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Saturday, April 1, 2006

Gadget Bag: Teleconverters

Increase the focal length of your telephoto lenses


A teleconverter offers photographers the means to shoot at increased telephoto focal lengths without buying a completely new lens. With 400mm and higher lenses costing thousands of dollars, an optical accessory that delivers an increased focal length at a fraction of the cost has its obvious appeal.

With magnification factors typically ranging from 1.4x to 2x, teleconverters increase the effective focal length of a lens. Modern teleconverters include high-quality optics and coatings that ensure sharpness, less flare and reduced optical aberrations when combined with a telephoto. Attach a 2x teleconverter to a 300mm lens and you have the equivalent of a 600mm lens.

A teleconverter (also called a tele-extender, converter or extender) is an optical device that fits between your camera and the lens. It optically magnifies a lens’ focal length. Most of today's teleconverters include electronic contacts that maintain numerous camera/lens functions, including autofocus, metering and special functions such as vibration control. The lack of such contacts results in the loss of autofocus and limits the use of exposure modes, leaving you with the option of manual mode.

Teleconverters are typically designed to work with specific lenses, primarily those with fixed focal lengths. Teleconverters are also optimized for lenses of a certain range, such as 200mm and below or 300mm and higher. Performance suffers otherwise. In some cases, there are physical obstructions incorporated in the converter and lens design that prevent the converter from being used with an incompatible lens. Check lens compatibility when considering any teleconverter.

The cost to be paid for the magnification a teleconverter provides is light loss. A 1.4x teleconverter results in a light loss of one stop, and a 2x teleconverter reduces the light reaching the sensor by two stops. So a 300mm ƒ/2.8 lens is transformed into a 420mm ƒ/4 with a 1.4x teleconverter and 600mm ƒ/5.6 with a 2x.

Although the light loss might seem a disadvantage, the benefits of having the increased focal length in the field can mean the difference between getting and not getting the shot. With today's digital cameras, it's an easy matter to increase the ISO sensitivity of the camera to compensate for the effective light loss.

Light loss is a problem for many variable-aperture zoom lenses. Attach a 1.4x teleconverter to a lens with a variable maximum aperture of ƒ/4-5.6, and your effective aperture becomes ƒ/5.6-8. This typically results in the loss of autofocus function, as most AF systems require a minimum light level of ƒ/5.6 to be consistently effective. Though there are fast zoom lenses that can be used with teleconverters, you generally can achieve better quality when using a converter with a fixed-focal-length lens.
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