When to use teleconverters?
Friday, June 11, 2010 at 10:02AM 
Canon 1.4x II teleconverter
What is a teleconverter?
A teleconverter is an adapter that is used in between your lens and your camera, and they typically magnify your focal length by 1.4x or 2x. If you have a Nikon lens you will get the best quality with a Nikon teleconverter, and the same goes for Canon and other lens manufacturers. I am not a fan of off-brand manufacturers, such as Kenko.
- Canon makes 1.4x and 2x teleconverters
- Nikon makes 1.4x, 1.7x and 2x teleconverters
- Other manufacturers make 1.4x teleconverters at a minimum
A 300mm lens with a 2x teleconverter becomes a 600mm effective focal length. A 400mm with a 1.4x teleconverter becomes a 560mm.
Canon 2x II teleconverter
When you use a teleconverter, you also lose light coming into the lens. With a 1.4x you lose 1 f/stop of light, a 1.7x you lose 1.5 stops, and with a 2x you lose 2 stops. With a 300mm f/2.8 lens mated to a 1.4x teleconverter you end up with a 420mm f/4 lens.
Nikon 1.7x TC-17EII teleconverter
When do we use a teleconverter?
You will want to use a teleconverter when you would like to have more focal length.
What are the downsides to using a teleconverter?
Unless you are using the best glass on the market, you are likely losing optical quality. You are also losing 1 to 2 stops of light, so you will need to make sure that you can afford the light loss. I am often shooting at ISO 1600 and above at the end of the day, and I usually do not use a teleconverter when I am at the edge of my minimum shutter speed. Additionally, some cameras cannot autofocus when a lens is wide open at f/8. For example, for Canon shooters with a 500mm f/4 and a 2x teleconverter, you will be at f/8 as your wide open aperture, and on some cameras you cannot maintain autofocus with that configuration. A big down side, for sure. Canon 1-series cameras can maintain autofocus at f/8, however only with the center AF point.
Kenko 1.4x teleconverter
Which lenses can be used with a teleconverter?
A rule of thumb is that fast telephotos are the only lenses that you can use a teleconverter with, and zoom lenses will not work. There are exceptions, but not many. Here is a short list of lenses where you can use a teleconverter, and there are certainly more:
- 70-200mm f/2.8
- 70-200mm f/4
- 300mm f/2.8
- 300mm f/4
- 400mm f/4 DO
- 400mm f/2.8
- 200-400mm f/4
- 500mm f/4
- 600mm f/4
You technically can use a teleconverter on the popular 100-400mm, but please do not attempt it. Yes, technically it will fit together, however the optical quality absolutely sucks. Really sucks. I see this attempted all of the time, and I cannot emphasize strongly enough that it isn’t worth the effort.
I always travel with at least a 1.4 teleconverter for my longest telephoto lens, so it is an easy for me to gain more focal length without carrying more lenses and more weight.