We haven’t talked much on this site about lens modifiers – things like teleconverters, extension tubes, and the like. Well – let’s change that with a discussion about teleconverters.
First let me say that teleconverters are pretty slick. They are a cost-effective way to increase the reach of your lens with minimal monetary outlay and only slightly more expense in its effect on your image. Let me explain.
Teleconverters tend to come in three flavors: 40% extension (1.4x), 70% extension (1.7x), and 100% extension (2x). So a 200mm lens with a 1.4x converter attached between the lens and the body becomes a 280mm lens. With a 1.7x converter it turns into a 340mm lens and with a 2x converter it’s a 400mm lens. That’s obviously great but that benefit comes with an in-camera expense. Teleconverters “eat” light. The extra glass inside the converter gives you focal length but takes effective aperture (and actually increases effective depth-of-field – something you may not want). A 1.4x teleconverter costs you a stop. A 1.7x teleconverter eats 1.5 stops and a 2x takes two whole stops with it.
Nevertheless, if you need a little extra reach, a 1.4x teleconverter seems to be a small price to pay in terms of image quality. There are a few other small gotchas to consider. First of all, teleconverters don’t work on all lenses. Before you buy, make sure you check with the manufacturer (via their web page) to see if the teleconverter will actually work with your lens. Generally teleconverters are intended for prime glass with a fixed aperture and you may get a mixed bag of results on lenses with variable apertures (i.e. zooms). The teleconverter can even eat enough light to prevent autofocus from working. Again check with the maker to see if this fact impacts you. It’s also possible your camera will take longer to find focus with a teleconverter onboard.
Because a teleconverter essentially magnifies what it’s seeing, it will also magnify any aberrations caused by your lens. With a good prime piece of glass however those are going to be small and you shouldn’t notice much of any degradation with a 1.4x converter. Obviously the smaller the teleconverter you can use, the better life will be all around for your image. Only go big if you need to go big.
Moose Peterson put up a short video on teleconverters today and he says they are a valuable tool that he feels does not degrade sharpness one bit (this article over at DPS tends to disagree a little bit but I’m trusting Moose).
The bottom line, I think, is that a good 1.4x teleconverter is a valuable addition to your lineup, especially if you have a fast prime or the ubiquitous 70-200 f/2.8 lens that Canon and Nikon love to sell. Turning that lens into a nearly 300mm f/4 lens seems to be a good idea for much less then the cost of a 300mm prime lens. Both Canon and Nikon make outstanding teleconveters and Sigma has a pretty good reputation in the field as well. If you shooting outdoors on a bright day, you just won’t really see a downside. If you’re shooting sports, a simple bump of your ISO should offset any loss of shutter speed and now you’re that much closer to the action.
And want to try before you buy? A teleconverter rental is an excellent idea. Borrowlenses.com has teleconveters for both Nikon and Canon systems (click on Extenders under the appropriate camera system).