I own all three Kenko Teleplus Pro teleconverters, the 1.4x, the 2x, and the 3x. I also own older Nikon 1.4x and 2x teleconverters. From my experience, the optical performance of the Kenko models is equal to the Nikons, and at a far lower price. Moreover, the Kenko Pro models have full metering capability with current DSLR models, plus screw-type autofocus capability that will work down to and even past f5.6. I have used the 1.4x teleconverter with my Nikon 80-400mm. VR zoom, and even at the equivalent of 560mm. f8, autofocus will function, albeit slowly. The VR function also works, albeit anemically.
Overall, for bird photographers looking to get more versatility and \"reach\" out of current telephoto lenses, these are the best available teleconverters (Tamron\'s \"Pro\" series apparently is identical, just with a different nameplate). I recommend them highly.
Most people believe that a 3x teleconverter is necessarily a kind of gimmick that can\'t possibly provide top-quality images. I have obtained some excellent pictures using mine, however, coupled with a 500mm. tele. That\'s 1500mm. of telephoto power!
The downside of these (and any) teleconverters is that they will magnify any optical flaw in the lenses being used. They work best on top quality, fast telephotos. If used with inexpensive optics, the results will be correspondingly disappointing. They also tend not to work well with zoom lenses. This is partly because zooms invariably have more optical compromises than primes, but also, I think, because adding five or seven additional optical elements to zooms already incorporating eighteen or twenty pieces of glass simply increases the likelihood of diminished optical performance.
Consequently, the 1.4x, for example, CAN be used with the popular \"reduced vibration\" telephoto zooms, but don\'t expect the same level of image quality one could achieve using a high quality, fixed focal length telephoto.
Another problem, of course, is that teleconverters reduce available light. A 1.4x will necessitate a 2x increase in exposure, a doubler requires opening up two full stops, and the 3x teleconverter requires three full stops additional exposure. This factor necessarily makes getting good images more difficult, the optical quality of the \"rig\" notwithstanding. If one is using a DSLR, the ISO setting can be increased. I have obtained quite satisfactory images using a Nikon 500mm. f4 lens with the Kenko Teleplus Pro 3x teleconverter with the ISO setting turned all the way up to \"High 2,\" which is, I believe ISO 6400. The noise level in these images is unavoidably significant, but using a noise reduction program like Neat Image, one can produce finished images that are surprisingly satisfactory.
Still, when using teleconverters, particularly the 2x and 3x versions, the necessary use of slower shutter speeds will always be an issue. Consequently, using a very sturdy tripod and tripod head is a good idea, and taking measures to minimize \"mirror slap\" is also recommended. The Nikon D100, for example, has a special anti-vibration setting that delays exposure a few seconds while mirror vibration dissipates. For some kinds of bird photography (i.e., a bird that isn\'t moving around), this is a good solution.
Overall, the judicious use of these teleconverters adds tremendous versatility to one\'s SLR rig, providing that the lens(es) you use are of high quality and that you take the appropriate measures to achieve the best possible exposures you can get, given the limitations imposed by the use of teleconverters.
