Teleconverters

Nikon has made two teleconverters for the Z-mount: a 1.4x and a 2.0x version.

Common question: does a teleconverter always degrade optical performance?

Answer:  yes. Three basic components are involved:

  1. More glass/Air surfaces — every glass/air transition results in a small amount of light not being transmitted through to the image sensor. In poorly made teleconverters, you could also get reflections internally that could reduce overall contrast and perhaps increase ghosting.
  2. More diffraction — you're increasing diffraction blur size, particularly when you push the Airy disc size to over 2x the photosite pitch, which is fairly typical with f/4 and higher lenses on high megapixel count cameras. At 45mp, a critical diffraction point is hit at f/5.6. On an f/5.6 lens with a 1.4x teleconverter, you'll push the diffraction blur width 40%, and the information content of the image is reduced by about 50% (1/1.4^2). 
  3. Not specific enough — technically, you could make a "paired" teleconverter that corrects specific known anomalies of a particular lens that could do the opposite of degradation. Unfortunately, matching TC and lens is generally not the case, and certainly not the case with add-on teleconverters.

That said, teleconverters can be a useful tool when used properly and if designed well. The TC-1.4x teleconverter on the 100-400mm f/4.5-6.3 VR S lens, for instance, produces a far better than expected result. 

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