Question:
What about teleconverters? Are they effective and if so what should I consider when buying one?
Answer:
The good news is that yes, teleconverters can be pretty darned effective. They can take our 200 mm telephoto lens and turn it into a 400 mm SUPER-telephoto.
However, the “rest of the story” (the bad news part 1) is that teleconverters are kind of expensive and are usually paired with fast (f/2.8 or better) lenses which are very expensive. The reason for this economic bad news is that teleconverters greatly reduce the amount of light getting into our cameras and it takes really expensive lenses to be fast enough (have a wide enough aperture) to deal with that.
Teleconverters cut down the light by 1-stop (-1 ev) for a 1.4X teleconverter and 2-stops for a 2X. So, this means that you have to be starting with a pretty fast lens BEFORE you attach the converter or you’re going to be literally working in the dark. The expensive, “pro,” f/2.8 lenses can handle this because they can operate at f/4 with a 1.4X or f/5.6 with a 2X - not too bad.
Unfortunately, less expensive lenses don't work so well. For example, consider that when you put that 2X teleconverter on a lens like an 18-200 mm, f/3.5 - f/5.6 zoom lens and zoom it to 200 mm (now the widest/fastest aperture is f/5.6) the best it could operate would be 2-stops slower or f/11. (e. g., aperture of f/5.6 + 1 stop = f/8; + 1 more stop = f/11). So, it's getting pretty dark in there with only 1/4 of the light.
Yes, you can boost the ISO to 1600 or something like that to compensate for the low light that’s reaching the image sensor but (here comes bad news part 2) the view finder may be very dark and autofocus will likely NOT get enough light to work.
The expensive solution would total out like this: the Nikon 2X teleconverter runs about $500; the 1.4X is about $400. These are paired with lenses like the 70-200mm f/2.8, VRII which sells for about $2300. (See what I mean about expensive!)
If you just can’t budget that kind of $$ (and not many people can) and are thinking about getting an after market teleconverter for your slower lenses, be sure to take your camera and lenses to the store and try it out on your gear to see if you like it and if your camera will work with it. Any reputable camera store will welcome your doing that.
What you want to check for is if autofocus will work. If not, is there enough light in the viewfinder to allow you to reliably focus manually? Try it inside the store and then walk outside and see how it works in full daylight. It might just do what you need. If it works OK then you are $$ ahead and can go out and enjoy the extra reach that the teleconverter provides.
