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Tuesday, July 30, 2013

Gadget Bag: Extend Your Reach


Tele-extenders give you an inexpensive way to boost your focal length

Labels: GearGadget Bag


Tele-extenders, also known as teleconverters or multipliers, are small optical accessories that mount between your lens and camera to extend your focal length. Tele-extenders come in several different multipliers, with the most common being 1.4x and 2x. The 1.4x will increase your focal length by 40%, making your 200mm lens act as a 280mm, while the 2x will increase focal length by 100%, turning the 200mm lens into a full 400mm.

This can have exciting implications for wildlife photographers. For one, having extended focal length that fits in a small camera bag gives you the tools to capture make-it-or-break-it moments while letting you be flexible and mobile. Not only do side-by-side comparisons of tele-extenders versus postprocessing crops show extenders to have a quality advantage in sharpness and precision, but tele-extenders are more budget-friendly than super-telephoto lenses, making experimenting and perfecting new types of birding or quick-action wildlife photography more accessible for exploratory photographers.

But while a tele-extender may sound like the perfect quick fix for any focal-length woes, it's important to know how to pair your extender with your lens to get the highest-quality images possible.

When light enters a tele-extender from the lens, the optics in the extender spread the light over a larger surface area. This magnifies the image, casting the central section over the camera sensor. At the same time, this dissipates some of the light from the edges of the image, decreasing the overall amount of light reaching the sensor.

Because of this, tele-extenders will decrease the maximum aperture ability of the lens. A 1.4x extender will always make the lens one stop slower. A 2x extender, in turn, will always make the lens two stops slower. For 1.4x extenders, it's best to pair them with at least ƒ/4 lenses, while 2x extenders are best paired with ƒ/2.8 lenses. This way, you're starting with workable combinations given variable lighting conditions.

This slower speed will affect the autofocus ability of the camera. For a camera to autofocus, it opens to its widest aperture, compares the angles of two paths to set the focus, then closes back to the set exposure. Prosumer-type cameras can generally autofocus up to ƒ/6.3, while pro DSLRs autofocus up to ƒ/8. Depending on the lens and lighting conditions you're working with, when you lose stops with the extender, you may lose the ability to autofocus.

Tele-extenders add an extra step between the lens and the camera, and in so doing also add an additional bend to the light. This may negatively impact the image sharpness and contrast, but the degree of impact often is dependent on the type of lens you're using with the extender. Images taken with wide-angle lenses are more adversely impacted than telephoto lenses—zoom lenses more than primes. The 2x extenders often have more noticeable effects than 1.4x extenders.

When you're using an extender, it's important to remember that as you're multiplying focal length, you're also multiplying the effect of camera shake. Some extenders have stabilizers built in, but using a tripod or monopod support will help you obtain sharp images.

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