A teleconverter is an economical adjunct that can extend the reach of your telephoto lens. Before purchasing one, you should understand the advantages and disadvantages of a teleconverter, as well as the differences between good and not so good designs.

First off, teleconverters come in 3 general flavors: 1.4x, 2x and 3x. (But you will find 1.5x and 1.7x as well.) Each magnifies the focal range of your lens by the corresponding conversion factor. A 200mm f/2.8 lens becomes a 280mm, 400mm or 600mm lens using the 1.4, 2 or 3x TC. It sounds great on paper! Teleconverters are relatively inexpensive and to buy a new 600mm lens will cost you thousands of dollars, so this is a no brainer, right?

Not so fast. You need to consider a few things:

1. You lose light with a teleconverter.

With a 1.4x TC, your 200mm lens (in my example) has an f/2.8 maximum aperture. When you add the 1.4x TC, now your lens will read a maximum f/4 on your camera's exposure display.

With the 2x TC, your f/2.8 becomes an f/5.6.....and has a 400mm focal length.

With a 3x TC, your nice fast f/2.8 becomes a slow f/8 with a 600mm focal length.

Now, if your lens is actually an f/4 or f/5.6 to start with, do the math to figure out how small your maximum aperture will be. An f/5.6 lens with a 3x TC will end up with an f/16 aperture. Pretty much useless.

Beyond f/5.6, with reduced light and contrast, the AF of your camera will not work; this is a major drawback to many lens + teleconverter combos.

2. You need faster shutter speeds with longer focal length lenses.

You just jacked your 200mm lens into a 600mm with your 3x TC. Antishake notwithstanding, a good rule of thumb for acceptable shutter speed in the film days (to minimize camera shake induced blur) was to take the reciprocal of your focal length. With the 200mm lens, you should shoot at 1/200 second as a minimum shutter speed. Your TC enhanced 600mm lens requires at least 1/600 second.....and don't forget your new fastest aperture is f/5.6 or......worse?? Hopefully, it's a sunny day!

You will need a very bright scene to make this combo work, or you'll need to pump up the ISO to a noise inducing 800 or 1600+.

3. You lose image quality with a teleconverter.

By adding air/glass surfaces between your lens and camera with the addition of a teleconverter, you degrade image quality. The degree of loss depends on a number of things (glass quality, design etc.) and in many cases, the degradation is minimal or even unnoticeable in better quality teleconverters. However, even the most inexpensive teleconverters will do a decent job if you are only printing 4x6 snapshots. You will notice a degraded image when you enlarge your prints to 8 x 10 or larger if you used poor quality optics. More magnification (either via teleconverter or in printing) will make your system's shortcomings more noticeable by magnifying flaws.

As a rule of thumb, you'll get the least loss of IQ (image quality) from a 1.4x TC, and the most from the 3x, for teleconverters of a given quality.

Your optics (and IQ) are only as good as the worst link in your camera system. Don't put a cheap teleconverter with your expensive telephoto lens.

3. Design Considerations.

Teleconverter design can have some effect as well. Cheaper 2x and 3x teleconverters are made with 4 optical elements while the better designs incorporate 7 elements and usually better quality glass, and as such, degrade IQ less. One caveat is the 1.4x design - it only requires 4 elements to reach its magnification.

Better designs incorporate LD (low dispersion), SLD (super low dispersion), SHQ (Super High Quality) etc. optics, which indicates a higher quality glass for improved image quality characteristics.

The best designs use APO (apochromatic) coatings and some manufacturers, like Minolta, Sony and Sigma, will match the optics of their high end APO teleconverters to specific premium lenses in their line ups for maximum optical performance. When these lens + TC combos are used, image quality loss is minimal to neglible. But there is still a price to pay for this convenience....this time it is in your wallet! These high quality teleconverters can cost over $300 used, and up to $600 each new.

4. Some lenses will not accept a teleconverter!

Do not use a teleconverter on a wide angle lens, or any lens where the rear element protrudes at the lens mount. Do not use a teleconverter on a lens that the lens manufacturer recommends against. When first attempting to mount a lens (especially a zoom) to your new teleconverter, be very aware and careful of the adjacent glass surfaces. If it appears that contact could be made, then don't tempt fate by mating the two units together. Many manufacturers will include a list of lenses that their teleconverter is designed to be used with.

OK, so which teleconverter should I buy?

You will have to balance your pocketbook with your shooting interests with your existing optics to answer that question.

If you absolutely need a 3x TC for surfing photos or some other long distance photography and a 600mm lens is outside your budget, then your alternatives are limited. Generally, I don't recommend buying a 3x TC because of the reduced light and IQ limitations.

There is no point in putting a cheap teleconverter with an expensive telephoto. You won't be happy with the result. Likewise a cheap lens with an expensive TC. There's just no point. The TC will likely cost as much as an average used consumer telephoto (eg. 400/5.6) which will give you equal or better IQ than the cheap lens with an nice TC.

Many of my customers are using a Minolta 70-210 f/4 (aka "The Beercan") and want a teleconverter for it. I generally recommend something like the Tamron MC-7 2X TC, or if you want something a little better, a Kenko Teleplus 2X TC, keeping in mind that at f/8, you may be in manual focus mode. A good quality 1.4x TC with this lens will keep the AF rolling and perhaps give you that little extra reach you are looking for with minimal loss of IQ.

Personally, I like the Kenko and Tamron teleconverters the best for use with the consumer grade lenses of the Minolta/Sony alpha mount system, but that is just my preference.

The Minolta/Sony APO teleconverters are a premium product with a very specific use. If you are shooting with APO lenses, obviously, you need to get the matched APO TC to get the best out of your system and if you have these lenses, you should use one of these specialized converters, which of course will work on your other lenses, design incompatibilities notwithstanding.

So there you have it in a nutshell. If you found this article interesting/informative/helpful/non-sleep inducing....click "YES!"