Perhaps the biggest downer about Fujifilm cameras is that the X series system is a closed one. While Sony has opened the gates so that Sigma, Tamron, and others can create lenses without reverse engineering their AF system, Fujifilm does not want to go this route.
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Recently, Sigma told the world that they would love to make lenses for the Fujifilm X mount platform. Still, because Fujifilm will not play nice and share their algorithms with third parties, Sigma said that they would have to walk away as they do not have the time to reverse engineer Fujifilm's AF system.