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This converter is the ideal companion to any of the professional nikon lenses that are rated at f2.8 or f4.
As it says on the box it will multiply the focal length of your lens by 1.4, so a 70-200 f2.8 lens will become a 98-280mm lens, and a 300 will become 420mm, if you are lucky to have a 500mm it will become a 700mm monster! The only downside is a loss of one stop of light, so f2.8 lens will become f4, and an f4 lens will become f5.6.
This converter will only autofocus with lenses that have an internal focusing motor such as AF-S and HSM. The construction of the converter includes a little metal tab which stops it attaching to non AF-S lenses too. This means it won't attach to the Sigma lenses. However, there is a little work around which can be found on the internet which involves filing off the tab, I would warn you that this will void warranties, and unless you are really sure in what you are doing don't even try! It will then fit and work just fine with sigma HSM lenses as well as nikon AF-S lenses. This is perfect for lenses like the sigma 500mm f4.5 lens which would otherwise not fit this converter.
Sharpness when using this converter is excellent, and I use mine paired with the nikon 70-200 f2.8 vr lens almost all the time. This combination makes a perfect lens for wildlife photography, with an extra little bit of 'reach' over the lens by itself. Most photo agencies do not allow upscaling of images so this is the way to give your lenses a bit of higher magnification without buying a bigger lens.
The build quality of this teleconverter is excellent. And the finish matches that on the new AF-S nikon lenses well. It is reasonably light in weight, especially when you think you would may need a whole new lens to replicate its effect.
The only downsides are that it is quite expensive, and that you have to modify it to work with non AF-S lenses such as those by sigma, tamron etc.