ƒ/5.6. If you attach a 2x converter to a 300mm ƒ/4 lens, you get a 600mm ƒ/8 lens, and only a few top pro DSLRs can autofocus at ƒ/8. Of course, you can focus manually, but the viewfinder image at ƒ/8 is somewhat dim.
The final drawback to using a teleconverter is reduced image quality. The converter adds more glass elements to the optical path, and this tends to reduce sharpness and contrast, and increase aberrations and vignetting. Higher-end converters matched to higher-end lenses minimize this—pros often use converters—but you probably don't want to put a cheapie 2x converter on, say, an 18-200mm superzoom if you want optimal image quality.
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