The dreaded back-focus and front-focus generally occur because the camera body is inaccurately manufactured so the light path to the AF sensor is different length to the light path to the sensor. There's also the problem that there may be focus issues. I wouldn't like to exaggerate the problem because most people have no problems with Sigma lenses on Sony/Minolta stuff but there's always that nagging doubt that a Sigma lens won't be compatible with a later release of body (because Sigma probably don't know all the potential of the mount). Sigma don't have a licence and rely on "reverse-engineering" to work out what all the pins do. Sony (and Minolta) charge money to license that information. Basically the A mount isn't an open system (unlike the 4/3 system) where anyone can get the specs for the electronic and mechanical interface.
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