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DDoublers can be very useful in the field, and save money over the purchase of a long lens. THere is no free lunch, though. Any softness in the main lens is magnified by the TC. There is always the matter of light loss when using a doubler or TC. A 1.4 TC loses one stop of light, while a 2x TC will cost 2 stops of light.
Kenko TC's can usually be used with any lens having a focal length of 100MM or longer, though it is always best to check clearance before mounting.
Nikon TC's tend to be a bit more lens specific. In other words, certain lenses need certain TC's.
Image quality can be a mixed bag. A 1.4 TC usually degrades the image very little, while the 2X TC can sometimes cause fairly heavy degradation. Much depends on the quality of the lens being used and the quality of the TC.