Ok, so in my quest for a longer lens suitable for budget bird photography, I've looked at mirror (catadoptric) lenses like the Sigma 600mm f/8, less expensive super telephoto lenses (including primes, zooms, and the used options), and now it is time to talk about teleconverters. The good news is that teleconverters cost much less than an equivalent lens, from $50-$300.
Honestly, there's a ton of information out there, so I've included some great links at the bottom of the post (start with the Bob Atkins page first!). Instead of doing a lot of testing, I'm going to give an overview of what's out there (and the prices) and the my conclusions. Keep in mind that my main lens I'd use a tele-converter with is my Canon 70-200mm f/4 USM. If I had a Canon 70-200mm f/2.8, 300mm f/4, etc, my decision would likely be different.
First, teleconverters are a trade-off. You gain focal length, which increases reach and magnification. In return, you lose sharpness and aperture (requiring faster shutter speeds, both to avoid camera shake at the increased focal length and to expose the image with more light). Typically, a 1.4X TC will decrease the aperture by one stop and 2X will decrease it by two stops (preventing some cameras from autofocusing). To compute the result, multiply the focal length of the original lens by the teleconversion factor and decrease the aperture by the resulting number of stops, i.e.: 200mm @ f/4 * 1.4X = 280mm @ f/5.6).
Second, teleconverters only work on high-quality lenses. Increasing focal length on a crappy lens will produce more magnification with even crappier crappiness than you started with. You can't make gold if you start with a hunk of lead. Because primes usually have better quality to begin with, teleconverters work better on primes than zooms.
Third, as the magnification of the teleconverter increases, image quality will decrease at a faster rate. 1.4X teleconverters work quite well with a minor decrease in image quality, 2X results in a significantly worse image, and 3X should only be used on the very best lenses, and even then, the image will be very soft and aberrations will become a problem. Tele-converters can also be stacked in some cases, with equivalent (or worse) image quality degradation compared to a single TC of that magnification.
Finally, just as with quality lenses, quality tele converters will give you much better results than cheap ones. It is worth investing in a good TC from the start so you don't need to buy another later. Be careful though, some of the the high-end TCs (especially those by Canon and Nikon) only work on certain lenses, typically longer telephotos. Check compatibility before you buy.
While this list is by no means comprehensive, these are the main options I would consider if I was buying a tele-converter tomorrow. For each option, I've linked the Amazon page(s) to the new price and estimated the used price over the last month (using Terapeak.com) where applicable. Mouse over the links to see the current price.
- Canon EF 1.4x II ($280 at Amazon, $175+ used) or Canon EF 2x II ($280 at Amazon, $230+ used on eBay). These are the Cadillacs of the EOS teleconverter world, expensive but the best. You know they'll work well as long as you have the right lens for them (primes 135mm and longer, the 70-200 series of zooms, and 100-400 zoom). IMO, the price premium isn't worth it unless you have plenty of money and/or multi-thousand dollar primes.
- Tamron-F 1.4x ($110 at Amazon, $50-90 used) or Tamron-F 2x ($150 at B&H, $50-100 used). These are the cheapest of the good TCs, and are relatively plentiful on eBay. In tests (cited below) they perform noticeably worse than the better quality TCs, but mostly on the edges (which is less of an issue on a non-full frame dSLR body like the 20D). The 1.4x is probably the best price/performance deal out there if you can get one used at a good price. The 2x should probably be avoided.
- Tamron SP 1.4x Pro ($185 at Amazon, $150 new on eBay) or Tamron SP 2x Pro ($205 at Amazon, $130-$150 used). It should mention that there aren't too many of these floating around used -- everyone likes them. The general consensus is also that these are near-identical optically to the Kenkos, so I'd be more inclined to grab a Kenko unless you find one of these used for a good price.
- Kenko Teleplus PRO 300 DG 1.4x ($195 at Amazon, $105 new on eBay) or Kenko Teleplus PRO 300 DG 2x ($205 at Amazon, $100-110 new on eBay). Supposedly near-identical to the Tamron SP, I think there is some serious value in the Kenko line. Notice the extreme price difference for a new lens on eBay! The older version is white (and not designated 'DG') while the newer version is black. Given a choice, go for the newer version, and buy them on eBay while the price is good!
Also note that, while I only list Canon-compatible teleconverters, Tamron and Kenko make mounts for all major brands and each major camera maker has their own high-quality teleconverter line. Be especially careful when choosing a Nikon TC though; they have two types of converters based on focal length (<=200mm and >=300mm).
One other comment: some bodies/tc combinations won't autofocus if the lens + tc result in an aperture smaller than f/5.6 (most notably the 10D/20D/etc line). To get around this in some cases, you can tape pins on the teleconverter to trick the camera into thinking it isn't there. Details are in the references below.
Before I impulsively bought the Sigma 600mm I was planning on picking up either a used Tamron-F 1.4x or, if I could find a good price, a used Tamron or Kenko 1.4X Pro model. If I found a used Tamron or Kenko Pro 2X I may have grabbed that instead, with the understanding that my Canon 70-200mm F/4 USM might have some trouble autofocusing and I'd have to go manual.
After this research, though, my mind has changed a bit. If I found a Tamron-F 1.4x for $60 or less I'd buy that to get started cheaply. Otherwise, I'd pick up a new Kenko 1.4x or 2x (depending on my mood/needs) off eBay because the price is REALLY, REALLY good right now. Sadly, I've blown most of my extra cash on the Sigma and 100mm macro lens, so I'll need to wait a bit. A teleconverter for the 70-200mm F/4 would fit really well into my lens line-up though; the 600mm for really far away stuff, the 70-200mm + teleconverter for medium range stuff (and I'd still have autofocus, at least with the 1.4x), the 70-200mm alone for normal tele, and the Tamron 17-50mm f/2.8 for wide angle.
Plus, a teleconverter is a heck of a lot easier to throw into a camera bag than a whole 'nother lens. So depending on how real-world experience with the Sigma works out, I might grab a TC in the near future.
- Bob Atkins's Teleconverters -- Are they useful? Great overview of teleconverters, the advantages and disadvantages of them, and lots of example pics for various lenses from cheap zooms to expensive prime telephotos. Doesn't talk much about the 2x and 3x though.
- A Tale of Two TCs, on Photo.net by Bob Atkins Essentially says the Tamron-F 1.4x is only slightly worse than the Canon 1.4x in real-world testing. The main differences are slight color balance, a less sharpness, and slightly worse (in both quality and quantity) chromatic aberration.
- 1.4x TC test: Tamron-F 1.4x vs Kenko 1.4x Pro Again, demonstrates an improvement with the pro version, but not a huge one. This page has the example photos. Also describes the taping technique for maintaining autofocus even with an adjusted aperture past f/5.6 on the 10D.
- Bob Atkins's TC Reviews Great overview of all available teleconverters, EXCEPT the Tamron and Kenko pro series. This page is mostly text.
- Photo.net Tamron 2X Pro with comments on quality Reviews are quite favorable.
- POTN thread comparing Canon to Kenko Consensus is the Canon is only marginally better. Includes a beautiful shot of a meat-eating bird.
- Sigma 1.4x review at Photography Review Some reviews on the Sigma 1.4x EX APO, not horrible, but enough for me to be concerned.