Can a teleconverter be used with any lens, and what does it do?

• A teleconverter is basically a magnifying lens used between the camera body and the existing lens, it is also called an Extender.

• Physical compatibility with a teleconverter depends on the lens being used and not the camera, but of course they need to be the same mount. A lens with a protruding rear element, which would mesh with the teleconverter front element, cannot be used.

  In general wide angle lenses cannot be used due to protruding rear elements.

• A 1.4x extender will cause your maximum aperture to be reduced by one stop, a 1.7x extender causes a one and a half stop decrease, and a 2x extender results in a 2 stop loss of light.

  A simple means to calculate your resulting F number is to multiply the Teleconverter's magnification by the lens' F number.

• The autofocus Teleconverters will not allow autofocusing less than f /5.6, the general rule to follow is lenses with a maximum aperture slower than f/ 2.8 can not be used with a teleconverter. Manual focusing is still possible.

  Some newer camera bodies offer f/8 autofocus, meaning that an f/4 lens with a 2X teleconverter attached (2 stops of light lost) will still allow autofocusing.

  It's probably best to buy a lens Teleconverter of the same brand as your lens so you can check the manufacturer's website for compatibility and to avoid warranty issues.

• You want to spend a fair bit on a quality Teleconverter since it will affect the quality of the images produced by the lens it is used with; expect reduced sharpness in any event.

  If you have a large lens collection one expensive extender can double your lens choices. If you have a small and inexpensive lens collection a moderate priced extender might be more budget conscious.

• Usually they are available in 1.4 to 2x magnification, with prices ranging from U$50 to over U$2K.

• Some lenses include an extender and it's possible to stack multiple extenders and get an acceptable result at a much lower cost and weight than an equivalent super telephoto lens.

  Canon has a Japanese patent for a 400mm lens with with a 2 stage extender allowing both 1.4 and 1.7 multiplication. Whether it will come to market remains to be seen.

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The advantages an extender gives you:

• A longer focal length (obviously)
• A better focal length/weight ratio
• A better focal length/price ratio (there are some exceptions)
• Double the compositional options
• A way to get to focal lengths that aren’t achievable with a prime lens on its own
• Shorter overall length when used with a long focal length lens
• Since you're only using the center of your lens any vignetting will be reduced

The disadvantages of using an extender are:

• Reduces AF tracking speed and accuracy
• Increases chromatic aberrations
• Decreases lens sharpness
• Decreases image contrast
• Since you're magnifying the central portion of your lens any flaws are magnified, so the quality of the lens being extended plays a greater role than the quality of the extender itself, though it's still important.

Further reading: See Dan Carr's article, The Ultimate Guide to Extenders or Teleconverters.