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Kenko Teleplus Pro 300 teleconverterslast updated: June 28, 2003
In recent years, Kenko has gained a reputation for building high quality teleconverters with an excellent price/performance ratio. They are the Teleplus Pro 300 serie, composed of a 1.4x and a 2x teleconverter (it seems that a 3x exists but I don't have information about it). They are made of high quality multicoated Hoya glass and are able to mantain autofocus and Matrix 3D capabilities with most of Nikkor lenses. Note that the same product is also branded Tamron SP-AF.
The older version, black coloured and named AF-d, is not able to mantain autofocus with AF-S lenses, while the newer one, grey coloured and named AF-s, is. Kenko says that their optical design has been optimized for use with telephoto lenses of 100mm and above, especially in the range from 200 to 500 mm; they should not be used with focals shorter than 50 mm. The barrel is made of plastic but both the front and rear lens mounts are in metal. Kenko says that the core is in metal too, but I did not verify it.
Every teleconverter works enlarging the central portion of the image projected by the lens; this means that the resolution is reduced (mostly at borders where the image could look 'soft') and that every defect of the lens (for instance chromatic aberration) is magnified. Other negative effects can be the reduction of contrast and light fall-off when the lens is used at full aperture. At last, depending on which lens you use, other annoying phenomena such as colour fringing may occur. Early teleconverters were very poor because they were not able to control all of these side effects, while recent ones, such as the Kenkos, show dramatic improvements. However, remember that if you want good results with a teleconverter you should start with a good quality lens. As a rule of thumb, prime lenses perform much better than zoom lenses, even if latest-generation zooms (such as the AF-S 80-200 f2.8 D) seems to work quite well with a good teleconverter.
AutofocusThe AF-d converters (black barrel) mantain autofocus with AF lenses, but not with AF-S lenses. The AF-s converters (grey barrel) properly autofocus with AF-S lenses too. For what concerns speed, I run the following test: I leave the front cap on the lens and measure the time the lens spends to go from infinity to minimum focusing distance (with any focus limiter disabled) and back to infinity. I use my F80 with a fresh set of AAA batteries. The results are:
Of course these information are not enough, since we are not considering how authoritative the AF is in the different cases; anyway the performances cannot be better since these times represent the rolling speed of the barrel. On the field, with Nikkor AF-S 300mm f/4D plus Kenko 1.4x I was able to track fast flying warplanes during an airshow on a sunny day with no problems at all. Out of about 300 shots, my F80 hunted only for 2% of the times. Keep in mind that when using a teleconverter, the depth-of-field is not as deep as you might expect: with a 2x the DOF is one half of the prime lens. For instance, using a 2x with a 300mm f/4 to achieve a 600mm and shooting at f/8, if your subject is five meters far, the DOF is about 6 cm! At f/11 is 9 cm; at f/16 is about 12cm. So, when shooting close subjects (such as small birds - see photo 0209329) be sure to use the smallest possible aperture, otherwise large portions of your subjects will be off-focus. When you first experience this very shallow DOF, it is very common to erroneously blame the teleconverter for producing a 'soft' image or the AF for not working properly.
Matrix 3DMatrix metering is preserved by both kinds of converter, and Matrix 3D will work if you are using a 'D' lens. But something funny occurs: if you use AF (non AF-S) lenses or the AF-d converter, your camera body will not show the actual aperture. For instance, if you use a 1.4x together with a AF 180 mm f/2.8, the body will still believe your combo is able to reach f/2.8 at full aperture, and not f/4 (which is the correct answer because the teleconverter introduces a loss of light). Nevertheless your TTL exposimeter will compute the proper exposure time, 'believing' that the loss of light is due to the external conditions, i.e. there is a weaker light out there. Most of the times this fact causes no harm; but since the Matrix fuzzy logic sometimes takes decisions depending on absolute light values (that is, measuring the effective light out there), it could be fooled, usually underexposing (see an article by Ken Rockwell about this topic). This might happen with images having complex lighting, which should not occur very often since you're using a long focal and a narrow field of view which isolates subjects from the context. When if you use the AF-s converter together with an AF-S lens, the correct aperture will be shown. Summing up:
Light fall-offLight
fall-off seems to be decently controlled by the Kenko 1.4x AF-s.
The picture below shows some preliminary tests run with the Nikkor
AF-S 300mm f/4D. Some fall-off occurs at full aperture (f/5.6);
it is greatly reduced at f/6.7 and almost invisible at f/8. From
f/9.5 and above it completely disappears. The test was run with
the following procedure: F80 in A mode, the subject was a clear
blue sky at noon, film was Fuji Velvia 50 pushed one stop at ISO
100 (this setting makes for very high contrast, so we can consider
the test a worst-case).
Nevertheless you must be careful when you meet the typical conditions that enhance the effect of light fall-off: when shooting against an almost uniform background such as blue sky, water pools, lakes or the sea (see photos 0204703 and 0204015). Note: you can click any of the following images to enlarge them (you need JavaScript).
The Kenko 2x seems more critical. I still have to run an objective test such as the one made for the 1.4x, but field experience says that light fall-off is quite annoying at full aperture (see photo 0204015). With subjects which do not present an uniform background the combo can be used with very good results (see photo 0202533). Anyway, stopping down you get great photos (see photo 0209329).
FeedbackDid you like this article? Do you think it was useful? Is there anything you would like to say? Is there any different test you would like me to try? Feel free to submit your thoughts at: webmaster@timelesswanderings.net. NOTE: if I don't reply within two days, try again at Fabrizio.Giudici@libero.it - it seems that I have some troubles with the former account.
LinksDon
Ferrario's test: Bruce
and Tamy Leventhal's test: Grizzly
Bear test: Manufacturer's
site: Ken
Rockwell on Matrix problems with third party teleconverters: Guy
Parsons' Testing Tamron SP AF Teleconverters:
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