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Last year I obtained an EF 100-400mm f/4.5-5.6L IS USM II lens and an f/1.4x III extender. The new series 3 extenders are much sharper than the previous versions.
I tried the 1.4x III on the lens with good results since the APS-C sensor gives me an additional 1.61x magnification. On the 100-400mm II lens with the APS-C crop factor and 1.4x III extender gives me about 901.6mm zoom (equiv).
