http://www.the-digital-picture.com/Reviews/Sigma-100-400mm-f-5-6.3-DG-OS-HSM-Contemporary-Lens.aspx schrieb:
Providing significantly more magnification capability to this lens are teleconverters. The 100-400 C is compatible with the Sigma TC-1401 1.4x and TC-2001 2x Teleconverters, respectively generating a 1.4x and 2x increase in both focal length range and MM values.
The 1.4x TC creates a 140-560mm lens, closely matching many of the other zoom lens options shown in the above table. Not so closely matching is the 1-stop-narrower max aperture this teleconverter creates, making this a dark f/7.1-9 lens. As mentioned earlier, many cameras require an f/5.6 max aperture for conventional phase detection AF to work and the best require f/8. Expect, minimally, focus hunting to be a common issue. Live View and other sensor-based AF systems can often AF with lenses as dark as f/11, so that option remains available.
The viewfinder is not terribly bright with an f/6.3 max aperture and it will noticeably darken with the 1.4x in place.
When the base image quality of a lens is magnified, defects are also magnified and I was not expecting very good results with teleconverters behind this lens. Surprisingly, this lens does not show much sharpness degradation with the 1.4x behind it and the added barrel distortion offsets some of the lens' native pincushion distortion. A downside is that lateral CA is increased.
Addition of the 2x TC makes this a very impressive 200-800mm lens. Much less impressive is the resulting f/10-13 max aperture and equally unimpressive is the sharpness of this combination that is diffraction limited wide open on many current and recent model cameras. Additional lateral CA and reduced barrel distortion are two more attributes of this combination.