Sometimes the longest telephoto lens in your bag still doesn’t deliver the reach you need. Yes, you can crop to get in tight enough on your subject, or deploy the proverbial foot zoom. If none of these options are viable, however, there remains a class of accessory that will increase the reach of your lens: the teleconverter.
Also known as “extenders,” teleconverters mount between your camera and lens and magnify the central portion of the image field of your lens. Except for a few outliers, they enable either a 1.4x or a 2.0x increase in focal length and, depending on manufacturer and model, range in price from $50 to $1,000. This price advantage over the high cost of ultra-telephoto lenses makes teleconverters an attractive option for many photographers. Not much larger than a standard 50mm lens, they also travel well in addition to maintaining a lens’s minimum focusing distance, enabling closer focus than would be possible with a longer focal length lens.
Magnification Factor
If you’ve decided the teleconverter is the way to go, the first aspect to consider is your desired magnification factor. Most teleconverters come with a 1.4x or 2x (or “doublers”) increase in focal length, with some manufacturers also producing 1.7x and 3x magnifications as well. In the case of a 70-200mm lens, a 1.4x teleconverter would convert the lens to a 98-280mm equivalent zoom, while a 2x teleconverter converts the same lens to a 140-400mm equivalent zoom.
OEM vs Third-Party Converters
Another consideration is the teleconverter’s make. Major lens manufacturers like Sony, Canon, Nikon, and others all produce their own proprietary teleconverters, which on a case-by-case basis allow for continued use of features like autofocus, image stabilization, and the recording of EXIF data when used with compatible lenses. These tend to be more expensive than teleconverters from third-party manufacturers, with whom electronic communication is often less precise or non-existent, requiring the user to make exposure and focus adjustments manually.
Potential Drawbacks to Using a Teleconverter
Perhaps the most significant drawback to using a teleconverter is the loss of light transmission. The degree of light loss is standard across all teleconverters - you lose 1 stop of light with a 1.4x teleconverter, 1.5 stops with a 1.7x teleconverter, and 2 stops of light when using a 2x converter. This manifests as a smaller maximum aperture, so in the case of the 70-200mm f/2.8 lens, a 2x teleconverter effectively becomes the equivalent of a 140-400mm f/5.6 lens. Of course, when paired with many of today’s mirrorless cameras with their improved ISO performance and image stabilization, the effect can be mitigated.
Another important effect of teleconverters is a reduction in the lens’s resolving power. The introduction of any additional lens element into an otherwise-optimized light path results in a loss of image detail, which in the case of teleconverters manifests as reduced sharpness, increased aberration, or loss of resolution. This is all before the introduction of slower shutter speeds and/or higher ISOs, which could potentially result in camera shake and increased noise.
Lastly, the teleconverter’s compatibility with both the lens and the camera body will impact its performance. Even in an optimal, fully-integrated system where the camera, lens, and teleconverter are from the same manufacturer, users can expect some reductions in autofocus speeds and performance. Furthermore, if the parallel alignment of the camera mount and lens isn’t at 100%, the image underperforms its sharpness potential. Again, fielding an OEM teleconverter will largely mitigate the likelihood of this issue, along with other issues of electronic communication. Finally, it’s important to keep in mind that not all lenses are candidates for use with a teleconverter, which are designed for a limited number of telephoto and telephoto zoom lenses and are totally incompatible with wide-angle lenses. Be sure to confirm the compatibility of the lens and the converter, even when they are both made by the same brand.
Drawbacks aside, teleconverters remain an attractive and affordable option for additional focal length. If the teleconverter is for you, then magnification, quality, and compatibility are the important factors to consider before making your purchase. With any luck, your new teleconverter will expand both your reach and creativity without breaking the bank or your back.
