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(Kenko, Tamron, Sigma, Spooginar, Quantary, Tokina, Albinar etc.)
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Many folks ask me about teleconverters. I personally have never tried one of these discount versions, however here are my suggestions.
Nikon makes no teleconverters that allow autofocussing with normal AF lenses. If you have this need you have to interbreed. I have a page about the use of teleconverters in general here.
Watch the matrix meter coupling. I suspect that the discount converters do not properly couple the effective apertures to the cameras and therefore will screw up the operation of the Matrix meter.
The range of f/stops available as indicated on your camera must be one or two stops slower than your lens, depending on the strength of the converter. If you can get the camera to indicate the same maximum aperture of your lens even if you are using the converter, you can screw up the operation of the matrix meter.
For example, if you have an f/4 lens and use a 1.4x converter you had better not be able to set any aperture larger than f/5.6 through the camera. If you can your converter is not calculating the data sent to the camera correctly.
Why? Simple. You know you lose a stop with a 1.4x converter and 2 stops with a 2x converter. Therefore an f/4 lens becomes effectively an f/5.6 with a 1.4x converter, and f/8 with a 2x. In motion pictures we call these "T" stops, for "true" or "transmissive" stops.
The Nikon TC-E series converters do this correctly. With an AF-S lens having f/stops from 2.8 through 22 I get an indicated range of f/4 - 32 with the TC-14E 1.4x and f/5.6 - 45 with the TC-20E 2x. Therefore the matrix meter works flawlessly as it always does.
If your discount converter messes this up, as most of them do, you run the risk of really bad exposures in some of the complex lighting situations for which most people take the matrix meter for granted.
Reports confirm that the Kenko PRO300 converter has this defect with conventional AF lenses, and that it does affect the matrix meter. In these cases you may want to revert to center weighted metering and the zone system or strong exposure compensation.
How do you check this? Simple again. Go find a bright beach, snow, or anything bright white in full sun. Photograph it with slide film with and without the converter and without any filters. Without the converter you'll get a white object. If the matrix meter is fooled you'll get grays instead of whites.
You will get the matrix metering symbol, but since you're not feeding the camera the correct lens data you may get poor exposures.
Kenko tried to brush this under the doormat when I asked, since their "pro" series converters about which most people ask have this problem. The guy tried to get me to believe that since the camera still reads light through-the-lens that the effect of the TC is compensated by magic. Well, yes it does, however you are fooling the matrix by a stop or two and may get a stop of under exposure due to this in bright situations. You will see the matrix indicator, but it may not be working correctly.
Don't trust anyone here. Go buy one and try it on some white subjects in daylight on slide (not print) film.
In fact, no one knows how well any converter will work with your lens and your camera until someone actually tries that exact combination. You have to be your own judge; the performance of the converter is meaningless since it will do very different things with different lenses.
Lenses and converters are sort of like people. Each one comes out a little different, and each works differently and differently with others. You have to try it to see how you like it.
If I was to buy one of these converters my first try would be the Kenko Pro series. Some of them supposedly work with all of the AF, AF-I and AF-S lenses. I'd carefully test their function with matrix metering. If they fail I'd either send them back or ask myself if I really want to fall back on center weighted metering if the matrix metering was unreliable.
The center weighted and spot meters require a great deal of skill to use and compensate for in contrasty light. The Matrix is so good that few photographers know how to use the old style meters well anymore.
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